Identification of yeast and plant salt stress tolerance determinants

The detrimental effects of high concentrations of salts (mainly Na +) on plants remains a major limitation to agricultural productivity in arid, semi-arid and irrigated agriculture. Currently, the use of genetic molecular systems is become an increasingly important tool in understanding basic cellular functions. Here the yeast, Saccharomyces cerevisiae , has been used as a molecular genetic system to identify salt tolerance determinants by gain-of-function complementation of salt sensitive yeast mutants with yeast or plant genes. In addition, genomic microarray analysis has been used to dissect components of pivotal salt stress signal transduction cascades in the model organism by monitoring global mRNA level changes, in response to salt treatment, between wild type cells and salt sensitive yeast mutants. Functional complementation of the salt sensitive yeast mutant, nls2, identified two categories of suppressors, SPK1 (Salt Protein Kinase) and SCP1 (Salt C-2 domain Protein), albeit neither is NLS2. SPK1 expression also increased Na+/Li+ tolerance of wild type yeast cells, suggesting it is a limiting factor in salt adaptation. A spk1 knockout resulted in reduced growth of yeast strains in the presence of salt. The salt sensitive phenotype of a spk1 and calcineurin null (cnb1) double mutant was additive, relative to either individual mutant, indicating that Spk1 functions independent of calcineurin to regulate ion homeostasis and salt tolerance. Expression of a tobacco cDNA library (constructed in a single copy yeast expression vector) in cnb1 resulted in the isolation of NtSLT1 (Nicotiana tabacum Sodium Lithium Tolerant) by functional complementation of salt sensitivity. NtSLT1 suppressed salt sensitivity of the yeast mutant only if a N-terminally truncated protein was produced. Truncation was also a conserved property for functional complementation by the Arabidopsis thaliana homolog AtSLT1, and is indicative that the N-terminus contains an auto-inhibitor. NtSLT1 increased salt tolerance of wild type yeast, and suppression of salt sensitivity was restricted to mutants that are defective specifically in components of the calcineurin signal pathway (cnb1, tcn1, ena1-4). Microarray analysis of wild type and yeast mutants (cnb1, hog1, spk1) cells revealed numerous genes that were regulated uniquely by each of the major hyper-saline stress signal pathway