IDENTIFICATION AND KINETICS OF ACCUMULATION OF PROTEINS INDUCED BY ETHYLENE IN BEAN ABSCISSION ZONES

A two-dimensional gel electrophoresis system that combines a cationic polyacrylamide gel electrophoresis at pH near neutrality with sodium d odecyl sulfate-polyacrylamide gel electrophoresis was used to analyze the spectrum of basic polypeptides that accumulate in bean (Phaseolus vulgaris) abscission zones after treatment with ethylene. Results show ed that, as abscission progressed, at least seven basic proteins accum ulated in the abscission zone prior to the accumulation of 9.5 cellula se. Six of the seven proteins correspond to pathogenesis-related (PR) proteins. Among them, two isoforms of beta-1,3-glucanase and multiple isoforms of chitinase were identified. A 22 kilodalton polypeptide tha t accumulated to high levels was identified as a thaumatin-like protei n by analysis of its N-terminal sequence (up to 20 amino acids) and it s serological relationship with heterologous thaumatin antibodies. A 1 5 kilodalton polypeptide serologically related to PR P1 (p14) from tom ato was identified as bean PR P1 (p14)-like protein. The kinetics of a ccumulation of glucanases, chitinases, thaumatin-like and PR P1 (p14)- like proteins during ethylene treatment were similar and they showed t hat PR proteins accumulated in abscission zones prior to the increase in 9.5 cellulase. Addition of indoleacetic acid, a potent inhibitor of abscission, reduced the accumulation of these proteins to a similar e xtent (60%). The synchronized accumulation of this set of PR proteins, early in the abscission process, may play a role in induced resistanc e to possible fungal attack after a plant part is shed. The seventh pr otein does not correspond to any previously characterized PR protein. This new 45 kilodalton polypeptide accumulated in abscission zones on exposure to ethylene concomitantly with the increase in 9.5 cellulase. Its N-terminal sequence (up to 15 amino acids) showed some homology w ith the amino terminal sequence of chitinase. Polyclonal antibodies ag ainst chitinase recognized the 45 kilodalton peptide, but polyclonal a ntibodies against the 45 kilodalton protein recognized chitinase weakl y. When abscission was inhibited by addition of indoleacetic acid, the accumulation of the 45 kilodalton protein was strongly inhibited (80% ). This result suggests that the 45 kilodalton polypeptide may play a more direct role in abscission.