Methyl jasmonate represses translation initiation of a specific set of mRNAs in barley

Jasmonic acid methyl ester (methyl jasmonate, JaMe) causes accumulation of novel abundant proteins in excised leaf segments of barley, and concomitantly represses synthesis of most pre‐existing (‘control’) proteins. The changes in control protein synthesis do not correspond to equivalent alterations at the in vitro translatable mRNA level, suggesting a post‐transcriptional mode of regulation. Methyl jasmonate did not interfere with the in vitro translation of either control or JaMe‐induced mRNAs. Polysome runoff translation, in combination with two‐dimensional separations of the products formed, however, revealed a reduced synthesis of control proteins in JaMe‐exposed leaf tissues, in contrast to a continued translation of these polypeptides in water‐treated leaf segments. In vitro translation of polysomal RNAs demonstrated the preferential association of control mRNAs with polysomes of water‐treated leaf tissues but not with polysomes of JaMe‐treated tissues. Polysomes isolated from the latter leaves contained primarily JaMe‐induced mRNAs, as shown by in vitro translation and Northern hybridization with gene‐specific probes. Treatment of JaMe‐incubated leaf tissues with cycloheximide prior to harvesting caused an increase of in vitro translatable control mRNAs recovered from polysomes, thus highlighting an impairment of control protein synthesis by JaMe at the level of translation initiation.