Droughtinduced Changes in Gene Expression

In response to drought, salt and other osmotic stresses gene expression profiles change. These changes have been documented at the RNA and protein levels by many research groups, including many cases for tomato and related species (Bray 1988; Cohen and Bray 1990; Ho and Mishkind 1991; Chen and Tabaeizadeh 1992a, b; Thompson and Corlett 1995; Jin et al. 2000). Many of the changes in gene expression appear to be regulated by the plant hormone abscisic acid (ABA). ABA-dependent and independent routes for tran-scriptional regulation of drought, salt, and cold temperature stresses have been well described for Ara-bidopsis and predicted for other plants (Nakashima and Yamaguchi-Shinozaki 2006). The first event is the perception of the water deficit state, followed by a signal transduction cascade that may be initiated by altered levels or sensitivities to ABA or through other non-ABA based paths. Altered patterns of transcription are then observed, presumably in response to increased levels of specific transcription factors. These new transcripts then result in the accumulation of proteins/enzymes that synthesize osmoticum, stabilize membranes, bind water, thicken wax layers, alter ion flux, close stomata, etc. Also, transcription factors orothertypesofchromatinproteinsmay represstran-

scription. Photosynthesis rates are reduced during drought stress. This physiological change is mediated at a number of levels, but in particular, transcription is reduced for the genes for light harvesting chlorophyll a-b binding protein (cab) and Rubisco small subunit (rbcS) during drought stress (Bartholomew etal. 1991).

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