multiple responses to stress, and the elimination of a single aspect of the stress response may not have a
significant effect on plant stress tolerance.
B. Introduction of Foreign Genes
When a function is understood that promotes adaptation to stress, genes that have been identified previ-
ously may be exploited to improve plant stress tolerance. Thus far, there is one example of this. A gene
isolated from E. coli, mtlD, which encodes mannitol-1-phosphate dehydrogenase, is involved in manni-
tol catabolism and leads to the production of fructose-6-phosphate. It was hypothesized that if this gene
were expressed in plants, the enzyme may catalyze the reverse reaction, resulting in the synthesis of man-
nitol-1-phosphate, which would be a substrate for general phosphatases, resulting in the synthesis and ac-
cumulation of mannitol [98]. Transgenic tobacco plants, with mtlDdriven by the 35S CaMV or NOS pro-
moter, resulted in the accumulation of mannitol in young leaves and roots [98]. After 30 days of exposure
to salinity, transgenic plants producing mannitol had a greater shoot height and root length than those of
control plants [99]. Therefore, this is the first example of a transgenic plant, altered with a microbial gene,
which has a greater resistance to plant osmotic stress. Further trials are required to determine the agricul-
tural applicability of these plants. Other genes might also be exploited, especially for other strategies of
osmotic adjustment. The use of these strategies in combination may improve stress tolerance of transgenic
plants as well as our understanding of the importance of water-deficit responses to plant adaptation to the
environment.
IV. ABA INDUCES SPECIFIC GENES DURING WATER DEFICIT
The concentration of ABA is altered by changes in the environment. Large increases in ABA concentra-
tion have been documented in response to drought stress, with lesser increases in ABA concentration oc-
curring in response to salt and low-temperature stress. These changes occur at the cellular level, but the
ABA that is transported throughout the plant at the whole plant level is also changed.
The ABA biosynthetic pathway occurs through the carotenoid biosynthetic pathway. The compound
9
- cis-neoxanthin is cleaved to result in the postcleavage intermediate to ABA, xanthoxin. Xanthoxin is
oxidized to ABA-aldehyde, which is converted to ABA by ABA-aldehyde oxidase [100]. The step in the
ABA biosynthetic pathway that is regulated by stress is likely to be the cleavage step, although this has
not been proved because this gene has not been isolated and/or an assay has not been developed for that
enzymatic step. The rate of ABA biosynthesis during stress is limited by the production of xanthoxin, not
the conversion of xanthoxin to ABA [100].
The trigger that is recognized by the cell to induce ABA biosynthesis is not understood. ABA accu-
mulation is correlated with a reduction in turgor to near zero [2]. Therefore, it is thought that the cellular
mechanism for turgor perception is linked to the ABA biosynthetic pathway through a signal transduc-
tion pathway. Inhibition of transcription and translation prevented ABA accumulation in response to
stress [10], indicating that these processes are required for the cell to recognize stress, or the ABA biosyn-
thetic enzymes must be synthesized for ABA to accumulate. More effort is needed to understand the
mechanism of stress-induced ABA accumulation.
Although it is not certain how ABA biosynthesis is controlled, it has been demonstrated that ABA is
part of the signaling mechanism during stress that induces specific genes. This has been demonstrated us-
ing mutants that are deficient in ABA biosynthesis [14,15,87]. For most studies it is convenient to apply
ABA and determine if the application of ABA causes an accumulation of specific transcripts. In the cases
where it does, those genes are found to be ABA responsive, indicating that gene induction may occur in
response to ABA. However, these studies do not prove that ABA is an endogenous signal used during
specific stresses to induce particular genes. Application studies indicate only that ABA is one of the sig-
nals that the gene is capable of responding to. Inhibitors of carotenoid biosynthesis result in a decreased
level of ABA [100], and responses of the plant that are reduced by inhibitor application have been used
to analyze the role of ABA. But as with the use of other inhibitors, effects that are not caused directly by
the reduction in ABA concentration may also occur, because carotenoid concentration is also reduced af-
ter application of these inhibitors. Mutants that are deficient in ABA biosynthesis and cannot accumulate
ABA during stress can be used to identify genes that require elevated levels of endogenous ABA for ex-
pression. Mutants in tomato, maize, and Arabidopsis, which have specific blocks in the ABA biosynthetic
744 BRAY