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 delta
1-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 contributes 20-40% of total P5CS
mRNA in plant tissues, but is solely responsible for the synthesis of abundant P5CS
mRNA in rapidly dividing cell cultures. Accumulation of AtP5CS transcripts is regulated in a tissue specific manner and inducible by drought, salinity, ABA, and to a lesser extent by
auxin. Induction of AtP5CS1
mRNA accumulation in
salt-treated seedlings involves an immediate early transcriptional response regulated by ABA signalling that is not inhibited by
cycloheximide, but abolished by the deficiency of ABA biosynthesis in the aba1 Arabidopsis mutant. However, inhibition of
protein synthesis by
cycloheximide prevents the induction of AtP5CS2
mRNA accumulation, and blocks further increase of AtP5CS1
mRNA levels during the second, slow phase of
salt-induction. Mutations 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 effect on
salt-induction of the AtP5CS genes.