Proline is a common compatible osmolyte in higher plants. Proline accumulation in response to water stress and salinity is preceded by a rapid increase of the mRNA level of Δ1-pyrroline-5-carboxylate synthase (P5CS) controlling the rate-limiting step of glutamate-derived proline biosynthesis. P5CS is encoded by two differentially regulated genes in Arabidopsis. Gene AtP5CS1 mapped to chromosome 2–78.5 is expressed in most plant organs, but silent in dividing cells. Gene AtP5CS2 located close to marker m457 on chromosome 3–101.3, and is responsible for the synthesis of abundant P5CS mRNA in dividing cells. Accumulation of AtP5CS transcripts is regulated in a tissue specific manner and inducible by drought, salinity, ABA, and to lesser extent by auxin. Induction of AtP5CS1 mRNA accumulation in salt-treated seedlings involves an immediate early transcriptional response regulated by ABA signaling. Inhibition of protein synthesis by cycloheximide affects the induction of AtP5CS mRNA accumulation. Mutations abal, abil and axr2, affecting ABA synthesis and perception in Arabidopsis, reduce the accumulation of both AtP5CS mRNAs during salt-stress, whereas ABA-signaling functions defined by the abi2 and abi3 mutations have no effect on salt-induction of the AtP5CS genes. Promoter regions of the AtP5CS genes has been cloned and the sequence of 1 kb. fragments has been determined. Sequence analysis of the promoter regions of the AtP5CS genes revealed the presence of putative conserved cis-acting elements, including a G-box. cDNA of AtP5CS1 has been placed under the control of different plant promoter sequences in binary expression vectors. Genetic transformation of tobacco and alfalfa resulted in the regeneration of transgenic plants containing increased internal proline content. Studies on the effect of proline accumulation on salt and drought tolerance is in progress.