MELBOURNE BIOLOGY NETWORK MEETING 28.5.02
SPEAKER: Dr Don Gaff, Monash University (retired).
Survival in Extreme Conditions – Resurrection Plants
Resurrection plants:
* leaves, shoots and roots survive air-drying. Water content drops to 4 – 8%
dry weight.
* drying takes 3 – 7 days after rain.
* rehydration takes 12 – 24 hours
* air-dried leaves survive 2 – 5 years
* there is no limit on the number of hydration/rehydration cycles that RPs can
survive.
Abscisic acid (ABA)
* in plants generally [ABA] with stress. There is a 40 – 100 times increase in
situations such as: drought, salt, cold, heat, low levels O2.
* ABA treatment increases drought resistance of these plants in general:
  root growth
 closes stomata
  shoot growth
  leaf hairiness
  leaf senescence (reduces number of leaves).
o Loss of chlorophyll
o Leaves shed (possibly through ethylene action).
Is ABA involved in Resurrection Plants?
If a small stem of Borya (RP) is air dried, it can be resurrected.
If a leaf is cut from the plant and dried quickly then it dies but the chlorophyll
stays.
If such a detached leaf is dried slowly over two days, chlorophyll is lost and
the plant stays alive.
[ABA] increases in Borya leaves as they dry. Also ABA treatment of wellhydrated Borya leaves allows them to survive subsequent drying.
Cf. Leaves of the South African resurrection grass Sporobolus stapfianus
can survive so long as they stay on the parent plant. If detached leaves are
dried they will always die.
The basic hypothesis: there must be a signal from the root system to the
shoot.
(8) Physiological changes
Response
(7) Specific protein
mRNA
(6) Activation of specific gene
(5) ABA-induced compounds
Signal transduction
(across cell to nucleus)
(4) ABA receptor on cell membrane
Leaf
Root
(3) ABA rises in xylem
(2) ABA secreted into root xylem
Transport of hormone
Perception stimulus
(organ response)
(1) Drying soil
Please note that some hormones (eg. ABA & gibberellins) move through
xylem as well as phloem.
It is the [ABA] at the cell membrane (cell wall space) that is important rather
than the [ABA] in the cells in inducing stomatal closure.
Some evidence indicates that detached leaves can produce their own ABA
(eg. Borya and Sporobolus, see above).
Then researchers started looking for another substance as a root signal. No
singal plant hormone gave enough improvement in drought tolerance of
Sporobolus leaves. Perhaps a particular mixture of several hormones is
required, cf. Experiments in South Australia with grape vines.
An experiment was done using split-root irrigation of a grape vine. The root of
a plant was split and watered on one side only using drip irrigation with the
other side remaining dry. The following was found:
 The water content in the foliage was the same.
 The stomata closed.
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



There was less growth of foliage.
Grape yield was reduced only a little.
Grape quality was much improved. This is good news for
reducing irrigation volumes.
No single hormone explains the postulated root signal.
As S. stapfianus (RP) is dehydrating the number of proteins produced in the
plant is increased. No explanation yet.
In a non-resurrection plant often studied by geneticists:
Arabidopsis thaliana – the Japanese showed that there are four different
drought-induced pathways that stimulate suites of genes (two pathways with
ABA). The pathways involve “transcription factors” that activate the genes.
Hormones such as ABA, auxin, ethylene and gibberellin stimulate the
transduction pathways (which can interact) and the system amplifies the
response.
Waterlogging (Plants generally)
In waterlogged soil plants: reduce [gibberellin] and [cytokinin] in the roots as
well as increase [ABA] and [IAA] in the roots. This leads to an increase in the
root/shoot ratio of cytokinin and IAA. Thus root growth is reduced.
Low cytokinin / high IAA tends to stimulate root growth.
Production of ethylene:
Methionine
SAM
Step stimulated by low O2
ACC synthetase
ACC (transported through plant)
ACC oxidase
Step impaired by low O2
Ethylene (C2H4)
SAM = S-adenylmethionine
ACC = 1-aminocyclopropane-1-carboxylate
Ethylene acts where it is produced so, by definition, it is not a hormone. That
is, it is not produced in one area and transported to another. Nevertheless
released as a gas it can cause growth response in other organs in other
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plants. Moreover, in combination with precursor ACC (transportable) it forms
part of a hormone-type system.
ACC is formed in waterlogged roots and transported up in the xylem. When
the ACC reaches aerated tissues, the oxygen present allows conversion of
ACC to ethylene.
Growth of new roots is stimulated in the aerated by the ethylene in
combination with a favourable cytokinin/IAA ratio.
In the new roots (in the aerated soil) most of the root cortex collapses and
large gaps are produced that fill with gas that allows a better supply of air.
Ethylene (from the ACC from the lower waterlogged roots) causes this
breakdown of the cortical cells. This is tissue with the large gaps is termed
aerenchyma.
Some ‘recent’ references on plant hormones
1st Year Uni: Knox B, Ladiges P, Evans B, Saint R (2002) “Biology” 2nd ed.
McGraw-Hill, Roseville NSW
Ch 24
pp 630-647
2nd/3rd Year Uni:
Salisbury F, Ross C (1992) “Plant Physiology” 4th ed.
Wadsworth, Belmont, Calif.
Ch 17-18
pp 357-406.
Atwell B, Kriedemann P, Turnbull C, eds. (1999) “Plants in Action” Macmillan,
S Yarra.
Ch 9
pp 284-306.
Starr C and Taggart R (2001) “Plant Structure and Function” 9th ed.
Thomson/Brook/Cole.
Anderson J and Beardall J (1991) “Molecular Activities of Plant Cells”.
Blackwell, Oxford.
www.plant-hormones.bbsrc.ac.uk/education/kenh.htm
www.biologie.uni-hamburg.de/b-online/e31/31h.htm
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