Degradation of ribulose‐1,5‐bisphosphate carboxylase and chlorophyll in senescing barley leaf segments triggered by jasmonic acid methylester, and counteraction by cytokinin

Abstract

Barley (Hordeum vulgare L. cv. Salome) primary leaf segments responded to the application of a putative plant growth regulator, ± jasmonic acid methylester (JA‐Me). with accelerated senescence, as indicated by the loss of chlorophyll and the rapid decrease in activity and immunoreactive protein content of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (RuBP carboxylase, EC 4.1.1.39). The senescence‐promoting action of JA‐Me differed in light and in darkness; e.g. the initial rates of chlorophyll and RuBP carboxylase breakdown were markedly higher in light than in darkness in the presence of 4.10−5M JA‐Me. Cytokinin (benzyladenine, 4.10−5M) stopped the loss of chlorophyll and RuBP carboxylase during senescence; however, the rapid drop induced by JA‐Me in the early phase of leaf segment senescence could not be prevented by concomitant or previous addition of BA. On the other hand, BA added 24 h after JA‐Me application resulted in a recovery of chlorophyll and RuBP carboxylase at the later stages, indicating a possible rapid inactivation of JA‐Me in the tissues. The activities of a number of other chloroplastic and cytosolic enzymes were not significantly altered in JA‐Me‐treated leaf segments compared with controls floated on water. Time‐dependent chlorophyll decrease in isolated chloroplasts did not change upon JA‐Me addition to the isolated organelles. It is suggested that JA‐Me acts on chloroplast senescence by promoting cytoplasic events which eventually bring about the degradation of chloroplast constituents.