Water scarcity imposes huge reductions in crop yield and is one of …

• Abstract
• Introduction
• Revising the constraints to photosynthesis and the regulatory systems operating under water deficits
• Applications to water-saving agriculture
• Conclusions
• References
• Table 1
• Table 2
• Figures

Most of the terrestrial plants have evolved either to escapedrought by appropriate phenology or to avoid drought, by developingstrategies 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 ofother stresses that often arise concomitantly with drought,and ultimately involve oxidative stress. Protective responsesat the leaf level must then be triggered quickly in responseto the stress effectors to prevent the photosynthetic machinerybeing irreversibly damaged. Therefore, signals are key playersin plant resistance to stress. It is now apparent that redoxsignals are early warnings, exerting control over the energybalance of a leaf, and alterations in the redox state of redox-activecompounds regulate the expression of several genes linked tophotosynthesis and other metabolic pathways. It is also knownthat plant responses to stresses arise from the interplay betweendifferent 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, ‘dryingwithout dying’ as described by Alpert and Oliver (2002),are not common in higher plants, genetic engineering to introducethese traits may be a way forward for marginal environments,complementing the breeding work and marker-assisted selectionfor tolerance that explores the natural allelic variation atgenetically identifiable loci. Moreover, QTL mapping alliedwith comparative mapping and map-based cloning in plants maybe 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 engineertolerance to multiple stresses. Engineering a single gene, suchas a Group 3 LEA gene or one affecting sugar metabolism, orplaying 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 possiblein the near future. It may be possible to achieve multiple tolerancemechanisms for one or more abiotic stresses, with sufficientsuccess for commercial exploitation through co-transformationor gene pyramiding. Moreover, the upstream targeting of regulatorynetworks may have a more consistent role in providing tolerance,either through protection or repair mechanisms. Advances inthe molecular biology of stress response in tolerant organismsare raising a number of possibilities concerning regulatorygenes that may be used in agricultural programmes, not onlyto ensure survival under water deficit but also to guaranteea reasonable productivity under reduced water availability.