similarity with fish antifreeze protein was noted [35]; however, there is no functional evidence for the role
of these proteins in a stress response.
A variety of genes that encode hydrophilic proteins are expressed in response to abiotic stresses.
Many of the protein families have different structures, leading to predictions that these proteins are in-
volved in ion sequestration, water binding, and other protective roles in the cytoplasm. At this time, bio-
chemical studies are needed to confirm these predictions.
B. Genes Encoding Enzymes
The activity or the amount of many enzymes has been shown to be altered during drought stress [37]. In
many cases the activity of an enzyme has been studied during stress but the gene has not been character-
ized. In this chapter, only enzymes in which the gene has been cloned and the mRNA corresponding to
that gene has been shown to be elevated in response to drought are discussed. In the coming years, the
number of enzymes that are cloned and shown to be regulated during stresses that impose water deficits
is sure to grow. Although an enzymatic function can be predicted from the deduced amino acid sequence
of a cloned gene, the role during stress may not be obvious or ensured by the presence of a transcript dur-
ing stress. Until additional experiments are done, it cannot be determined if the activity of these enzymes
promotes stress adaptation. Enzymes induced by water stress and involved in osmolyte accumulation,
protein degradation, CAM, ion transport, and signal transduction are described.
- Osmolyte Accumulation
During periods of water deficit, compatible solutes accumulate in the cell, resulting in a lower cellular
osmotic potential. If the cellular water potential is more negative than that of the cells environment, wa-
ter will be taken up by the cell. This process, called osmotic adjustment, may occur in the field after a
long-term drought stress [38]. Compatible solutes, or osmolytes, include inorganic ions; organic ions;
soluble carbohydrates, including polyols; amino acids, particularly proline; and quaternary ammonium
compounds such as betaines [37]. Several genes have been identified which code for enzymes that may
be involved in the accumulation of osmolytes during osmotic stress; these include enzymes in the
biosynthetic pathway of proline [39–41], glycine betaine [42], and polyols [43,44]. The plant biosyn-
thetic pathway for proline contains two genes; one is a bifunctional enzyme, ^1 -pyrroline-5-carboxy-
late synthetase, which has both -glutamyl kinase and glutamic- -semialdehyde dehydrogenase activi-
ties [40], and the other is ^1 -pyrroline-5-carboxylate reductase (P5CR) [39–41]. Both of these genes are
induced by salt stress, indicating that they may play a role in osmotic adjustment. In pea, P5CR is
induced in roots, but not in shoots, of pea seedlings in response to salt stress [41]. The gene coding
for betaine aldehyde dehydrogenase, the last step in glycine betaine synthesis, has been isolated and
is induced by salt stress [42]. A gene encoding myo-inositolO-methyl transferase (imt1) was isolated
from the facultative CAM plant Mesembryanthemum crystallinumand is induced by salt stress [44],
drought, and low temperature [45]. This enzyme is involved in synthesis of the polyol, pinitol. Tobacco
plants transformed with imt1driven by the 35S promoter accumulated ononitol, an osmolyte not syn-
thesized in wild-type tobacco. Although stress studies were not reported, these results indicate that the
enzyme encoded by imt1is a step in the biosynthetic pathway of pinitol [46]. Aldose reductase, an
enzyme involved in sorbitol synthesis, has been identified in barley seeds, although this gene is not
expressed in dehydrated barley leaves [43]. An NADPH-dependent aldose reductase is induced by
ABA in bromegrass suspension cells with increased freezing tolerance [47]. In response to a number
of stresses, enzymes involved in osmolyte accumulation are induced. These enzymes are probably
|involved in the accumulation of osmolytes, which promotes uptake of water into the cells through
osmotic adjustment.
Vegetative storage protein genes, vsp, are induced by environmental stresses such as water deficit
and wounding. The VSPs have been shown to accumulate preferentially in the vacuoles of paraveinal
mesophyll cells of soybean. It has been shown that two of the proteins, VSPand VSP, are acid phos-
phatases with the highest substrate specificity for tetrapolyphosphates [48]. Staswick et al. [49] reported
that although the VSPs have acid phosphatase activity, they are not the major acid phosphatase in leaves.
DeWald et al. [48] propose that VSPs are involved in amino acid uptake and temporary sequestration of
amino acids in paraveinal mesophyll cells. Therefore, these proteins may play a role in osmotic adjust-
ment during stress.
ABIOTIC STRESSES AND ABSCISIC ACID 739