Cellular Reactions to Drought Stress
At the cell level, water deficit is not only a result of drought, but also of salt stress and of frost. The loss of up to 90% of the cell water is a necessary process during seed and fruit ripening and thus is a regular component of a plant's development. Plants have, therefore, developed biochemical mechanisms to cope with drought stress. Our understanding of the molecular processes triggered by drought and of the adaptation to water shortage is, however, still patchy. There is no doubt that each cell has more than one mechanism for reacting to drought; there is, so to say, a multiple safety net. However, not all changes in cell biology occurring upon drought stress are specific to that situation. Some strains are primarily targeted to other stressors, e.g. metabolic changes triggered by cold or pathogen attack can also be effective to cope with dehydration stress. In addition to specific stress-directed reactions, desiccation interferes with the normal housekeeping machinery of the cell. This holds in particular for poikilo-hydric plants (see Chap. 188.8.131.52) where photosynthesis is reduced in order to minimise the danger of photooxidation. In the resurrection plant Craterostigma plantaginea expression of genes of the photosynthetic machinery is attenuated, probably concomitantly with the induction of genes whose products - proteins and metabolites
- increase drought resistance of cells directly or indirectly. A key role in coping with dehydration stress is attributed to the phytohormone abscisic acid (ABA; see Fig. 1.5.3 and Box 1.5.3), which triggers fast reactions as well as long-term adaptation. Complying with the above-mentioned complexity of stress-induced cellular reactions, ABA-independent reactions are also well known. Current knowledge shows cellular responses to drought to be no less complicated than the responses to heat stress.
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