- Proteases
Proteases have also been shown to be induced by abiotic stresses. A thiol protease is induced in pea by
water deficit [50]. Two additional proteinases, cysteine proteinases, were identified in drought-stressed
Arabidopsis[51]. These genes have the catalytic sites typical of cysteine proteinases and have amino-ter-
minal signal peptides. They are not induced by ABA or temperature stress but are strongly induced by salt
and drought stress [51]. These genes are similar to a cysteine protease induced by low temperature in
tomato [52]. Their function during drought stress is not confirmed, but they may be involved in the degra-
dation of polypeptides denatured during stress, processing of precursor proteins to the mature form, or
degradation of vacuolar proteins, after which the amino acids may be used in synthesis of stress-induced
proteins or osmotic adjustment [50,51].
- Induction of CAM
In plants that have the capacity for Crassulacean acid metabolism (CAM), a switch from C3 metabolism
to CAM may occur during periods of stress or in response to development. In Mesembryanthemum crys-
tallinum, there is a 10- to 20-fold increase in phosphoenolpyruvate carboxylase (PEPCase) activity in re-
sponse to salt stress [53]. PEPCase is the enzyme responsible for the primary fixation of CO 2 into ox-
aloacetate, which is subsequently converted to malate during CAM. In M. crystallinum, PEPCase is
represented by two genes, one of which is salt stress induced. However, induction does not occur in salt-
stressed suspension cultures of M. crystallinum[54]. CAM coupled with stomatal conductance provides
a means for the improved stress tolerance that may occur in specialized plants.
- Plasma-Membrane H-ATPase
During salt stress or water deficit, the concentration of ions in the cytoplasm must be controlled. The
import of ions into the cytoplasm must be limited to the capacity to compartmentalize Naand Cl
into the vacuole. Active transport of ions is driven by an Helectrochemical gradient that is generated
by the plasma-membrane H-ATPase [55]. Carriers or channels facilitate the active or passive trans-
port of these ions, with active transport driven by the H-ATPase. During development of water deficit
in soybean seedlings, there is an increase in H-ATPase mRNA levels in the roots only. This correlates
with the ability of growth maintenance in this organ; in shoots in which growth is not maintained dur-
ing water stress, there is not an increase in H-ATPase mRNA levels [56]. This may indicate that
growth during stress is dependent on increased activity of ATPase and is associated with ion transport.
In the halophyte Atriplex nummularia, it has been demonstrated that the plasma-membrane H-ATPase
is regulated by NaCl [57]. The ATPase activity may have an important role in stress tolerance and
should be studied further.
- Protein Kinase
For plants to respond to the environment, mechanisms must have evolved that signal changes from the
environment at the cellular level. The pathway of information transfer from the environment to the cell,
resulting in alterations in gene expression, is the signal transduction pathway. Many different component
pathways may be required to achieve alterations in gene expression. In one type of signal transduction
pathway, a protein kinase controls activation or deactivation of proteins by phosphorylation. A cDNA,
PKABA1, corresponding to a protein kinase that is induced by ABA, has been isolated [58]. The deduced
amino acid sequence has 12 catalytic subdomains found in serine/threonine protein kinases that are
thought to be critical for phosphorylation. Two mRNAs hybridize to PKABA1 in dehydrated seedlings,
and the accumulation of these mRNAs corresponds to an increase in ABA concentration. The rate of ac-
cumulation is similar to the accumulation of other drought- and ABA-induced genes; therefore, it is pro-
posed that this kinase is involved in the phosphorylation of other ABA-induced gene products [58]. Al-
though there are many other possibilities for the action of this gene product, the function cannot be
determined until its substrate specificity is characterized.
C. Genes That Have Other Predicted Cellular Functions
For a plant to survive periods of stress, many developmental, physiological, and metabolic functions may
need to be altered. A number of unique changes in gene expression have been identified, using gene
cloning techniques. Many of these would not have been identified using other types of studies. Genes
740 BRAY