Drought or Water Deficit Stress Resistance Summary

The genetic basis for abiotic stress resistances and tolerances in wild tomato species are quantitatively inherited; consequently it is unlikely that a single transgene expressed in cultivated tomato will confer agronomically relevant drought or salt tolerance. A complete understanding of the biochemical and physiological basis of abiotic stress resistances is essential for the development of crops able to yield har-vestable product, i.e., tomato fruit, when grown with reduced water or poor quality water. With competing demands for agricultural water by urban development, water availability will continue to be a limitation for crop production. To best utilize a molecular approach, a full understanding of the physiological basis for the drought resistant phenotype is critical.

The best genetic sources of drought resistance for cultivated tomato are wild tomato species. There are two species with very different mechanisms for drought resistance; S. chilense invests in root growth, while S. pennellii regulates stomatal aperture efficiently during drought stress. Drought-induced alterations in gene expression, most commonly assayed at the transcriptional level, have been observed in a number of systems. Identification of genes associated with drought responses in plants, particularly in tomato, has been achieved through a variety of differential screens and comparisons. Inter-specific comparisons with drought-resistant species have been especially informative. The diversity of well characterized phenotypes in wild tomato species coupled with molecular genetic approaches are likely to result in a full understanding of the variety of ways that plants continue to grow in the face of limited water.

The molecular analysis of drought and salt responses in tomato and related species has identified a number of interesting genes and identified regions of the genome associated with selected stress resistance phenotypes. A candidate gene analysis for those regions or a biochemical or physiological assignment for these loci has yet to be accomplished; the full power of a genomics analysis of these responses has yet to be seen. The comparative analyses possible between cultivated tomato and wild tomato species that have evolved to survive in arid and semi-arid environments continues to be a productive research strategy. If these analyses are conducted using global gene expression (e.g., microarray) technology, then the results may give a more conclusive understanding of genetic responses during conditions of drought stress. Comparison of the genetic network for gene expression in resistant and susceptible genotypes will provide the missing information to understand the different strategies tomato has evolved to grow in environments with reduced or poor quality water. Some of these adaptive responses may be useful to plant breeders developing tomato genotypes for fruit production in areas likely to experience drought or reduced water availability.

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