example, Ismail et al. [110] reported cosegregation of dehydrin alleles with chilling tolerance in cowpea
seedling emergence.
Constitutive expression of specific dehydrins has been reported in several species [90,107,111,112].
Thus, it is possible that some dehydrins fulfill a role(s) in normal growth and development. Alternatively,
they may be synthesized as a constitutive defense against rapid changes in water status.
Metabolic or Osmotic Adjustment Proteins. See section on salinity stress.
Transport Proteins. Transport proteins include those involved in ion transport (see section on
salinity) and those involved in water transport (aquaporins). Aquaporins have been characterized at both
the transcriptional and translational levels, with differential expression reported [113]. Although water
deficit and salinity have been shown to induce aquaporin transcripts, the overall function of aquaporins
in regard to lowered water status as well as normal growth and development is still unresolved [113].
Heat Shock Proteins. Water deficit has also been shown to induce several classes of HSPs, in-
cluding HSP70 [88,89]. They probably play roles similar to those needed during elevated temperatures
(see earlier).
Protein Degradation. Ubiquitin and polyubiquitin are inducible by water deficit [114,115].
Given that proteins can be denatured by dehydration, these polypeptides probably play a role similar to
that seen during elevated temperature (see before). It has also been shown that proteases can be drought
inducible [114,115]. It is likely that the proteases either destroy denatured proteins or recycle amino acids
for proteins needed in response to water deficit.
Lipid Transfer Proteins. Lipid transfer proteins (LTPs) catalyze the transfer of several classes
of phospholipids and/or glycolipids between membrane vesicles (in vitro) or their deposition in the cell
wall [116]. LTPs have been shown to be induced by water deficit in the aerial portions of tomato and pea
[117,118]. The data indicate a need to either increase membrane fluidity or decrease water loss via in-
creased epidermal impermeability. These proteins are also inducible by low temperature and salinity
stress [116].
- Salinity Stress
PHYSIOLOGY. Although salinity stress is related to water deficit by a decrease in water status, the
presence of excess ions also appears to be detrimental to many plant processes. Thus, plants subjected to
salinity stress appear to face two stresses at the same time. Plants vary in their ability to survive salt stress,
with tolerant plants generally either sequestering ions in the vacuole or synthesizing osmotically active
compounds (i.e., osmoregulation). Nontolerant plants typically attempt to exclude excess ions via active
transport. In the case of nontolerant Arabidopsis, a Ca^2 sensor has been discovered that may be impor-
tant for active transport [119]. Liu and Zhu [119] noted that Ca^2 fluxes detected by this sensor may po-
tentiate the regulation of Kand Natransport systems, as occurs with animal homologues.
MULTIPLE PROTEIN RESPONSES. Like water deficit, salt stress results in a general decrease in
protein synthesis (e.g., Ref. 120), which is correlated with a loss of polysomes in vitro [121]. In turn, many
proteins and transcripts have been reported to increase or be synthesized de novo in response to salt stress.
Many of these proteins or transcripts are also inducible by water deficit or ABA.
OSMOTIC ADJUSTMENT. Another aspect shared by salt stress and water deficit is osmotic adjust-
ment, wherein organic or inorganic osmotically active solutes (osmolytes) are accumulated. This accu-
mulation creates a lower solute potential, which allows a plant cell to maintain a higher water content than
in the absence of these osmolytes. Many different organic molecules have been described as accumulat-
ing during water deficit or salt stress, including quaternary amines, polyols, and sugars [122], as well as
inorganic Kand Clions [123]. The ability of these organic molecules to balance ions sequestered in
the vacuole and to stabilize enzymes incubated with salt solutions has resulted in describing these com-
pounds as compatible solutes. Although osmotic adjustment does occur in response to water deficit or
salinity stress, Hare et al. [124] contend that osmolyte accumulation is generally insufficient to lower so-
lute potential significantly. In contrast, they suggest that the primary benefits of such osmolytes are
metabolic in nature, either as compatible solutes, sensors of photosynthate partitioning, or buffering re-
dox potentials. Further research is required to assess this hypothesis.
Sugars. Two sugars have received attention, sucrose and trehalose. Sucrose synthesis is well char-
acterized, and induction of sucrose synthase and sucrose-phosphate synthase transcription and translation
has been observed in response to water deficit in several species [114]. In contrast, genes for trehalose
668 ARTLIP AND WISNIEWSKI