METHYL JASMONATE REPRESSES TRANSLATION INITIATION OF A SPECIFIC SET OF MESSENGER-RNAS IN BARLEY

Jasmonic acid methyl ester (methyl jasmonate, JaMe) causes accumulatio n of novel abundant proteins in excised leaf segments of barley, and c oncomitantly represses synthesis of most pre-existing ('control') prot eins. The changes in control protein synthesis do not correspond to eq uivalent alterations at the in vitro translatable mRNA level, suggesti ng a post-transcriptional mode of regulation. Methyl jasmonate did not interfere with the in vitro translation of either control or JaMe-ind uced mRNAs. Polysome runoff translation, in combination with two-dimen sional separations of the products formed, however, revealed a reduced synthesis of control proteins in JaMe-exposed leaf tissues, in contra st to a continued translation of these polypeptides in water-treated l eaf segments. In vitro translation of polysomal RNAs demonstrated the preferential association of control mRNAs with polysomes of water-trea ted leaf tissues but not with polysomes of JaMe-treated tissues. Polys omes isolated from the latter leaves contained primarily JaMe-induced mRNAs, as shown by in vitro translation and Northern hybridization wit h gene-specific probes. Treatment of JaMe-incubated leaf tissues,with cycloheximide prior to harvesting caused an increase of in vitro trans latable control mRNAs recovered from polysomes, thus highlighting an i mpairment of control protein synthesis by JaMe at the level of transla tion initiation.