METHYL JASMONATE-REGULATED TRANSLATION OF NUCLEAR-ENCODED CHLOROPLASTPROTEINS IN BARLEY (HORDEUM-VULGARE L CV SALOME)

The naturally occurring plant growth regulator (-)-jasmonic acid methy l ester (JaMe) induces the formation of novel abundant proteins in exc ised barley leaf segments. Concomitantly, this substance depresses the translation of most preexisting (''control'') leaf mRNAs, including t hose for nuclear-encoded chloroplast proteins such as the small subuni t of ribulose-1,5-bisphosphate carboxylase/oxygenase (SSU, rbcS gene p roduct) and several light harvesting chlorophyll protein complex apopr oteins (LHCPs, cab gene products). The changes in protein synthesis ob served for SSU and LHCPs did not correspond to equivalent alterations in the rbcS and cab transcript levels. Analysis of polysome-associated in vitro translatable and hybridizable mRNAs, however, demonstrated a restriction of rbcS and cab transcripts to smaller polysomes in JaMe- exposed leaf tissues, in comparison to water-treated tissues. Since tr eatment of JaMe-incubated leaf segments with cycloheximide prior to ha rvest led to a shift of both transcripts toward larger polysomes, a ho rmone-induced impairment of chain initiation is assumed to lower trans lation of SSU and LHCP in situ. In contrast, the mRNA for plastid leuc yl-tRNA synthetase (LRS1, lrs1 gene product) neither changed its abund ance nor its association with polysomes in JaMe-treated leaves and was translated into the corresponding polypeptide. Together, our results highlight a remarkable variability of nuclear gene expression in respo nse to plant growth regulators of the methyl jasmonate type.