DIFFERENTIAL EXPRESSION OF 2 P5CS GENES-CONTROLLING PROLINE ACCUMULATION DURING SALT-STRESS REQUIRES ABA AND IS REGULATED BY ABA1, ABI1 ANDAXR2 IN ARABIDOPSIS

Proline is a common compatible osmolyte in higher plants. Proline accu mulation in response to water stress and salinity is preceded by a rap id increase of the mRNA level of Delta(1)-pyrroline-5-carboxylate synt hase (P5CS) controlling the rate-limiting step of glutamate-derived pr oline biosynthesis. P5CS is encoded by two differentially regulated ge nes in Arabidopsis. Gene AtP5CS2 mapped to chromosome 2-78.5 is expres sed in most plant organs, but silent in dividing cells. Gene AtP5CS2 l ocated close to marker m457 on chromosome 3-101.3 contributes 20-40% o f total P5CS mRNA in plant tissues, but is solely responsible for the synthesis of abundant P5CS mRNA in rapidly dividing cell cultures. Acc umulation of AtP5CS transcripts is regulated in a tissue specific mann er and inducible by drought, salinity, ABA, and to a lesser extent by auxin. Induction of AtP5CS1 mRNA accumulation in salt-treated seedling s involves an immediate early transcriptional response regulated by AB A signalling that is not inhibited by cycloheximide, but abolished by the deficiency of ABA biosynthesis in the aba1 Arabidopsis mutant. How ever, inhibition of protein synthesis by cycloheximide prevents the in duction of AtP5CS2 mRNA accumulation, and blocks further increase of A tP5CS1 mRNA levels during the second, slow phase of salt-induction. Mu tations abi1 and axr2, affecting ABA-perception in Arabidopsis, reduce the accumulation of both AtP5CS mRNAs during salt-stress, whereas ABA -signalling functions defined by the abi2 and abi3 mutations have no e ffect on salt-induction of the AtP5CS genes.