The Molecular Basis of ABA-Mediated Plant Response to Drought - Abiotic Stress: Plant Responses and Applications in Agriculture - page 118

Evaporation is a main determinant of leaf temperature. There is a direct relationship be‐
tween leaf temperature, transpiration rate and stomatal conductance [159-161]. Drought-tol‐
erant genotypes can maintain a higher stomatal conductance and also a higher rate of
photosynthesis, as was mentioned above, thus these genotypes could be identified as having
a lower canopy temperature than the sensitive genotypes [162-163].
7. GM crops - are they a solution?
Genetic modification of crops is a controversial issue. Some aspects of genetic modification
that have potential to improve drought tolerance in crops are presented here. Biotechnologi‐
cal approaches may involve the overexpression of genes related to osmotic adjustment,
chaperones and antioxidants [reviewed in 164-165]. Also, ectopic expression or suppression
of regulatory genes, such as genes that encode transcription factors, is widely used [166]. Re‐
cent studies on rice led to the identification of genes involved in three pathways that can be
manipulated in order to improve drought tolerance in crops: the gene that encodes β-caro‐
tene hydroxylase, which confers drought resistance by increasing xanthophylls and ABA
synthesis [167], the DST1 ( DROUGHT AND SALT TOLERANT 1 ) gene that regulates stoma‐
tal closure and density under drought stress [168] and the TLD1 / OsGH3.13 ( INCREASED
NUMBER OF TILLERS, ENLARGED LEAF ANGLES, AND DWARFISM ) gene whose down-
regulation enhanced drought tolerance in rice [169]. Although several genes that can im‐
prove the drought tolerance of crops have already been identified, progress in the
commercialization of the traits controlled by these genes has been slow [165]. One of the
genes that has been successfully introduced into a crop plant and that gave improved
drought tolerance in field trials was the gene encoding Cold Shock Protein B (CspB) RNA
chaperone from Bacillus subtilis . The CspB gene is important in the ability of bacteria to adapt
to cold, and its overexpression in plants was shown to provide drought tolerance in Arabi‐
dopsis, rice and maize [170]. Results from field experiments showed that a maize line ex‐
pressing the CspB gene had a higher yield under water deficit conditions than the control
and expressed a yield equivalent to the control under non-stressed conditions. Tests are in
progress in 2012 on commercial farms, [171; http://www.monsanto.com/products/Pages/
corn-pipeline.aspx#firstgendroughttolerantcorn]. The value of a biotechnological approach
to improving crop yields under drought stress conditions is becoming evident with the first
demonstrations of improved drought tolerance in crops in the field (reviewed in [171]).
8. Conclusions and perspectives
In order to achieve a full understanding of drought-response mechanisms in plants and to
make use of this understanding to produce crops with improved drought tolerance, there is
a need to combine the data derived from different studies. Detailed analyses of the networks
of protein interactions, the co-expression of genes, metabolic factors, etc. should provide in‐
sights into the key regulators of drought response [172-173]. Biotechnological approaches