Methyl Jasmonate-Regulated Translation of Nuclear-Encoded Chloroplast Proteins in Barley (hordeum Vulgare L. Cv. Salome).

The naturally occurring plant growth regulator (-)-jasmonic acid methyl ester (JaMe) induces the formation of novel abundant proteins in excised barley leaf segments. Concomitantly, this substance depresses the translation of most preexisting ("control") leaf mRNAs, including those for nuclear-encoded chloroplast proteins such as the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (SSU, rbcS gene product) and several light harvesting chlorophyll protein complex apoproteins (LHCPs, cab gene products). The changes in protein synthesis observed 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 treatment of JaMe-incubated leaf segments with cycloheximide prior to harvest led to a shift of both transcripts toward larger polysomes, a hormone-induced impairment of chain initiation is assumed to lower translation of SSU and LHCP in situ. In contrast, the mRNA for plastid leucyl-tRNA synthetase (LRS1, lrs1 gene product) neither changed its abundance 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 response to plant growth regulators of the methyl jasmonate type.