synthesis have only recently been described in many species, with trehalose capacity probably present in
all angiosperms [125]. Crowe et al. (Ref. 108 and references therein) have established that these sugars
stabilize membranes and proteins in the presence of low water potentials and may play a crucial role in
plant survival during decreased water status.
Quaternary Amines. The most studied molecules of this class are proline and glycine betaine.
Numerous studies have demonstrated that proline accumulates in response to water deficit or salinity
stress. Arguments have been made that the generally inhibited metabolism resulting from these stresses
reduces the demand for proline in protein synthesis. However, sufficient evidence indicates that proline
accumulation is active rather than passive [126]. Hu et al. [127] showed that mRNA for the enzyme ^1 -
pyrroline-5-carboxylate synthetase is increased in salt-stressed roots, and Delauney and Verma [126] sug-
gested that it may be a rate-limiting step for proline synthesis. Delauney and Verma [126] also indicated
that^1 -pyrroline-5-carboxylate reductase activity and mRNA increase with salt stress, but there is doubt
as to whether it is involved in the NaCl-dependent regulation of proline synthesis. Glycine betaine origi-
nates from choline rather than via amino acid biosynthesis and is found in 10 flowering plant families.
McCue and Hanson [128] have shown that betaine dehydrogenase (BADH), the last step in the synthesis
of glycine betaine, is salt inducible at both the protein and mRNA levels in sugar beets and spinach. ABA
can also stimulate BADH protein and mRNA synthesis but at lower levels than via salt stress [129].
Ca^2 -ATPase. Nahas been implicated in some of the difficulties faced by plants during salt stress. It
reportedly displaces Ca^2 from membranes, possibly reducing membrane stability [130]. It is more likely,
however, that the primary injuries are from displaced Ca^2 increasing the cytoplasmic Ca^2 , which could
cause a disruption of signal transduction pathways requiring regulated levels of the ion [131]. Wimmers
et al. [132] have shown that the mRNA for a Ca^2 -adenosinetriphosphatase (ATPase) is increased in
abundance in response to elevated NaCl concentrations in tomato. They suggested that this ATPase may
act to maintain proper levels of Ca^2 , thus mitigating the effects of Na.
- Cold Stress
PHYSIOLOGY. Cold stress may be considered as a composite of two separate stresses: chilling (gen-
erally, temperatures from 4 to 15°C) and freezing. Chilling stress has in general been attributed to effects
at the plasma membrane, manifested by electrolyte leakage from tissues (e.g., Ref 133). Williams [134]
summarized data that indicated that leakage could be due to phase transitions caused by the presence of
minor lipid components in the membrane or, alternatively, failure to seal critical intrinsic membrane pro-
teins into the cell membrane by non–bilayer-forming lipids. Another explanation, that of lipid peroxida-
tion (e.g., during photoinhibition), does not appear to be a cause of leakage. Hodgson and Raison [135]
demonstrated that neither superoxide dismutase activity nor lipid peroxidation appears to increase at
moderate photon flux levels.
Freezing stress appears to be the result of two components. The first is intracellular, in which ice
crystals can pierce the plasma membrane (immediately lethal). The second is extracellular, in which the
low water potential of ice in the intercellular spaces and cell wall can remove water from the cell (i.e.,
desiccation). Plants that achieve freezing tolerance apparently mitigate intracellular ice formation through
biochemical means (i.e., changes in proteins and carbohydrates) [136]. Desiccation via freezing is ame-
liorated by both biochemical and biophysical changes, particularly in woody plant species [137].
Tolerance to chilling is apparently a prerequisite for tolerance to freezing. Chilling tolerance is an in-
ducible response, dependent on day length and temperature [138,139], and is accompanied by an increase
in the ABA content of cells (e.g., Ref. 140). Low temperatures also induce numerous proteins or their mR-
NAs, and evidence exists that some of these proteins are necessary for chilling tolerance. For example,
Mohapatra et al. [141] compared alfalfa cultivars and cold-inducible gene products. They found a high
correlation coefficient between the LT 50 (the temperature that is lethal to 50% of the treated plants) and
the relative amounts of a particular low temperature–induced mRNA.
COMPARISONS WITH HEAT SHOCK. Unlike heat shock, general protein synthesis does not ap-
pear to cease in response to chilling (Ref. 136 and references therein). In addition, there appears to be lit-
tle conserved with heat shock in terms of the types of synthesized proteins [136]. However, some of the
HSPs, or their transcripts, are also cold inducible [142–147]. Jaenicke [148] indicated that the stability of
proteins is limited by both high andlow temperatures. Hence, the presence of HSPs may not be that un-
usual. However, conclusive proof of a role for HSPs during low-temperature stress is currently lacking.
INDUCTION OF PROTEINS IN RESPONSE TO STRESSES 669