Most of the terrestrial plants have evolved either to escape
drought by appropriate phenology or to avoid drought, by developing
strategies that conserve water or optimize its acquisition.
This requires early warning systems and different types of signalling.
In general, plants also have to cope with the interaction of
other stresses that often arise concomitantly with drought,
and ultimately involve oxidative stress. Protective responses
at the leaf level must then be triggered quickly in response
to the stress effectors to prevent the photosynthetic machinery
being irreversibly damaged. Therefore, signals are key players
in plant resistance to stress. It is now apparent that redox
signals are early warnings, exerting control over the energy
balance of a leaf, and alterations in the redox state of redox-active
compounds regulate the expression of several genes linked to
photosynthesis and other metabolic pathways. It is also known
that plant responses to stresses arise from the interplay between
different signalling pathways.
The importance of the long-distance signalling for the plantfeed-forward response to water stress is acknowledged, namelythe role played by chemical signals synthesized in the rootsand transported to the shoot via the xylem sap. Novel managementtechniques that exploit the knowledge of plant's long-distancesignalling are increasingly being applied to get improved planttrade-off between carbon assimilated and water used, while sustainingyield and improving the quality of the crop products.
On the other hand, because drought-tolerance traits, ‘drying
without dying’ as described by Alpert and Oliver (2002)
are not common in higher plants, genetic engineering to introduce
these traits may be a way forward for marginal environments,
complementing the breeding work and marker-assisted selection
for tolerance that explores the natural allelic variation at
genetically identifiable loci. Moreover, QTL mapping allied
with comparative mapping and map-based cloning in plants may
be used to screen genes important in the response to stress.
The molecular understanding of stress perception, signal transduction,
and transcriptional regulation of these genes, may help to engineer
tolerance to multiple stresses. Engineering a single gene, such
as a Group 3 LEA
gene or one affecting sugar metabolism, or
playing a role as an anti-oxidant, proved to alter metabolism,
but in most cases only led to marginal stress improvement. However,
recent advances suggest that rapid progress will be possible
in the near future. It may be possible to achieve multiple tolerance
mechanisms for one or more abiotic stresses, with sufficient
success for commercial exploitation through co-transformation
or gene pyramiding. Moreover, the upstream targeting of regulatory
networks may have a more consistent role in providing tolerance,
either through protection or repair mechanisms. Advances in
the molecular biology of stress response in tolerant organisms
are raising a number of possibilities concerning regulatory
genes that may be used in agricultural programmes, not only
to ensure survival under water deficit but also to guarantee
a reasonable productivity under reduced water availability.