Drought Responsive Genes

By screening samples using microarrays, approximately 130 drought responsive genes were identified in Arabidopsis (Reymond et al. 2000; Seki et al. 2001). These drought responsive genes were placed into functional classes using the predicted amino acid sequence of the gene to identify possible gene functions; drought responsive genes were found in eleven of the 13 possible functional classes (Bray 2002).

A search of GenBank (March 2007) for drought responsive genes returned the following numbers. For "Lycopersicon" in the organism field and ABA, drought, or salt stress, there were 18, 12, and 16 entries, respectively. This search excluded ESTs. When the organism field was "Arabidopsis", there were 160, 138, and 51 entries for the same terms, respectively. If we presume that the number of genes annotated in Arabidopsis is close to the true number of drought-responsive genes, this suggests that ~10% of the drought responsive genes in tomato have been identified. In potato, over 1,400 transcripts have been identified as unique to potato leaf or root tissue under abiotic stress (Rensink et al. 2005). This set of transcripts included genes expressed in response to cold, drought, salt, and heat stress. While there was good overlap with genesexpressedasstressresponsivein Arabidop-sis within these potato transcripts, there were also a number of transcripts that did not match any entry in an in-house non-identical amino acid database (Rensink et al. 2005). A number of the stress responsive transcripts in the potato set matched Arabidopsis genes with no functional annotation, which may help improve the annotation of the Arabidopsis genome.

The current challenge is to identify genes that confer drought- or salt-resistant phenotypes; multiple genes are predicted to be key as both phenotypes in tomato are inherited as quantitative traits. Further complicating the challenge is the stage-specific nature of the adaptive stress responses, seed germination vs. vegetative growth vs. fruit growth. The assays or screens that compare gene expression within one species with and without the stress are likely to identify genes expressed in all cells in response to the stress (e.g., gene products involved in osmotic adjustment mechanisms of cells). Drought resistance strategies that involve adapted or unique developmental patterns of gene expression or pre-existing states (constitutive expression) are not likely to be identified by screening differential treatments. Identification of these genes will rely on identification of the gene pro ducts associated with the QTLs for the trait, or by gene expression studies that include comparisons between genotypes as well as comparisons between environmental conditions over time in the drought or abiotic stress cycle. Microarrays or other high-throughput technologies to assay gene expression, in conjunction with sophisticated modeling of genetic networks, will undoubtedly be required for successfully carrying out these types of studies.

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