STRESS-INDUCED CHANGES IN PLANT GENE-EXPRESSION - PROSPECTS FOR ENHANCING AGRICULTURAL PRODUCTIVITY IN SOUTH-AFRICA

The natural environment of southern Africa is characterised by great c limatic diversity and marked inter-seasonal variability in climatic co nditions. It is estimated that less than 20% of South Africa by area y ields mean annual primary agricultural outputs in excess of 40% of its potential. Environmental stress is the primary cause of the differenc e between observed and potential agricultural yields. It also accounts for considerable inter-annual variability in yield and thereby poses a threat to food security. Drought, in particular has played a dominan t role historically in limiting agricultural growth. Since the adverse influence of abiotic stress on agricultural output leads to an almost continuous drain on state finances, the development of stress-toleran t varieties of agriculturally important crops is likely to be of great economic importance. Although plant responses to environmental stress involve adaptation at several levels of organisation, they all must u ltimately have a genetic basis. Several genes are now known to respond to different stresses commonly encountered in agriculture. The geneti c manipulation of these genes holds considerable promise as a first st ep towards increasing environmental tolerance and is well within the s cope of plant biotechnology currently at our disposal. This review att empts to assess the feasibility of using such an approach as part of a long-term sustainable development strategy in southern Africa and to indicate strategies that might be considered to enhance stress toleran ce of crops of agricultural importance. In the light of increasing evi dence implicating the existence of a stress response system in plants, it is proposed that overlapping responses to different environmental stresses may be mediated by common cellular signal transduction pathwa ys. Recent studies of signalling cascades in higher plants have identi fied ion channels, intracellular signalling proteins, and second messe ngers as critical components mediating early events in signal transduc tion. In the long term, targeting the genes encoding components of str ess-related signal transduction pathways might be more profitable than manipulation of genes involved in intermediary metabolism. Transgenic crops altered as a result of stress-related cellular signalling may d isplay increased ability to adapt to several stresses experienced conc urrently in the field Since the molecular details of stress-related si gnal transduction in plants remain largely unknown, however characteri sation of the molecular signals involved in stress perception and the molecular events that specify the expression of stress tolerance will be necessary to provide a sound basis on which such strategies can be founded.