Identification of new up-regulated genes under drought stress in soybean nodules.

Legumes/rhizobium biological N(2) fixation (BNF) is dramatically affected under abiotic stress such as drought, salt, cold and heavy metal stresses. Nodule response to drought stress at the molecular level was analysed using soybean (Glycine max) and Bradyrhizobium japonicum as a model, since this symbiotic partnership is extremely sensitive to this stress. To gain insight into molecular mechanisms involved in drought-induced BNF inhibition, we have constructed a SSH (Suppression Subtractive Hybridisation) cDNA library from nodular tissue of plants irrigated at field capacity or plants water deprived for 5 days. Sequence analysis of the first set of 128 non redundant ESTs using protein databases and the Blastx program indicated that 70% of ESTs could be classified into putative known functions. Using reverse northern hybridization, 56 ESTs were validated as up-regulated genes in response to drought. Interestingly, only a few of them had been previously described as involved in plant response to drought, therefore most of the ESTs could be considered as new markers of drought stress. Here we discuss the potential role of some of these up-regulated genes in response to drought. Our analysis focused on two genes, encoding respectively a ferritin and a metallothionein, which are known to be involved in homeostasis and detoxification of metals and in response to oxidative stress. Their spatiotemporal expression patterns showed a high accumulation of transcripts restricted to infected cells of nodules in response to drought.