Proline Accumulation and Salt - Stress - lnduced Gene Expression in a Salt - Hypersensitive Mutant of Arabidopsis '

The sosl mutant of Arabidopsis thaliana i s more than 20 times more sensitive to NaCl stress than wild-type Arabidopsis. Because proline (Pro) i s generally thought to have an important role in plant salt tolerance, the sosl mutant and the wild type were compared with respect to their capacity to accumulate Pro under NaCl stress, and sosl mutant plants accumulated more Pro than the wild type. The P5CS gene, which catalyzes the rate-limiting step in Pro biosynthesis, i s induced by salt stress to a higher level in sosl than in the wild type. Although a defective high-affinity K uptake system in sosl causes K deficiency and inhibits growth in NaCI-treated plants, this decrease i s not a sufficient signal for Pro accumulation and P5CS gene expression. Not all salt-stress-induced genes have a higher level of expression in sosl. The expression levels of AtPLC and RD29A, which encode a phospholipase C homolog and a putative protective protein, respectively, are the same in sosl as in the wild type. However, the expression of AtMYB, which encodes a putative transcriptional factor, i s induced to a much higher level by salt stress in sosl. Thus, the SOSl gene product serves as a negative regulator for the expression of P5CS and AtMYB, but has no effect on AtPLC and RD29A expression.