Ptant, Cetl and Environment {^995) 18, 801-806
Enhancement of seed germination in high salinity by
engineering mannitol expression in Arabidopsis ttiaiiana
J. C. THOMAS,'-^* M. SEPAHI,' B.
ARENDALL'
& H. J. BOHNERT''"
' Department of Biochemistry, and Departments of ^ Plant Sciences, and ^Molecular and Cellular Biology, The University of
Arizona, Biosciences West, Tucson, AZ 85721, USA
ABSTRACT
The bacterial gene mtlD, which encodes mannitol 1-phosphate dehydrogenase (E.C.I.1.1.17), was transformed into
Arabidopsis thaliana and expressed under control of the
CaMV 35S promoter. AfrfD-transformants accumulated
mannitol, a sugar alcohol that is not normally found in
Arabidopsis. Amounts of soluble carbohydrates, sucrose,
glucose, fructose, myo-inositol and mannitol were determined in different tissues of wild-type and transgenic
plants. We estimated that less than 1% of the carbon
assimilated was converted into mannitol by the transgenic
plants. The establishment of individual transformed lines
(after self-crossing three times) resulted in high and low
mannitol-producing lines which were stably maintained.
The presence of mannitol did not alter plant appearance
or growth habit. When MtlD-expressing seeds and control
seeds (T3 generation) were imbihed with solutions containing NaCI (range 0 to 400 mol m~'^), transgenic seeds
containing mannitol germinated in medium supplemented
with up to 400 mol m"'' NaCI, while control seeds ceased
germination at 100 mol m"^ NaCI. It is doubtful whether
the ability to germinate in high salt was a result of an
osmotic effect exerted by elevated levels of mannitol, considering that mannitol concentrations were in tbe mol m~^
range in seeds. A specific effect of polyols, for example on
the integrity of subcellular membranes or enzymes, cannot be excluded.
Key-words; metabolic engineering; polyol; sugar alcohol.
Abbreviations; CaMV35S, promoter for Cauliflower mosaic
virus 35S RNA; K,,,"*. kanamycin resistance; nitlD, gene
encoding Mtl-DH frotn Escherichia coli; Mtl-DH, matinitol
1-phosphate dehydtogenase (E.C. 1.1.1.17); uidA, gene
encoding /3-glucuronidase from E. coli.
INTRODUCTION
The accumulation of polyols in response to stresses that
affect water availability and the accumulation of other
*Correspondence and present address; John C. Thomas, Department of Natural Sciences, University of Michigan (Dearborn),
Dearborn, MI 48128-1491, USA.
© 1995 Blackwell Science Ltd
metabolites, such as proline and glycine-betaine, have
been described in many studies (Flowers et al. 1977; Ford
1984; Thomas et al. 1992; Vernon & Bohnert 1992;
Delauney & Verma 1993; Hanson etal. 1994). Accumulation of these compounds is strongly correlated with resistance or tolerance of plants to abiotic stresses that affect
water availability. Proposed mechanisms of action of these
accumulating substances include their action in osmotic
adjustment, and/or their action as compatible solutes or
osmoprotectants (McCue & Hanson 1990).
In osmotic adjustment during NaCI stress, for example,
an accumulating metabolite on a mole-by-mole basis
would replace other compounds within the cell. Sodium or
other toxic compounds might be sequestered into the vacuole, if the metabolite were strictly confined to the cytoplasm. If found in the cytosol, organelles and vacuole, the
metabolite would increase the osmotic pressure of the cell.
For example, when mistletoe taps into the host phloem,
polyol accumulation provides the parasite with a high
osmotic pressure (Richter & Popp 1992). Thus, when acting as compatible solutes during osmotic adjustment,
accumulating metabolites are considered non-inhibitory to
cellular metabolism and their effect might be exerted at
high or moderately high concentrations. Osmoprotectants
could even act at low concentrations by protecting specific
structures or enzymatic processes, by exerting regulatory
effects on ion or water uptake or transport, or by stabilizing multi-subunit enzyme complexes or membranes
(Smirnoff & Cumbes 1989; Sommer etal. 1990- Smirnoff
1993).
,o,;j-.
rrjn.
We have begun to analyse the effects of one class of
putatively protective metabolite, polyols, on the performance of plants under stress (Tarczynski et al. 1992; Tarczynski et al. 1993; Vernon et al. 1993). A bacterial gene,
mtlD, encoding mannitol 1-phosphate dehydrogenase,
which catalyses the production of the acyclic polyol mannitol, was transfeiTed to tobacco. We showed that the
transgenic plants accumulated mannitol. These plants
under controlled growth conditions were able to tolerate
moderate salt stress better than control plants. The function(s) of the polyol, however, and the mechanism(s)
through which mannitol might act are still unknown.
Instead of monitoring whole-plant performance under
stress, a different investigation was performed here. By
801
802
J. C. Thomas et al.
engineering mtlD into Arabidopsis thaliana and selfmg
selected transformed lines, large populations of plants
were grown. Mannitol production by itself did not alter the
appearance and growth of these plants relative to control
plants. We analysed the distribution of sugars and polyols
in different tissues and in seeds of MtlD and control Arabidopsis. The results indicated that high NaCI was less
stressful at germination even at relatively low concentrations of mannitol in the germinating MtlD seeds and
seedling when compared to controls. All lines germinated
equally well on 100 mol m""^ NaCI, in contrast to wildtype. Different lines, however, showed differences in the
maximal concentration of NaCI tolerated. Since the
amount of accumulated mannitol in seeds was lower than
that required to exert an osmotic effect, we suggest that
this polyol might act as an osmoprotectant rather than as a
compatible solute in Arabidopsis thaliana during germination under non-physiological salt conditions.
, >ififo fitd
tions, up to 500 seeds were sown onto petri dishes and getmination was scored after 7 d of germination under conditions as described above. Seeds were scored as germinated
when the radicle was at least 5 mm long and the cotyledons
had begun to unfold.
Seeds were surface-sterilized and germinated in previously sterilized 1:1 vermiculite/ potting soil. Individual
plants were grown in 5 cm containers (24 °C; 24 h light at
150-250 jimoX nf^ s"' photon flux density). Different plant
lines were grown in a growth chamber and roots, leaves,
stalks, flowers and siliques were harvested at the same time
for sugar, proline and polyol analysis.
HPLC analysis
The analysis of sugars and sugar alcohols was performed
accordingto Adams era/. (1992, 1993, 1995).
MATERIALS AND METHODS
RESULTS
Gene constructs, transformation, and
regeneration of plants
The gene construct used consisted of a CaMV 35S promoter transcriptionally fused to a short segment upstream
of the start codon of the bacterial mtlD gene as described
(Tarczynski et al. 1992). After root transformation of Arabidopsis thaliana (Landsberg erecta) a number of individual transformants were selected on 50 /ig cm"-* kanamycin
(Valvekens et al. 1988). The first generation was allowed
to .set seed in tissue culture and Tl seeds were recovered.
From these seeds, plants were grown to maturity and selfing was repeated. Progeny were selected on 100 /ig cm"-^
kanamycin and populations of plants and T3 seeds were
established. From such T3 seeds, plants were grown and
analysed for the presence of mannitol and sugars in different organs.
The bacterial gene mtlD (Davis et al. 1988) was engineered to contain a CaMV 35S promoter/enhancer element
and the 3' end of the NOS (nopaline synthase) gene as
described (Tarczynski (?r a/. 1992). Arabidopsis thaliatia
(ecotype Landsberg erecta) were transformed using root
explants (Valvekens et al. 1988). Young roots were incubated with Agrobacterium tumefaciens containing the
35S-mtlD gene for 30 min. After washing and co-cultivation for 2-3 d, the roots were transferred to liquid medium
containing 500 /lg cm~" vancomycin and 50 fig cm '^ kanamycin and washed several times. The roots were then
placed on solid medium with vancomycin and kanamycin
(24-27 °C, 16 h light at 150-200 /imol m"^ s'' photon flux
density) for 2-3 weeks. Green shoots which emerged were
transferred to growth medium (200 /ug cm "* vancomycin,
50 /lg cm'"* kanamycin). Plants were allowed to set seeds
in sterile culture. Viable seeds were recovered from mote
than 20 individual transformation events and seeds were
germinated on 100 /lg cm^"' kanamycin. Subsequent green
K,,,'* progeny were grown to maturity and selfed. This step
was repeated once to obtain the T3 generation seeds. As a
control, the gene encoding ^-glucuronidase (uidA) (Jefferson et al. 1987) under 35S promoter control was transferred into Arabidopsis using the same procedure.
Growth parameters and germination studies
Surface-sterilized seeds were placed individually on agar
containing MS-salts, 1% sucrose, 0-1 mg dm""^ thiamine
HCl and 0 8% Bacto agar. NaCI was added at different
concentrations (0 to 400 mol m"^). Approximately 100
seeds were placed on each 8 cm petri dish and incubated in
a growth room (24-27 °C, 16h light at 150-200 /imol m~^
s photon flux density). To obtain data on larger popula-
Extracts from wrrfA-transformed control and /«r/D-transfonned T3 plants were analysed by HPLC at high pH with
pulsed amperometric detection of hydtoxyl gtoups
(Adams et al. 1992, 1993, 1995). The uidA-expressing
(GUS expression; Jefferson et al. 1987) plants showed
sugar profiles identical to those of wild-type plants
(Fig. la). Extracts frotn mr/D-transformed plants showed a
typical distt ibution of sugars and contained one additional
prominent peak, with a retention time -2-2 min, not found
in non-transformed controls (Fig. lb). NMR analysis of
collected material from this peak (data not shown) indicated that the material in this peak was mannitol. The
amount of mannitol in individual lines was variable, similar to findings with transgenic tobacco (Tarczynski et al.
1992; 1993). Levels ranged ftom 0-05 to 12 00 /imol g~'
fresh weight (Table 1). This amount did not seern to be
correlated with the amounts of the other sugars. Table 2
shows this distribution of sugars and mannitol in different
tissues of transformed plants from a representative experiment. Wild-type values for sugars are comparable, varying
from 1 to 17 /itnol g ' fresh weight for sucrose. In leaves
of transgenic mtlD plants, the amount of sucrose ranged
from 0 4 to 1 5 /imol g"' fresh weight, that of glucose from
© 1995 Blackwell Science Ltd, Plant, Cell and Environment, 18,801-80
Engineering of mannitol expression in Arabidopsis
nA 200
nA 200
M
100
9.6
I
0
6
Mm
6
Figure 1. HPLC profiles of soluble low-tnolecular-weight
carbohydrates in wild-type and transgenic Arabidopsis thatiana
expressing the bacterial mannitol I-phosphate dehydrogenase
gene. The analysis of sugars, proline and polyols was performed
according to Adams et al. (1992). I, inositol; G, glucose; F.
fructose; S, sucrose; M. niannitol; P. proline (detectable levels
only under stress, panel c). (a) Control plants containing 35S-«iVM
gene, (b) Plants containing the ?i5S-nittD Gene, (c) Plants
containing the mtlD gene after 5 d in 200 mol m""" NaCI.
0-6 to 2-7 /imol g ' fresh weight, and that of fructose from
0 2 to 13 /lmol g~' fresh weight. All samples were taken
during the second half of the light period. The analysis of a
large number of control or mannitol-containing plants
showed that notmal sugar and tnannitol amounts were
within a range of approximately 0 to 10 /imol g~' fresh
weight with large variations depending on plant age and
tissue sampled. There was no significant difference
between the lines.
Under stress conditions, plants growing in the presence
of 200 mol m""* NaCI showed changes in sugars (Fig. lc).
The amounts of sucrose and reducing sugars declined drastically, depending on the severity of the stress and also on
the time during development at which the stress was
© 1995 Blackwell Science Ltd, f/fld/, Cell and Environment, 18.801-806
803
applied. As a result of the decline in sugar content, the relative contribution of mannitol to the osmotic pressure
increased and several other, minor carbohydrate peaks,
whose nature we have not studied, increased. In addition,
proline was easily detected under stress conditions
(Fig. lc). The identification of the material in this peak
(retention time 3-45 min) as proline is based on the retention time in comparison with proline standards, re-chromatography of the material on a cation HPLC system (W.
Jones, The University of Arizona, personal communication), and on the fact that the material could be removed by
desalting of the extracts (data not shown).
To test for the ability to germinate under stress conditions, control and mannitol-expressing seeds were surfacesterilized and placed on agar containing levels of NaCI
ranging from 0 to 400 mol m"-' NaCI (Fig. 2). The control
seeds, which had been transformed with a 35S-uidA gene,
generally ceased germination at levels higher than 100 mol
m"-', similar to teports for wM-type Arabidopsis (Saleki et
al. 1993). Lines which contained mannitol always germinated better than controls on 100 mol m~-^ NaCI. At
200 mol m"'' NaCI some lines were comparable to wildtype germination on 100 mol m""" NaCI, and a few lines did
germinate, albeit poorly, on 400 mol m^"* NaCI (Fig. 2).
Figure 3 provides an impression of the germination and
growth of 35S-M(V/A-expressing and 35S-tntlD expressing
Arabidopsis thaliana populations in the absence and presence of NaCI. Non-transfotmed wild-type plants behaved
identically to the 35S-uidA plants (data not shown). Germination on agar supplemented with MS salts and that on
agar supplemented with I % sucrose were identical, apart
from differences in the overall germination of the lines, in
the absence of NaCI (Fig. 3, top row). In the presence of
200 mol m"^ NaCI, 35S-M(<a'A plants did not germinate at
Germination 0 mol m"^ NaCI
Germination 100 mol m"^ NaCI
Germination 200 mol m'^ NaCI
Germination 400 mol m"-^ NaCI
0.0%
121.la
M3
M5
Plant Line
M6
Figure 2. Germination of wild-type and Iransgenic Arabidopsis
Ihaliana in the absence or presence of NaCI. Percentages given are
with tespect to the total number of seeds. The experiment was
performed two times with different seed batches. Seeds from line
M2 only showed some germination on 400 inol m ' NaCI.
804
J. C. Thomas et al.
Table 1. Mannitol in Arabidopsis thaliana expressing 35S-mtlD and 'i5S-uidA
Plant
clone
Lower
leaves
Upper
leaves
Ml
0-05 (0-08)
2-50 (0-21)
0-17 (0-05)
1-75 (0-21)
0-24 (0-12)
5-80 (1-60)
3 07(1-10)
4-05 (0-63)
0 32 (0-08)
1-59 (014)
0-66 (014)
0-43 (0-07)
0-28 (0-14)
5-63 (0 92)
3-14(1-04)
2-99 (0-29)
ND
ND
ND
ND
ND
ND
ND
ND
M2
M3
M4
M5
M6
M7
M8
M9
12M-a
12M-b
12M-C
Stem
Flower
Silique
ND
ND
ND
ND
ND
0-10(0-04)
0-72(0-10)
0-56 (0-06)
ND
ND
3-0(0-70)
2-68 (0-40)
ND
ND
ND
ND
ND
0-21 (0-03)
ND
7-47(1-10)
5-90 (0-50)
NT
ND
ND
ND
ND
ND
7-37(1-10)
12-00(2-20)
NT
ND
ND
ND
ND
NT
0-84(0-15)
7-20(0-55)
1-70(0-61)
0-21 (0-10)
0-70 (0-09)
ND
Germinated on
200 trol tir^^ NaCI
Seeds
ND
10-05(0-97)
9-95(1-90)
4-25 (0-35)
ND
ND
ND
ND
0-51 (0-05)
0-36 (0-05)
NT
NT
NT
NT
NT
ND
ND
NT
NT
Extracts of soluble carbohydrates were made frotn T3 pooled plants of independent K^," sample lines. Lines 121-1 are lines from independent
transformation events with a 35S-uidA construct (T3 after Km" selection). Data are expressed as /(mol mannitol g~' fresh weight (or dry
weight for seeds). NT: not tested; ND: none detected. Standard ertors are given in parentheses. In this experiment NaCI was used at 200 mol
m"^ in agar with MS salts. The experiment was performed three times with different seed batches.
'
!
• . ' • / •
all (Fig. 3, bottom row, right), while 35S-mtiD line M2
(Table 1) germinated at approximately 40% (Fig. 3, bottom, middle), and line M3 (Table 1) germinated poorly
(Fig. 3, bottom, left) at approximately 14%. Transgenic
plants germinating in 200 mol m "* NaCI could be rescued,
if they were removed from the plates within approximately
2 weeks. However, rescue was no longer possible once
plants had developed more than approximately four true
leaves on medium containing 200 mol m"'' or higher NaCI.
DISCUSSION
Figure 4 outlines the pathway which results in the accumulation of mannitol in a plant that does not normally synthesize this polyol due to mtlD expression. In E. coli (Fig. 4,
inset), from which the mtlD gene is derived, the pathway
proceeds through the uptake-coupled phosphorylation of
mannitol to mannitol 1-phosphate (M-IP). The MTL-D
enzyme converts M-IP to fructose 6-phosphate (F-6P)
which becomes available for catabolic reactions. In transgenic mtlD plants, the enzyme converts part of the F-6P to
M-1P utilizing cytosolic NADH + H"^. M-1P is dephosphorylated, probably through the action of a broad-substrate
phosphatase which we have not characterized. Indeed, elevated levels of M-IP or reduced levels of F-6P have not
been detected in transgenic tobacco (R.G. Jensen, The
University of Arizona, personal communication).
Once produced, we think that mannitol is confined to the
cells in which it is made or that it leaves cells slowly. No
information has been obtained on whether mannitol may
be transported out of the cells. An alternative is that mannitol may be re-phosphorylated and may again enter the
pathway to sucrose. Experiments which address this question are underway. We surmise that mannitol is slowly
catabolized because accumulation reaches a plateau, and
the plateau value is not significantly increased when
enzyme amounts are increased by crosses of different lines
(data not shown). The localization of mannitol in cells is
not known. If it were confined to the cytosol, a concentration
of mannitol of approximately 100 to 150 mol m"^ would
result in some lines.^'However, if mannitol were to move into
the vacuole, concentrations in total cell water might be as
low as 10 mol m"''.
Considering that mannitol accumulation and the ability
of a transgenic seed to germinate in elevated NaCI levels
were coincident, how might mannitol act during germination at high salinity? Osmotic adjustment as the mechanism of mannitol action is conceivable only when mannitol is restricted to the cytoplasm. Even under these
circumstances, the amount of mannitol, equivalent to
approximately 100 mol m"'', is barely high enough to fulfil
the requirement of an osmolyte for osmotic adjustment.
Table 2. Representative determinations of principal sugars in
leaves of control (pBI 121 -1) and MtlD (M)-transformed
Arabidopsis
Sample
Mannitol
Glucose
Fructose
Sucrose
121-1-a
12M-b
ND
ND
0-6
1-5
1-3
1-7
0-9
0-6
2-7
1-0
3-9
1-7
2-7
1-4
0-7
0-7
0-8
I-O
1-7
1-0
0-7
0-7
0-5
Ml
M2
M3
M4
M5
M6
M7
1-7
0-8
1-3
0-2
0-6
0-3
0-8
0-6
0-4
0-8
1-5
1-0
0-4
Data are from a single, representative experiment, and are
presented in /lmol g"' fw for each sugar/polyol. ND: not detected.
© 1995 Blackwell Science Ltd, Plant, Cell and Environment, 18, 801-80
Engineering of mannitol expression in Arabidopsis
Figure 3. Photograph of petri dishes showing Arabidopsis seedlings germinating in the absence (top row) or presence (bottom row) of
200 tnol m""* NaCI. The dishes on the right (top/ bottotn) are 35S-uidA. those in the middle (top/ bottom) are line M2, and those on the left
(top/bottom) are line M3.
although mannitol could act in concert with the proline
that is accumulating as part of the normal water and salinity stress response of Arabidopsis. Solutes that accutnulate
in response to an osmotic challenge range from inorganic
and organic cations and anions to specific amino acids,
sugars, sugar alcohols and tertiary ammonium or sulphonium compounds (Yancey et al. 1982; Smirnoff 1993;
Hanson et al. 1994). Solute accumulation is found in bacteria, fungi, plants and animals, indicating that most or all
organisms are capable of osmotic adjustment to sotne
degree. Possibly, the marginal increase of mannitol
achieved with the present gene constructions elevates
osmolyte concentration beyond a threshhold value that
confers increased tolerance.
In a germinating seed, the concentration of mannitol
detected was higher than in turgescent mesophyll cells. It
may be that this increase in osmolarity provides an advantage during germination, either by facilitating water uptake
or by preventing, in the short term, excessive sodium
influx. In both scenarios the tnovement of solutes across
membranes is involved. Mannitol and other osmoprotective substances may influence the transport of metabolites
and solutes across metnbranes in a specific way. There
have been several suggestions about possible functions of
osmoprotectants: radical scavenging, protection of
enzymes, enzyme complexes or membranes, and a sink for
ATP
NADPH
I
4
pholotynthatfs
chloroplast
Figure 4. Engineered pathway dependent on the action of
mannitol I -phosphate dehydi ogenase in transgenic Arabidopsis
thaliana. For explanations, see the Discussion.
© 1995 Blackwell Science Ltd. Plant, Cell and Environmem, 18. 801-806
805
806
J. C. Thomas et al.
To '
photosynthetically assimilated carbon under stress (Lewis
& Smith 1967; Briens & Larher 1983; Smirnoff & Cumbes
1989; Krall etal. 1989). Production of mannitol could promote seed germination in several ways, for example by
conferring resistance to NaCI toxicity and/or by allowing
seeds to absorb greater levels of water. Clearly, more work
is needed to investigate the mechanisms by which osmoprotectants act.
ACKNOWLEDGMENTS
We thank Pat Adams and Wendy Jones for HPLC analyses, and Richard G. Jensen and Pat Adams for suggestions
on the manuscript. This work was supported by grants
from DOE (Biological Energy Research) and U.S. Department of Agriculture (Plant Responses to the Environment)
to H.J.B. and from Arizona Agricultural Experiment Station to H.J.B. and J.C.T.
REFERENCES
Adams P., Thomas J.C, Vernon D.M.. Bohnert H.J. & Jensen R.G.
(1992) Distinct cellular and organismic responses to salt stress.
Plant and Celt Physiotogy 33, 1215-1223.
Adams P., Zegeer A., Bohnert H.J. & Jensen R.G. (1993) Anion
exchange separation and pulsed amperometric detection of inositols from flower petals. Analytical Biochemistry 214, 321-324.
Adams P., Jones W., Bohnert H.J. & Jensen R.G. (1995) Water
availability and osmotic adjustments in the ice plant: tissue
specificity of sodium chloride, proline and polyol accumulation.
Ptanta, in press.
BHens M. & Larher F. (1983) Sorbitol accumulation in Plantaginaceae: Further evidence for a function in stress tolerance.
Zeitschrift fur Pflanzenphysiologie 110,447^58.
Davis T, Yamada M, Elgort M.G. & Saier M.H. (1988) Nucleotide
sequence of the mannitol (mtl) operon in Escherichia coli.
Motecutar ,^4icrobiotogy 2, 4 0 5 ^ 12.
Delauney A.J. & Verma D.P.S. (1993) Proline biosynthesis and
osmoregulation in plants. Plant Journals, 215-223.
Flowers T.J., Troke P.F. & Yeo A.R. (1977) The mechanism of salt
tolerance in halophytes. Annual Reviews of Plant Physiology 28,
89-121.
Ford C.W. (1984) Accumulation of low molecular weight solutes
in water-stressed tropical legumes. Phytocheniistry 23,
1007-1015.
Hanson A.D., Rathinasabapathi B., Rivoal J., Burnet M. Dillon
M.O. & Gage D.A. (1994) Osmoprotective compounds in the
Plumbaginaceae: a natural experiment in metabolic engineering
of stress tolerance. Proceedings of the National Academy of Science USA 91, 306-3\0.
Jefferson R.A. (1987) Assaying chimeric genes in plants: The Gus
gene fusion system. Plant Molecular Biology Reporter 5,
387^05.
Krall J.P. Edwards G.E. & Andero C.S. (1989) Protection of pyruvate. Pi dikinase from maize against cold lability by compatible
solutes. Plant Physiology 89, 280-285.
Lewis D.H. & Smith D.C. (1967) Sugar alcohols (polyols) in fungi
and green plants. I. Distribution, physiology and metabolism.
New Phytologist 66, 143-184.
McCue K.F. & Hanson A.D. (1990) Drought and salt tolerance:
towards understanding and application. Trends in Biotechnology
8,358-362.
Richter A. & Popp M. (1992) The physiological importance of
accumulation of cyclitols in Vi.scum atbum L. New Phytotogist
121,431-438.
Saleki R, Young P.G. & Lefebvre D.D. (1993) Mutants oi Arabidopsis thatiana capable of gertnination under saline conditions. Ptant Phy.tiotogy 101, 839-845.
Smirnoff N. (1993) The role of active oxygenin in the response of
plants to water-deficit and desiccation. New Physiologist 125,
27-58.
Smirnoff N. & Cumbes Q.J. (1989) Hydroxyl radical scavenging
activity of compatible solutes. Phytochemistry 28, 1057-1060.
Sommer C, Thonke B. & Popp M. (1990) The compatibility of Dpinitol and lD-1-O-methyl-tnucoinositol with malate dehydrogenase activity. Botanica Acta 103, 270-273.
Tarczynski M.C, Jensen R.G. & Bohnert H.J. (1992) Expression of
a bacterial mtlD gene in transgenic tobacco leads to production
and accumulation of mannitol. Proceedings of ttie National
Academy of Science USA 89, 2600-2604.
Tarczynski M.C. Jensen R.G. & Bohnert H.J. (1993) Stress protection of transgenic tobacco by production of the ostnolyte mannitol. Science 259, 5d'8-510.
Thomas J.C, DeArmond R.A. & Bohnert H.J. (1992) Influence of
NaCI on growth, proline. and phosphoenolpyruvate carboxylase
levels in Mesembryanthemum crystaltinuni suspension cultures.
Ptant Physiology 98, 626-631.
Valvekens D., van Montagu M. & Van Lijsebettens M. (1988)
Agrobacterium tumefacien.s-mediate.d transformation of Arabidopsis root explants using kanamycin selection. Proceedings
of the National Academy of Science USA 85, 5536-5540.
Vernon D.M. & Bohnert H.J. (1992) A novel methyl transferase
induced by osmotic stress in the facultative halophyte Mesembryanthemum crystattinum. The EMBO Journat 11, 2077-2085
Vernon D.M., Tarczynski M.C. Jensen R.J. & Bohnert H.J. (1993)
Cyclitol production in transgenic tobacco. Plant Journal 4,
199-205
Yancey P., Lark M., Hand S., Bowlus R. & Somero G. (1982) Living with water stress: evolution of osmolyte systems. Science
217, 1214-1222.
Received 9 September 1994; received in revisedform 21 December
1994; accepted for pubtication 18 January 1995
© 1995 Blackwell Science Ltd, Plant, Cell and En vironment, 18, 801 -806