Plant Science 176 (2009) 187–199
Contents lists available at ScienceDirect
Plant Science
journal homepage: www.elsevier.com/locate/plantsci
Review
Vegetative desiccation tolerance: Is it a goldmine for bioengineering crops?
Ottó Toldi a,*, Zoltán Tuba b,c, Peter Scott d
a
Agricultural Biotechnology Centre, P.O. Box 411, H-2101 Gödöllő, Hungary
Institute of Botany and Ecophysiology, Szent István University, H-2103 Gödöllő, Hungary
c
Plant Ecology Research Group of Hungarian Academy of Sciences at Department of Botany and Plant Physiology,
Faculty of Agriculture and Environmental Sciences, Szent István University, Gödöllö, H-2103 Gödöllö, Hungary
d
School of Life Sciences, University of Sussex, Brighton BN1 9QG, UK
b
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 28 June 2008
Received in revised form 2 October 2008
Accepted 3 October 2008
Available online 19 October 2008
In desiccation-tolerant plants vegetative organs can dry to about 4–13% relative water content, while
desiccation-sensitive plants die when their relative water content drops below 20–50%. Desiccationtolerant plants have different stress-adaptation strategies that provides good basis for their
catalogization. Thus, these plants may be subdivided into homoiochlorophyllous and poikilochlorophyllous types according to the status of their photosynthetic apparatus upon dehydration. During
dehydration homoiochlorophyllous species retain their photosynthetic apparatus and chlorophylls in a
readily recoverable form, whereas in poikilochlorophyllous species desiccation results in the loss of
chlorophyll, which must be resynthetized following rehydration. Desiccation-tolerant plants can be
subdivided also on the basis of differences in the molecular mechanism of desiccation tolerance. Fully
desiccation-tolerant plants are able to withstand rapid drying and possess constitutive tolerance, while
modified desiccation-tolerant species are able to survive slow drying and possess inducible tolerance.
Mechanisms proposed to explain the ability of vegetative desiccation-tolerant plants to survive
desiccation include sucrose and trehalose accumulation, accumulation of stress proteins, increased
folding ability of cell wall structures and accumulation of membrane stabilizing polyphenols and
antioxidants. However, any of the above mechanisms may not have the same effect in desiccationsensitive plants, such as crop plants. The utility of using genes from the desiccation-tolerant plants lies in
the prospect of finding novel ways to maintain productivity of crop plants following periods of drought by
either broadening the limits of cell maintenance to encompass lower water potentials and/or repairing
damage as it occurs allowing for broadening of the sensitivity range. The question is how it can be
achieved technically, because the weakest point within the whole high-throughput technologies is the
limited capacity of conventional genetic transformation techniques. However, despite the limitations,
conventional genetic transformation is still an unavoidable part of the functional genomics and
molecular breeding programmes. Since resurrection plants show extreme sensitivity and vulnerability in
tissue culture, lessons learned from their genetic transformation can be extended over other recalcitrant
species.
ß 2008 Elsevier Ireland Ltd. All rights reserved.
Keywords:
Resurrection plants
Glutathione (GSH)
ABA
Raffinose family oligosaccharides (RFOs)
Poikilochlorophyll
Homoiochlorophyll
Contents
1.
2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.
Fully desiccation-tolerant plants versus modified desiccation-tolerant plants . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.
Homoiochlorophylly and poikilochlorophylly: two alternatives in the modified desiccation tolerance . . . . . .
1.3.
Evolutionary aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sugars serve as osmoprotectants, components of the biological glass, and mobilizable energy sources during stress
Proteins closely associated with desiccation tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
* Corresponding author. Tel.: +36 28 526166.
E-mail address: toldi@abc.hu (O. Toldi).
0168-9452/$ – see front matter ß 2008 Elsevier Ireland Ltd. All rights reserved.
doi:10.1016/j.plantsci.2008.10.002
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O. Toldi et al. / Plant Science 176 (2009) 187–199
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4.
5.
6.
7.
Abscisic acid-based stress signaling in vegetative desiccation tolerance: whether specific or follows the drought-sensitive scheme? . . . .
Glutathione might synchronize different stress-avoiding processes during desiccation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Genetic transformation of desiccation-tolerant plants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusion and future aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction
1.1. Fully desiccation-tolerant plants versus modified desiccationtolerant plants
A pyrrhic victory is a victory so costly that it is almost
equivalent to a defeat. It derives from the battle won by King Pyrus
of Epirus over the Romans at Asculum in 279 BC. Noting the heavy
losses his own side had taken, he is reported to have said: ‘‘One
more such victory and I am lost.’’ This could be one’s first
conclusions after reading the review entitled ‘‘Constraints of
tolerance: why are desiccation-tolerant organisms so small or
rare?’’ [1] Although the initial evolution of vegetative desiccation
tolerance was a crucial step required for the colonization of the
land by primitive plants, it came at a cost [2]. In desiccationtolerant plants vegetative organs can dry to about 4–13% relative
water content, while desiccation-sensitive plants die when their
relative water content drops below 20–50%. Probably because of
the extra energy-requiring protection and repair mechanisms, the
interrelated features of metabolic rates, biomass production and
competitive abilities are usually lower in desiccation-tolerant
plants as compared to desiccation-sensitive plants. The most
serious consequence of this conclusion is that a fully desiccationtolerant crop plant would have little agricultural value. However,
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this should not deflect us from asking two important questions:
what can we learn from desiccation-tolerant plants, and to what
extent will we be able to utilize this knowledge for bioengineering
increased transient drought tolerance in crop plants? For that we
need to select essential components of desiccation tolerance that
are readily transferable to non-tolerant systems. There are already
examples where outcomes of targeted studies in desiccationtolerant plants are going to be directly utilized to genetically
engineer crop plants [3]. For example, as a part of an ongoing
project, some desiccation tolerance-related genes of Xerophyta
viscosa are to be expressed in maize. It is not known whether the
transfer of protective proteins, osmolyte sugars, signal metabolites
or antioxidants from desiccation-tolerant species will result in
crop plants with increased abiotic stress tolerance.
A vast majority of plant tissues are sensitive to dehydration. The
tissue is damaged and will ultimately die once the water content of
the tissue falls below a certain percentage. Ironically most plants
possess at least one stage of their life cycle where at least some
tissues or cells can survive severe dehydration. For many plant
species this is limited to seeds, pollen or in dormant buds. During
maturation, these organs lose most of their water and enter a
dormant state and then can remain apparently inactive for long
periods. For example seeds can lie in this state for centuries, seeds
of sacred lotus (Nelumbo nucifera); 75% germinated after 1300
Fig. 1. Classification of vegetative desiccation-tolerant plants according to their drought-adaptation strategies.
O. Toldi et al. / Plant Science 176 (2009) 187–199
years of storage [4]. However, most plants lack this ability in
organs such as leaves. Desiccation-tolerant plants are able to do
this. In this small group of plants the mature leaves, roots and
shoots can lose up to 95% of their water. This results in a shrivelled
dried plant that is actually still alive. The loss of more than the half
water content is enough to kill the tissue of most plants.
Desiccation-tolerant, or in other words poikilohydric plants,
possess mechanisms to protect them in the dried state. Higher
desiccation-tolerant plants are also referred to as resurrection
plants, because they can be resurrected by rehydration. Desiccation tolerance is also common in many algae and among
microorganisms, such as rotifers, nematodes and tardigrades
and in Crustacea of seasonal water bodies. It is thus a widely
expressed potentiality of living organisms.
Desiccation tolerance occurs throughout the plant kingdom. It
is commonplace among lichens, relatively common in mosses and
is found sporadically among vascular plants from a range of
families. Desiccation tolerance of the vegetative tissues in vascular
plants has been demonstrated in some 350 species, making up less
than 0.2% of the total flora [5] but the list is constantly being
extended. Desiccation tolerance is thus thinly and unevenly,
scattered amongst vascular plants. Virtually all vascular plants
have desiccation-tolerant spores (including pollen) or seeds, so the
potential for desiccation tolerance is probably universal.
The majority of vegetative desiccation-tolerant plants are found
in the less complex clades that constitute the algae, lichens, and
mosses. These plants are also called fully desiccation-tolerant
plants (Fig. 1), because they withstand the total loss of free
protoplasmic water [6]. The internal water content of these plants
rapidly equilibrates to the water potential of the environment, as
they possess very few of morphological or physiological adaptations for the retention of water. As a result of this, many lichens,
algae, and desert bryophytes have rapid drying rates, i.e., reaching
air dryness within an hour. Desiccation of such plants can even be
achieved in a few minutes in a lyophilizer, which indicates that an
inducible protection mechanism is not necessary for survival in
this group of lower desiccation-tolerant plants [2]. This strengthens the hypothesis that the primitive mechanism of tolerance
probably involves a low intensity, but constitutively functioning
protection mechanism that is coupled with active cellular repair
[2]. A larger and more complex group of vegetative desiccationtolerant plants are named modified desiccation-tolerant plants.
The vascular desiccation-tolerant plant species belong to this
group [7]. They dry more slowly by an array of morphological and
physiological mechanisms that retard the rate of water loss to the
extent required to establish tolerance. Available evidence concerning desiccation tolerance of modified desiccation-tolerant
plants strongly suggests that they utilize preventive mechanisms
that rely heavily on inducible cellular protection systems [8,9].
1.2. Homoiochlorophylly and poikilochlorophylly: two alternatives in
the modified desiccation tolerance
Desiccation-tolerant plants may be subdivided also into
homoiochlorophyllous and poikilochlorophyllous types (Fig. 1).
During desiccation homoiochlorophyllous species retain their
photosynthetic apparatus and chlorophylls in a readily recoverable
form, whereas in poikilochlorophyllous species desiccation results
in the loss of chlorophyll, which must be resynthetized following
rehydration [10]. Much has been published on the photosynthetic
responses of homoiochlorophyllous desiccation-tolerant plants,
especially on the cryptogamic plants [7]. A great deal of research
has been conducted on the tolerance limits of homoiochlorophyllous desiccation-tolerant cryptogams and phanerogams [7,11].
However, until our recent studies [7,10,12–15] little was known
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about the ecology, ecophysiology and distribution and abundance
of poikilochlorophyllous desiccation-tolerant plants.
Poikilochlorophylly, the phenomenon of loss of chlorophyll
during desiccation was first described in Carex physodes from
central Asia [16]; then it was reintroduced [17], and was regarded
as a special phenomenon in certain desiccation-tolerant monocotyledonous plants [18]. In contrast, the rate of chlorophyll loss in
the homoiochlorophyllous desiccation-tolerant plants varies from
species to species, and it is influenced by environmental factors,
however the rate of chlorophyll loss in photosynthetically active
plants does not exceed the critical physiological amount. In other
words, homoiochlorophyllous desiccation-tolerant plants cannot
survive the complete or even the physiologically dangerous partial
loss of their chlorophyll content. Poikilochlorophylly evolved as a
different evolutionarily strategy [10,14,19]. It is based on the
dismantling of internal chloroplast structure by an ordered
deconstruction process during drying, and its resynthesis upon
rehydration by an ordered reconstruction process [13]. These
processes can thus be thought of not only as being superimposed
on an existing cellular protection mechanism of vegetative
desiccation tolerance [19] but also as a distinct new class of
desiccation-tolerance strategy [7,14]. The selective advantage of
poikilochlorophylly, in minimising photo-oxidative damage and
not having to maintain an intact photosynthetic system through
long inactive periods of desiccation, presumably outweighs the
disadvantage of slow recovery and the energy costs of reconstruction. Taxonomically poikilochlorophyllous desiccation tolerance
appears to be restricted to the monocots [18]. Poikilochlorophylly
is currently known in eight genera of four families (Cyperaceae,
Liliaceae/Anthericaceae, Poaceae and Velloziaceae). Most occupy
the almost soil-less rocky outcrops known as inselbergs, in
strongly seasonal tropical and sub-tropical climates [5]; the best
studied physiologically are the African Xerophyta scabrida, X.
viscosa and Xerophyta humilis and the Australian Borya nitida [7].
The homoiochlorophyllous desiccation-tolerant and poikilochlorophyllous desiccation-tolerant strategies solve the same
ecological problem, but cover a broad temporal range of adaptation
[10]. The homoiochlorophyllous desiccation-tolerant ferns and
angiosperms are generally adapted to longer drying–wetting
cycles than mosses and lichens, but to more rapid alternations of
wet and dry periods than the poikilochlorophyllous desiccationtolerant monocot species [10,11,20,21], though some can survive
desiccation for very long periods of time. The poikilochlorophyllous desiccation-tolerance strategy has evolved in habitats where
the plants remain in the desiccated state for 8–10 months. Under
these conditions it is evidently more advantageous to dismantle
the whole photosynthetic apparatus and reconstitute it after
rehydration. Of course there is variation within each category and
the categories overlap in their ecological adaptation, and two or
more may coexist in one habitat, e.g. on inselbergs [22]. Both ends
of this ecological spectrum have particular points of interest. There
is probably a trade-off between the ‘cost’ of protection and repair
to the photosynthetic apparatus if this is kept in a quickly
recoverable state through prolonged periods of desiccation, and
the ‘cost’ of reconstituting the photosynthetic apparatus de novo
[10].
1.3. Evolutionary aspects
The early evolution of vegetative desiccation tolerance is
thought to be a critical step in the colonization of the land by
primitive plants [2]. As plant species evolved, vegetative desiccation tolerance was lost as increased growth rates, competitive
ability, structural and morphological complexity involving
mechanisms that conserve water within the plant became valuable
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O. Toldi et al. / Plant Science 176 (2009) 187–199
traits for plants. Genes that had evolved for cellular protection and
repair were recruited for different but related processes such as
response to water stress and the desiccation tolerance of
reproductive organs like seeds, pollens, dormant buds and
protocorms. However, their expression in vegetative tissues is
rare, and must have re-evolved independently in Selaginella, in
ferns, and at least eight times in angiosperms [19]. The only major
class of vascular plants that does not have a representative species
that has desiccation-tolerant vegetative tissues is the gymnosperms (a taxonomic group consisting of the phylogenetically
distinct cycads, conifers, and genetophytes), a fact may signify a
minimum size limitation for desiccation tolerance, which members of this group exceed. We continue this speculation to assert
that modified desiccation tolerance typical of angiosperms is not a
developmental continuation of the more primitive full desiccation
tolerance, but it evolved from that programmed into seed
development. However, to answer the question, why was it an
evolutionary advantage to re-introduce vegetative desiccation
tolerance, instead of surviving unfavourable conditions as a seed,
still remains an enigma. Moreover, we know that the natural
habitats of vegetative desiccation-tolerant plants are often
deficient in mineral and organic nutrients [22]. This could suggest
that the resurrection cycle requires fewer nutrients from the
environment than that of completing the whole developmental
process from a seed. The other hypothesis is that vegetative
desiccation tolerance evolved from the response of desiccationsensitive plants to abiotic stresses such as cold, salt and drought
[23]. Desiccation-sensitive plants use an interconnected signaling
network to activate a common repertoire of responses to abiotic
stress [24]. These responses appear to overlap with those described
for desiccation-tolerant plants during extreme water loss as they
include accumulation of compatible osmolytes, and the upregulation of antioxidants and antioxidant enzymes [24]. Small-scale
microarray analysis of X. humilis, an indigenous Southern African
resurrection plant, has revealed that dehydration-upregulated
cDNAs included known stress-responsive genes encoding metallothioneins, galactinol synthases, an aldose reductase and a
glyoxalase [25]. A large number of genes encoding late embryonic
abundant proteins, dehydrins and desiccation-related proteins
were also identified, suggesting that proteins that provide
mechanical and antioxidant protection against water loss dominate the mRNA population in desiccated X. humilis leaf tissue [25].
Under this scenario, it was predicted that there should be
significant overlap between genes that are introduced in response
to abiotic stresses and desiccation tolerance [24]. This coincides
with the conclusion of those who suggest that desiccation
tolerance in vegetative tissues in Craterostigma plantagineum is
probably not due to structural genes that are unique to
resurrection plants, but that relevant genes are also present in
the genome of non-tolerant plants [26]. The difference between
tolerant and non-tolerant plants probably resides in gene
expression patterns, meaning that desiccation-tolerant plantspecific regulation of a common set of genes is probably
responsible for vegetative desiccation tolerance. However, there
are notable reports on large-scale partial sequencing of randomly
selected cDNA clones or expressed sequence tags (ESTs) in
different desiccation-tolerant plants that has led to a contrasting
hypothesis. A complementary DNA library was constructed from
the dehydrated microphyll fronds of the resurrection plant
Selaginella lepidophylla and used to generate an EST database.
ESTs were obtained for 1046 clones representing 874 unique
transcripts. Putative functions were assigned to 653 of these clones
after comparison with protein databases, whereas 212 sequences
were significantly similar to known sequences whose functions are
unclear and 181 sequences having no similarity to known
sequences [27]. Small-scale microarray analysis of Tortura ruralis,
a bryophyte model for genomic level investigations, showed that
only 29% of the desiccation-upregulated cDNAs had significant
similarity to previously identified nucleotide and/or peptide
sequences [28]. In an earlier report cold-plaque screening was
used to analyze 200 cDNA clones from C. plantagineum leaves that
had been either dried for 1 h or totally dried down [29]. One half of
the sequences showed no significant similarity to those in public
databases. A comparison of these reports suggests that considerable progress was achieved in exploring gene functions, but the
high proportion of unknown sequences and sequences without
known function still makes it difficult to establish provisional
pathways.
Desiccation-tolerant plants are important constituents in many
ecosystems from the tropics to the polar arctic region. For example,
lichens and mosses play a major role in temperate semidesert
grasslands, in cool and hot deserts, and importantly in tundra and
polar/arctic vegetation. These plants dominate under unfavourable
climate conditions, where the normal homoiohydric plants
maintain much of their biomass below the soil surface, succumb
to stresses and/or are unable to establish themselves. Large pools
of nutrients and carbon accumulate in desiccation-tolerant plants
in these extreme ecosystems, and therefore significant aspects of
ecosystem function depend on their physiological response,
production and turnover pattern. Thus, any analysis of the
terrestrial vegetation warrants investigation of the desiccationtolerant plants. But the desiccation-tolerant plants do have a direct
practical significance, also: the understanding of mechanisms of
desiccation tolerance will be useful in the future to modify species
by the powerful technique of genetic engineering, to develop crops
that are tolerant of the harmful effects of drought.
Desiccation tolerance is qualitatively different from drought
tolerance as ordinarily understood in vascular-plant physiology;
indeed desiccation tolerance could be seen as a droughtadaptation mechanism. Desiccation-tolerant plants are not simply
an odd sideline from mainstream homoiohydry [7]. They are an
adaptive optimum in particular ecological situations, and (as
‘normal’ vascular plants) can be only understood fully in the
context of a wide and multidimensional field of physiological and
ecological possibilities. Most vascular desiccation-tolerant plants
function as normal (‘homoiohydric’) vascular plants until water
becomes limiting. Like winter annuals and desert ephemerals (and
other mesophytes), they then dry out within a few hours or days.
The difference is that instead of dying and re-establishing from
seed, their fall-back is to survive in a desiccated, but still viable
vegetative state.
Wild species have been chosen as crops because of their high
productivity, and bred to make them more so. Because crops are in
general desiccation-sensitive, they must either be watered or
grown where there is no drought. Aridity thus remains the greatest
enemy of agriculture. In addition, there is a necessary functional
conflict between high productivity and desiccation tolerance.
Although there are other protective processes, three major
interacting biochemical mechanisms for recovery from desiccation-induced injury are emphasized in this review. Firstly, it was
proposed that non-reducing sugars facilitate tolerance to desiccation by protecting membranes and proteins [22,30–32] and by
inducing vitrification [32]. Secondly, another mechanism is based
on the capability to scavenge desiccation-induced free radicals.
Desiccation tolerance has been correlated with an increased alarm
status of both the non-enzymic (reduced glutathione, ascorbic
acid, tocopherols) and enzymic components of the antioxidant
defense system [21,33,34]. The third mechanism includes proteins
such as the late embryogenesis abundant (LEA) proteins [20,26]
and LEA-related proteins such as dehydrins and rehydrins [2,38].
O. Toldi et al. / Plant Science 176 (2009) 187–199
2. Sugars serve as osmoprotectants, components of the
biological glass, and mobilizable energy sources during stress
A fundamental component of desiccation tolerance appears to
be sugars [22,35,36]. In all of the modified desiccation-tolerant
plants studied to date, drying induces a major change in
carbohydrate metabolism, which may be directly related to
desiccation tolerance. Sucrose is the only free sugar available for
cellular protection in fully desiccation-tolerant mosses, including
Tortula ruraliformis and T. ruralis [37,38]. The amount of this sugar
in T. ruralis gametophytic cells is approximately 10% dry weight,
which is sufficient to offer membrane protection during drying, at
least in vitro. Moreover, neither drying nor rehydration in the dark
or light, results in a change in sucrose concentration on a dry
weight basis, suggesting that it is important for cells to maintain
sufficient amounts of sucrose [37]. The lack of a dehydrationspecific increase in soluble sugars during drying appears to be a
common feature of fully desiccation-tolerant mosses that maintain
high sucrose content constitutively [38]. In contrast to fully
desiccation-tolerant plants, modified desiccation-tolerant plants
accumulate high concentrations of sucrose during the dehydration
phase (Fig. 2) in all species when this has been studied [39–47]. In
tandem with specific proteins [48], sucrose is among the sugars
that probably stabilize drying cells both by direct interactions with
Fig. 2. Sucrose contents in leaves of desiccation intolerant (S. pyramidalis) and
desiccation-tolerant plants (C. plantagineum, M. flabellifolia, R. nathaliae, R. myconi,
H. rhodopensis, X. villosa, T. jacquemontii, S. stapfianus) in hydrated and dehydrated
stages.
191
macromolecules and membranes and by reversibly immobilizing
the cytoplasm to become an extremely slow-flowing glass-like
(vitrified) liquid [47]. Many isolated enzymes, dried in the
presence of sucrose, remain stable in the dried state [32]. Parallel
work on isolated membrane vesicles also supports the view that
sucrose preserves the integrity of the lipid bilayer during
dehydration [31]. Sucrose accumulation occurs relatively late in
the dehydration time course, initiated usually at relative water
content below 60%, in some the majority occurs at 20%. The
consensus is that the accumulation of sugar is one of the last
preparatory steps in the cell protection pathway when the cell is
fully committed to a period of quiescence.
To understand the biochemical basis for the stress-specific
switch in sugar metabolism, the expression of genes encoding
sugar-metabolizing enzymes was investigated in a higher plant
model C. plantagineum. This plant possesses a set of genes encoding
isoenzymes of sucrose synthase [49] and sucrose phosphate
synthase [42]. A characteristic rise in transcript levels of class-I
sucrose synthase genes was observed in response to dehydration
or abscisic acid (ABA) treatment, which indicates an increased
glycolytic demand. This coincides with the finding that the
transcript levels of cytosolic glyceraldehyde-3-phosphate dehydrogenase (GAPDH) also rapidly increase in abundance during
dehydration or ABA treatment [50]. The increase in class-I sucrose
synthase and GAPDH mRNA levels directly correlates with higher
protein and enzyme contents. As both sucrose synthase and
GAPDH are glycolytic enzymes, these results imply that enhanced
rates of glycolysis are one of the immediate cellular responses to
water deficit. Besides these enzymes, the transketolase–transaldolase functional enzyme complex might contribute to the
conversion of the eight carbon carbohydrate, 2-octulose, to sucrose
in C. plantagineum [51] and, alternatively, it might enhance
pentose-to-hexose flux providing more hexose for glycolysis. This
may be a mechanism by which the plant cell prepares for a demand
of ATP and NADH during recovery [49]. Sucrose phosphate
synthase behaves differently. The activity of this enzyme increases
in C. plantagineum leaves together with sucrose accumulation
during dehydration [42]. However, during the periods when the
highest extractable sucrose phosphate synthase activity was
found, the transcript levels were at their lowest, suggesting
additional regulatory mechanisms. The increased activity of
sucrose phosphate synthase may reflect the activation state of
the enzyme rather than just the amount of protein [42]. This
coincides with the recent consensus that sucrose phosphate
synthase is a subject of strong allosteric regulation [52]. The actual
activity of an allosterically regulated enzyme by positive and
negative effectors mirrors cellular homeostasis and thus, allows
continuous physiological adjustment.
The most potent negative effector of sucrose phosphate
synthase is the signal metabolite fructose-2,6-bisphosphate
(Fru-2,6-P2), which is an essential regulator of sugar metabolism
in fungi, plants and animals [53]. Plants use Fru-2,6-P2 to switch
between direct fuel use and storage, which is somewhat
comparable to the role of the compound in animals and humans
[54]. In plants Fru-2,6-P2 contributes both to coordination of
sucrose synthesis with the rate of CO2 fixation [52] and indirectly
to the control of assimilate partitioning between sucrose and
starch [55]. Fru-2,6-P2 allosterically inhibits cytosolic fructose-1,6bisphosphatase (cytFBPase), which catalyzes an irreversible
regulatory step in the sucrose synthesis pathway. A decrease in
the level of Fru-2,6-P2 increases the flux towards sucrose by
releasing cytFBPase from an inhibited state, which in turn,
stimulates sucrose phosphate synthase, another key enzyme in
the sucrose synthetic pathway [52]. At the same time, an increase
in the level of Fru-2,6-P2 increases the 3PGA:Pi ratio in plastids that
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O. Toldi et al. / Plant Science 176 (2009) 187–199
leads to a rise in ADPglucose pyrophosphorylase (AGPase) activity
and consequently to an elevated flux towards starch [55,56]. Thus,
manipulation of the endogenous levels of Fru-2,6-P2 provides a
testing system for molecular and metabolic processes that are
based on photosynthate partitioning. The upregulation of Fru-2,6P2 synthesis suppresses sucrose accumulation in photosynthesizing leaves during the light period. Sucrose synthesis is enhanced
following downregulation of Fru-2,6-P2 synthesis. This approach
has already been utilized to examine Crassulacean acid metabolism (CAM) in Kalanchöe daigremontiana [57]. We created
transgenic C. plantagineum plants with elevated levels of Fru2,6-P2 (O. Toldi, P. Scott, unpublished results) with the view of
suppressing sucrose accumulation during dehydration. All strong
expressors of the mammalian 6-phosphofructo-2-kinase gene (6PF-2-K), which synthesizes Fru-2,6-P2, were unable to ‘resurrect’
during rehydration. At the same time, wild type plants and
transgenic controls, carrying empty vectors, started the recovery
process when irrigation was continued (O. Toldi, P. Scott,
unpublished results). This demonstrates well that sucrose
accumulation is an indispensable mechanism within the vegetative desiccation tolerance at least for modified desiccation-tolerant
species. However, when Fru-2,6-P2 levels were downregulated by
the introduction of its Fru-2,6-P2ase-mediated catabolism, plants
became even more sensitive to drought. The high sucrose content
throughout the diurnal cycle gave a rise to an increased turgor
pressure in leaves, stomatal conductance, therefore, remained also
high during the dehydration phase, resulting in an uncontrolled
transpiration. Thus, manipulation of sucrose metabolism in the
view of engineering crops for drought tolerance does not seem to
be a viable approach. Most likely that it would interfere with
osmoregulation, metabolism and the biomass production.
However, there are other components of the photosynthetic
carbohydrate metabolism with a more realistic agronomic
potential. Raffinose family oligosaccharides have long been
suggested to act as antistress agents in both generative and
vegetative tissues [3,58,59]. They are the most widely distributed
non-structural carbohydrates in the plant kingdom, occurring in a
wide variety of species [60]. As non-reducing carbohydrates they
are good storage compounds, being able to accumulate in large
quantities without affecting primary metabolic processes [3].
Research in seeds [61,62] and root systems [63,64] has revealed
strong correlations between accumulation of raffinose family
oligosaccharides, primarily raffinose, stachyose, and verbascose,
and desiccation tolerance. We propose that the functional
identification of galactinol synthase (XvGolS), catalyzing the first
committed step towards raffinose family oligosaccharides from X.
viscosa [3], opens the possibility to introduce a new protective trait
into crop plants.
These contrasting stories about the different agronomic
potentials of sucrose and raffinose family oligosaccharides shed
light on a third important target of molecular breeding efforts. The
non-reducing disaccharide trehalose also serves as a protectant
against a variety of stresses in different organisms [65] and
significantly contributes to vegetative desiccation tolerance in
plants [8,66,67]. Trehalose is among the most chemically
unreactive sugars and its strong stability is a result of very low
energy (1 kcal mol 1) of the glycoside oxygen bond joining the two
hexose rings [68]. Thus, trehalose certainly will not interfere with
primary metabolism and the energy homeostasis. Nevertheless,
results from the introduction of genes encoding trehalosesynthesizing enzymes to certain crop plants are contradictory.
In some instances, transgenic plants had increased tolerance to
abiotic stressors that were associated with sustained plant growth
[69]. Three different approaches resulted in improved drought
tolerance without causing growth defects. First a double construct
carrying the Saccharomyces cerevisiae trehalose-6-phosphate
synthase and trehalose-6-phosphate phosphatase genes driven
by a constitutive promoter (AtRbcS1A) were introduced into
tobacco [70]. In the second approach, a trehalose-6-phosphate
synthase gene was driven by AtRAB18 [70] and rd29A [71] stressinducible promoters. The third strategy was the use of a
constitutive AtRbcS1A promoter together with a transit peptide
in front of the coding sequence of the S. cerevisiae trehalose-6phosphate synthase, which directed the enzyme to the chloroplast
[70]. However, although the majority of transgenic plants were
more or less drought tolerant, they were stunted with different
morphological abnormalities [72]. One of the reasons for this can
be the use of strong constitutive promoters instead of stressinducible ones [70,71]. However, it cannot be generalized, because
drought-specific expression of the S. cerevisiae trehalose-6phosphate synthase gene in potato also resulted in drought
tolerance, but slow growing, retarded plants [73]. Recent microarray data on plants modified in the trehalose biosynthesis
pathways indicate that many of the trehalose-inducible genes
are associated with jasmonic acid and ethylene-dependent stress
signaling pathways [74,75]. The overexpressed Arabidopsis trehalose-6-phosphate synthase influences sugar sensing and photosynthate partitioning [76,77]. Whatever the basis for these
observations, trehalose is an important component of the sugarsignaling pathway in higher plants [78]. Engineering trehalose
metabolism with the aim of producing crops more tolerant to
abiotic stressors may result in severe interference with sugar and
other stress signaling pathways.
3. Proteins closely associated with desiccation tolerance
Some proteins specific to desiccation also have a major role in
cellular protection and recovery in vascular plants [79]. It is
important to determine the roles of these proteins attempting to
understand the mechanism by which modified desiccationtolerant plants establish tolerance [2]. Several of the cDNA clones
isolated from the desiccation-tolerant Craterostigma tissues are
putatively related to late-embryogenesis abundant proteins
(LEAs), proteins responsive-to-ABA, and dehydrins [9,20]. These
are all implicated in cellular protection during seed desiccation and
water stress [80,81]. Similarly, several cDNAs representing dryinginduced transcripts in Sporobolus stapfianus also code for proteins
related to LEAs [82]. However, LEA genes have been cloned not only
from desiccation tolerant ones but also from many plant species. At
least six different groups of LEA proteins have been identified on
the basis of expression pattern and sequence [83]. As their name
suggests, they are produced in abundance during late embryo
development, comprising up to 4% of cellular protein [84] with
maximum expression post-abscission during desiccation [85].
Their expression is coincident with the acquisition of desiccation
tolerance in seeds and pollen, as well as desiccation-tolerant
plants. Many LEA proteins are also induced by cold, osmotic stress
or exogenous abscisic acid, or can even be expressed constitutively
[86]. They have been variously proposed to protect cellular
structures from the effects of water loss by acting as a hydration
buffer, by sequestering ions, by direct protection of other proteins
or membranes, or by renaturing unfolded proteins [80,83]. These
suggested functions have been made on a largely intuitive basis
and are supported by relatively little evidence [86]. However, some
effect on stress tolerance seems apparent because LEA proteins
from tomato, wheat and barley confer increased tolerance to
osmotic or freezing stresses when introduced into yeast [87–90].
Barley LEA protein improved tolerance to water deficit in
transgenic rice [91] and wheat [92]. Furthermore, an algal LEA
protein diminished freezing damage to lactate dehydrogenase in
O. Toldi et al. / Plant Science 176 (2009) 187–199
vitro [93]. Chimeric LEA genes have already been extensively
utilized in molecular breeding programmes [91,92]. However,
their real contribution to drought and desiccation tolerance can be
evaluated only when stress-tolerant crops carrying LEA genes will
be commercially available for users in the realistic hope of
profitable production. The integration of LEA-mediated stress
responses will be necessary to identify of regulatory elements with
synchronizing role. This could be complemented by a one-by-one
functional analysis of LEA genes to identify the ones most specific
to desiccation tolerance. LEA proteins can also be processed into
small polypeptides that have activity in preventing protein
aggregation upon drought stress [48]. An additional option that
LEA genes provide is that their promoter elements can be used for
the transcriptional modulation of other resistance genes allowing
stress specific expression [94].
LEA proteins are not the only proteins important in the
acquisition of dehydration or desiccation tolerance. There is
growing evidence that small heat-shock proteins play a key role
perhaps by virtue of their reported chaperone-like activity. LEA
proteins can prevent protein aggregation [48,95], which could
mean they have similar properties to other chaperonins and that at
the very least it would indicate that chaperonins might be a good
target for a biotechnology approach to dehydration tolerance.
4. Abscisic acid-based stress signaling in vegetative desiccation
tolerance: whether specific or follows the drought-sensitive
scheme?
Abscisic acid is thought to play a co-ordinating role in the
activation of tolerance genes in response to desiccation [2].
Exposure of C. plantagineum plants and callus to exogenous abscisic
acid induces genes that are otherwise activated by dehydration
[9,26]. Callus of this plant is not inherently desiccation tolerant, but
becomes so if treated for 4 days with abscisic acid before drying.
Numerous new proteins, typical of abiotic stress, are synthesized
when abscisic acid is applied to non-stressed tissues [96]. For
example, a novel stress-induced antioxidant has been identified in
X. villosa [97] and a new gene, encoding XvSAP1 protein, which is
inducible by exogenous abscisic acid and thought to play a role in
membrane stabilization during water deficit [98]. The XvSap1
protein displays greater than 55% overall identity with the
TaCOR413 and is consequently clustered with the COR413
proteins. XvSap1 is regulated by environmental stress and abscisic
acid that it is able to increase desiccation and salt tolerance when
expressed in Arabidopsis thaliana [99,100]. qPCR analysis indicated
that XvSap1 mRNA was upregulated at 60% relative water content.
Thereafter, expression decreased but increased again at 15%
relative water content. This would suggest that XvSap1 is involved
in the initial and late stages of dehydration protection. Genes
responsive to abscisic acid usually contain abscisic acid-responsive
elements consisting of the (C/T) ACGTGGC consensus sequence and
are transactivated by bZIP transcription factors [101]. Three classes
of transcription factors have been characterized in C. plantagineum:
a heat-shock transcription factor [29], two members of the
homeodomain Leu zipper family [102], and three MYB genes
[103]. Transgenic Arabidopsis lines overexpressing the heterologous Craterostigma MYB transcription factor gene CpMYB10 were
drought stress-tolerant, glucose-insensitive and abscisic acidhypersensitive [104]. Although stress tolerance can be increased
by manipulation of abscisic acid-transduced stress responses, it
seems that undesired interference with other signaling processes
is highly probable.
In addition, several desiccation-induced genes, expressed early
in the drying process, are not inducible by abscisic acid. This raises
the possibility that other signaling pathways are also involved in
193
desiccation tolerance in Craterostigma [105]. In other words, stressrelated gene expression responses utilize multiple signaling
pathways [106]. Other desiccation-tolerant plants also have
independent signaling cascades [107,108]. Abscisic acid has not
been found in the desiccation-tolerant moss, T. ruralis [2].
However, fluoroscopic evidence was delivered that abscisic acid
helps induce desiccation tolerance in the afromontane moss
Atrichum undulatum [109]. Abscisic acid also acts as a signal for
increased non-photochemical quenching during water loss,
decreasing the production of harmful singlet oxygen [109].
Abscisic acid has a wide range of effects, therefore will elicit
many processes not specific to desiccation tolerance. Abscisic acid
is a cell cycle inhibitor [110], regulates stomatal movement [111],
suppresses growth [112] accelerates accumulation of storage
compounds [113], induces development of storage organs in vitro
[114,115], initiates dormancy and water loss [116] in most of the
plant tissues where it was examined. It is used exogenously for
maturation of somatic embryos [117], to induce differentiation of
protocorms and embryogenic structures in dedifferentiated cell
cultures [118], as well as to harden plant tissues prior to low
temperature storage [119]. Looking at the problem just superficially, it seems that abscisic acid does the same things in
vegetative desiccation-tolerant plants as it does in non-tolerant
plants. However, integrated redirection of complex molecular
events and metabolic processes might require such a multitargeted
signal like abscisic acid. Cell cycling and growth must be slowed
down to survive dehydration, while accumulation of energy
storing compounds and building units of essential metabolites and
macromolecules must be accelerated. It is hard to find a
satisfactory consensus in relation to the complex role of abscisic
acid in the regulation of desiccation tolerance. These preliminary
explanations are descriptive and based mostly on opinions and not
hard evidence. Our belief is that abscisic acid integrates the signal
transduction processes during dehydration phase that make the
survival of vegetative desiccation possible both on cellular and
whole plant levels. This would mean that the regulatory role of
abscisic acid both spatially and temporally is limited to certain
steps where integrated actions are required during the dehydration. If it is so, is there another regulator that replaces abscisic acid
during the rehydration phase? Do the recovery of normal
functions, acceleration of cell cycling and growth certainly requires
different signals?
5. Glutathione might synchronize different stress-avoiding
processes during desiccation
Chloroplasts are particularly sensitive to oxidative damage
during the dehydration phase [11,34]. In addition to switching on
physiological stress-avoiding mechanisms, angiosperm desiccation-tolerant plants employ physical means to prevent free-radical
formation. As we discussed previously chlorophyll is degraded and
thylakoid membranes are dismantled during drying in poikilochlorophyllous desiccation-tolerant plants [120]. There are indications that the degradation of vast amounts of chloroplast
membranes provides carbon for the dehydration-specific protection processes in this group of desiccation-tolerant plants through
the re-activated peroxisomal glyoxylate cycle (O. Toldi, et al.,
unpublished results). Homoiochlorophyllous species retain chlorophyll [10], but use various mechanisms to prevent light–
chlorophyll interaction while plants are dry [121]. However, the
active photosynthesis [122,123] and the high concentration of
chlorophyll during desiccation is a source for the production of
potentially harmful reactive oxygen species [34]. As the complete
recovery of photosynthetic ability has central impact on the
resurrection from the dehydrated state, effective protection of the
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O. Toldi et al. / Plant Science 176 (2009) 187–199
thylakoid membranes and important photosynthesis-associated
enzymes are indispensable in homoiochlorophyllous desiccationtolerant plants. To counteract the toxicity of reactive oxygen, a
multicomponent antioxidative defense system, including both
non-enzymic and enzymic constituents, are present in plant cells.
There is one abundant representative of this defense system, the
tripeptide thiol glutathione, which is involved in the oxidative
stress responses in nearly all desiccation-tolerant plants where it
was examined. Transcriptome [20] and proteome level examinations [124] revealed that genes encoding enzymes involved in
glutathione metabolism are among the genes most sensitive to
dehydration. As a result, the dehydration process could be
characterized by significantly increased glutathione levels in the
desiccation-tolerant Myrothamnus flabellifolia [34] and Boea
hygrometrica [124]. Desiccation of S. stapfianus results in increased
glutathione reductase activity [125], reaching a maximum midway
during rehydration in Craterostigma wilmsii [21]. The total
glutathione content does not vary, but the glutathione status,
i.e. the proportion of its reduced (GSH) and oxidized (GSSG) forms,
does in three species of desiccation-tolerant lichens as representatives of fully desiccation-tolerant lower plants [126]. Glutathione, as an important redox buffer, that protects many
cellular components [127] and the thiol status of proteins against
oxidative stress [128]. Glutathione can also detoxify hydrogen
peroxide by participating in the ascorbate/glutathione cycle or in
the reaction catalyzed by glutathione peroxidase [129]. Hydrogen
peroxide is especially toxic in the chloroplast, because even at low
concentrations it inhibits the Calvin-cycle enzymes possessing
exposed sulphydryl groups such us NADP+-dependent glyceraldehyde-3-phosphate dehydrogenase and the plastidic form of
fructose-1,6-bisphosphatase, hence reducing CO2 assimilation
[130]. This might provide an explanation why glutathione is so
important in the recovery of photosynthesis during the rehydration phase. Although its specific role in vegetative desiccation has
not yet been established, it appears more than coincidental that
glutathione and genes encoding enzymes of its metabolism are so
frequently implicated by molecular and biochemical analyses.
Exogenously applied glutathione suppresses tissue necrosis and
hyperhydricity in two out of three successfully tissue cultured and
transformed desiccation-tolerant plants (Fig. 3) [131–133]. We
suppose that glutathione not only acts as an antioxidant during the
subsequent dehydration–rehydration cycles, but it also can
synchronize different recovery processes, just as abscisic acid
integrates protection processes during the dehydration phase. Let
us consider the arguments for it. Numerous physiological functions
are attributed to glutathione in plants [134]. Glutathione is
synthesized through two sequential ATP-dependent reactions that
yield g-glutamylcysteine from L-glutamate and L-cysteine, followed by the formation of glutathione by addition of glycine to the
C-terminal end of g-glutamylcysteine [135]. Besides being an
indicator of oxidative stress [136], glutathione is the exclusive
storage and transport-form of the reduced sulphur regulating
Fig. 3. Dramatic effects of the exogenously applied glutathione on the survival rate and plant development in tissue cultures of R. myconi (A and B) and C. plantagineum (C and
D). Tissue necrosis (A) and hyperhydricity (C) decrease the efficiency of the whole tissue culture procedure. Non-viable R. myconi leaf segments show a fatal and general tissue
toxicosis (A), while C. plantagineum plantlets display typical symptoms of hyperhydricity (C). Addition of filter sterilized glutathione (200 mg/L) into the tissue culture media
has resulted in nearly 100% survival rate and vigorous leaf segment expansion in R. myconi (B) and healthy, true-to-type plantlets in C. plantagineum (D). Bars represent 1 cm.
O. Toldi et al. / Plant Science 176 (2009) 187–199
interorgan sulphur allocation [137]. Glutathione has also been
shown to act as a regulator of gene expression [138], is the
precursor of phytochelatins, which bind heavy metals [139], and is
a substrate for the glutathione S-transferases, which catalyze the
conjugation of glutathione with potentially dangerous xenobiotics
such as herbicides [140]. How can the aforementioned functional
abilities help a plant to survive desiccation? First, glutathione
synthesis depends on the adenylate status of the cells (proportion
of ATP versus ADP plus AMP), therefore it is likely that glutathione
levels serve to sense energy provision. Second, glutathione is also
an indicator of carbon and nitrogen metabolism, because
glutamate availability results in kinetic limitations to its synthesis
[141]. The third parameter which is mirrored in the actual level of
glutathione is the efficiency of photosynthesis, because glutathione synthesis requires photorespiratory glycine. This means
that glutathione levels can be upregulated when the photorespiratory activity is high [135]. Photorespiration results from the
oxygenase reaction catalyzed by ribulose-1,5-bisphosphate carboxylase/oxygenase [142]. During this metabolic process, CO2 and
NH3 are produced and ATP and reducing equivalents are
consumed, thus making photorespiration a wasteful process.
However, precisely because of this inefficiency, photorespiration
could serve as an energy sink preventing the over-reduction of the
photosynthetic electron transport chain and photoinhibition,
especially under stress conditions that lead to reduced rates of
photosynthetic CO2 assimilation [142]. Fourth, glutathione has a
capacity to neutralize not only reactive oxygen species, but also
other metabolic byproducts that might accumulate during
dehydration that otherwise could result in tissue toxicosis upon
rehydration.
Taken together, although most of the above arguments are just
speculations, glutathione has all the potential to be involved in the
synchronization of repair processes allowing recovery from
desiccation in tolerant plants. Through the actual levels of
glutathione the nutritional and energy status of the cells can be
sensed together with the efficiency of CO2 fixation. Besides the
sensory role, glutathione participates in detoxification processes
directly, preventing the accumulation of toxic metabolic byproducts associated with the dehydration process such as, for
example, PUFA hydroperoxides. Our belief is that engineering
glutathione metabolism could lead to the creation of crop plants
that are more tolerant to dehydration–rehydration cycles.
6. Genetic transformation of desiccation-tolerant plants
Databanks only provide information about putative function of
genes on the basis of sequence similarities. In vivo-proofs must be
obtained after examining transgenic plants possessing upregulated or downregulated expression of the genes associated with
desiccation tolerance [143]. For this purpose, in vitro plant
regeneration protocols had been established in resurrection
195
plants such as C. plantagineum [131,144], R. myconi [132], H.
rhodopensis [145] and Lindernia brevidens [94] of which Craterostigma, Ramonda and Lindernia have been successfully transformed [94,131,133,144].
Vegetative desiccation-tolerant plants possess an extreme
sensitivity to tissue culture-level manipulations during the establishment of their plant regeneration and genetic transformation
systems. These tissues all displayed frequent evidence of necrosis,
development of hyperhydrated leaves and secretion of polyphenols
into culture media under suboptimal conditions. Therefore, socalled ‘low stress’ transformation technologies were established for
nearly all resurrection plants [146] involving the development of
non-lethal selection strategies and the application of modified tissue
culture media supplemented with decreased amounts of macro- and
microelements, and different antioxidant agents.
An additional increase in the transformation frequency was
obtained when the colonization of the wounded surfaces by the
transforming Agrobacterium cells was enhanced by biochemical
pre-induction of the vir genes. This included the application of low
pH liquid medium consisting of acetosyringone, aldose-type sugar
source and organic nitrogen during the infection phase. These
modifications together with the appropriate tissue culture system
resulted in reliable methods for genetic transformation of
resurrection plants. This knowledge is now ready to be extended
over other recalcitrant species.
7. Conclusion and future aspects
Genes with potential role in triggering desiccation tolerance can
be identified by transcriptome and proteome analysis, and
metabolic processes that are involved in the functioning stress
responses can be encompassed by metabolic profiling in an ideal
world. We propose that performing such complete analyses of
resurrection plants both technically and intellectually is still a
complicated task. However, partial profiling of the transcriptome
and proteome has already resulted in a provisional hierarchy
among stress avoiding mechanisms and thus elucidating candidate
genes for molecular breeding programmes (Table 1).
Although sucrose plays an indispensable role in establishing
vegetative desiccation tolerance, manipulation of its metabolism
in the view of engineering crops for drought tolerance does not
seem to be a viable approach. Both upregulation and downregulation of its synthesis could mean a serious threat to the fine
balance in osmotic adjustment, energy metabolism, growth and
development as has been demonstrated. Enhancing trehalose
synthesis in transgenic plants does not seem to interfere with the
primary metabolism as does sucrose, but with jasmonic acid/
ethylene-signaling. This can result in false signals that can perturb
plant responses in general.
In contrast to sucrose and trehalose, there are other components of the photosynthetic carbohydrate metabolism with a
Table 1
Advantages and disadvantages of the most abundant protective agents associated with vegetative desiccation tolerance in terms of the practical application.
Protection agent
Metabolism
Advantage
Disadvantage
Sucrose
Trehalose
Carbohydrate
Carbohydrate
Carbon and energy source, osmoprotectant
Osmoprotectant, signal metabolite
RFOs
Carbohydrate
LEAs
Protein
Osmoprotectant, antistress agent,
metabolically inactive, synergistic effects
Hydration buffer, protein and membrane protectant
Possible interference with growth and osmoregulation
Possible interference with sugar sensing and
photosynthetic performance
None
Glutathione
Tripeptide
ABA
Mevalonate
Antioxidant, integrated sensor,
signal metabolite, synergistic effects
Growth regulator, signal metabolite
The most specific and most determinative LEAs
are still missing
None
Possible interference with plant development
and growth, contrasting effects
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O. Toldi et al. / Plant Science 176 (2009) 187–199
greater agronomic potential. Large quantities of raffinose family
oligosaccharides accumulate without affecting primary metabolic
processes and signaling cascades. Over-expression of genes in crop
plants with determinative role in the metabolism of raffinose
family oligosaccharides seems a good choice for future practical
applications. For example, a dehydration-specific aldose reductase
implicated in sorbitol synthesis [147] and galactynol synthase
(XvGolS), catalysing the first step in the biosynthesis of raffinose
family oligosaccharides [3] could be among the first desiccation
associated genes with potential to be applied in molecular
breeding programmes.
LEA genes, which are believed to play a crucial role in
providing tolerance to different abiotic stressors, have already
been quite extensively utilized in molecular breeding programmes. However, their practical contribution has not been
tested under real field situations. Further studies are necessary
to identify the most effective types of LEA genes. In the
meantime, the utilization of their promoter regions opens the
possibility to drive the expression of different antistress genes
stress-specifically.
Glutathione has primarily been evaluated as a potent detoxifying agent. We are certain that glutathione is much more than that.
It is potentially involved in the synchronization of recovery
processes allowing resurrection from the desiccation state in
tolerant plants. Besides being a sensor and antioxidant, glutathione
pleiotropically influences signaling processes similarly to abscisic
acid. Its different functions can act synergistically while the
different functions of abscisic acid have both negative and positive
impact on the productivity. On this basis, our belief is that stressspecific engineering glutathione metabolism could lead to the
creation of crop plants that are more tolerant to dehydration–
rehydration cycles.
Now there are signaling and metabolic processes to modify and
there are genes to achieve these modifications. The question is
how, because the weakest point within the whole highthroughput technology is the limited capacity of conventional
genetic transformation techniques. Therefore, the development of
alternative in planta transformation appears promising, at least in
the case of some model plants [148] and is currently available for
both monocotyledonous and dicotyledonous plants. The advantage of this technique is that the time consuming and laborious in
vitro tissue culture procedures can be replaced by immersing
plants at a suitable developmental stage directly into Agrobacterium suspension cultures, which is followed by screening T1
generation for transgenic individuals. This method only works in a
few species. Until in planta transformation techniques are
available, the conventional ways of genetic transformation
through in vitro tissue culture systems are still vital in terms of
functional genetics [143]. However, stable transformants are not
always needed for analysing gene function. Large-scale transient
gene expression assays, based on inoculation of plants by viral
vectors, can be also performed at both the protoplast and the
whole plant levels [149,150]. This method could also mean a
reasonable
alternative
to
conventional
transformation
approaches and thus can accelerate cataloging genes associated
with vegetative desiccation tolerance.
Acknowledgements
This project has been funded by grants of OTKA (The Hungarian
Scientific Research Fund) T-043444 and OECD (Organisation for
Economic Co-operation and Development) Co-operative Research
Programme to OT, and Intergovernmental Chinese-Hungarian (TéT
CHN-7/2005), Indian–Hungarian (TéT Ind/2006) Science and
Technology projects to ZT and are gratefully acknowledged.
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