From an expression-based reverse genetic study to the functional characterization of two determinants of osmotic stress tolerance.

Adaptation to osmotic stress requires an extensive alteration of gene expression. Previously, we identified several genes regulated in cells adapted to polyethylene glycol (PEG). Here, the functional role of fifty of these genes was verified. Using a large-scale phenotype screening, we have identified two genes: the splicing factor IAG1 (INSENSITIVE TO ABA IN GERMINATION1) and the putative TOR-pathway component XSA1(EXTRA SENSITIVE TO ABA1). IAG1 is induced upon long-term exposure to abscisic acid (ABA) and PEG and is mainly expressed in trichomes and stomata, organs controlling transpiration. Germination analysis of plants with altered expression of IAG1 and protein interaction with the splicing factor SUA, suggest that IAG1 may be involved in premRNA splicing of effectors of ABA response leading to germination inhibition. XSA1 possibly affects pathways in ABA-mediated response to stress. XSA1 is expressed in vascular tissues and is up-regulated by long-term exposure to NaCl and ABA. xsa1-1 is ABA hypersensitive, indicating alteration in ABA biosynthesis and/or perception. Taken together, our results reveal promising mechanisms of plant adaptation to osmotic stress.