Plant cellular and molecular responses to high salinity

Plant responses to salinity stress are reviewed with emphasis on molecular mechanisms of signal transduction and on the physiological consequences of altered gene expression that affect biochemical reactions downstream of str ess sensing. We make extensive use of comparisons with model organisms, hal ophytic plants, and yeast, which provide a paradigm for many responses to s alinity exhibited by stress-sensitive plants. Among biochemical responses, we emphasize osmolyte biosynthesis and function, water flux control, and me mbrane transport of ions for maintenance and re-establishment of homeostasi s. The advances in understanding the effectiveness of stress responses, and distinctions between pathology and adaptive advantage, are increasingly ba sed on transgenic plant and mutant analyses, in particular the analysis of Arabidopsis mutants defective in elements of stress signal transduction pat hways. We summarize evidence for plant stress signaling systems, some of wh ich have components analogous to those that regulate osmotic stress respons es of yeast. There is evidence also of signaling cascades that are not know n to exist in the unicellular eukaryote, some that presumably function in i ntercellular coordination or regulation of effector genes in a cell-/tissue -specific context required for tolerance of plants. A complex set of stress -responsive transcription factors is emerging. The imminent availability of genomic DNA sequences and global and cell-specific transcript expression d ata, combined with determinant identification based on gain- and loss-of-fu nction molecular genetics, will provide the infrastructure for functional p hysiological dissection of salt tolerance determinants in an organismal con text. Furthermore, protein interaction analysis and evaluation of allelism, additivity, and epistasis allow determination of ordered relationships bet ween stress signaling components. Finally, genetic activation and suppressi on screens will lead inevitably to an understanding of the interrelationshi ps of the multiple signaling systems that control stress-adaptive responses in plants.