Genetic analyses are increasingly being applied to explore the mechanisms of plant tolerance to environment stresses. With many mutants recovered from various genetic screens, positional cloning of the mutations has become inevitable. Here we described the methodology for positional cloning of an Arabidopsis salt tolerance gene, SOS3. The sos3 mutant plants are hypersensitive to Na~+ stress and unable to grow under low K~+ conditions. However, high Ca2+ in the medium suppressed these defects in the sos3 mutant. Positional cloning of the SOS3 gene revealed that SOS3 encodes a calcium-binding protein with similarities to the B subunit of calcineurin and animal neuronal calcium sensors. The results revealed a critical component mediating the widely recognized interaction of K~+, Na~+, and Ca~(2+) in higher plants.
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