THE ROLE OF SENSITIVITY TO ETHYLENE IN POLLINATION-INDUCED COROLLA SENESCENCE SYNDROME

Non-pollinated cyclamen flowers produce very little ethylene and are insensitive to ethylene. Pollination renders the flowers sensitive to ethylene, thus cause them to respond to internal and external ethylene.

Following pollination a signal is transmitted from the gynoecium to the perianth inducing increased sensitivity to ethylene which precedes by a few hours the increase in ethylene production. The increase in ethylene sensitivity was not dependent on ethylene production as it occurred also in flowers treated with inhibitors of ethylene biosynthesis. The pollination-induced climacteric rise in ethylene synthesis is a response to the basic level of ethylene, triggered by the early increase in ethylene sensitivity.

We tried to evaluate the involvement of several steps in this signal transduction pathway in pollination-induced ethylene sensitivity of Phalaenopsis flowers.

Treatment of cut Phalaenopsis flowers with cholera toxin or guanosine-5–0-(3-thiotriphosphate), compounds that modulate GTP-binding protein activity, increased the sensitivity of the flowers to ethylene. Guanosine-5–0-(2-thiodiphosphate) which does not affect the activity of GTP-binding proteins, had no affect on the sensitivity to ethylene. Western blot analysis of microsomal proteins, revealed that a peptide with a molecular mass of ca 42 kDa cross-reacts with antibodies against well-conserved amino acid sequence of mammalian G-proteins. Calcium ions, known co-factors of protein kinases, also increased the sensitivity of the flowers to ethylene, while EGTA, a chelator of calcium, decreased it. Protein phosphorylation in petal microsomal membranes was doubled in the presence of calcium ions. Ten hours after pollination, at the peak of ethylene sensitivity, a significant increase was measured in the binding of GTP to the membranes. This increase is unrelated to the effect of pollination on ethylene production, since it occurred also in flowers treated with AOA. Protein phosphorylation in flowers increased significantly following pollination, with a single peptide of ca 30 kDa most heavily phosphorylated. These observations indicate a direct involvement of GTP-binding proteins, calcium and protein phosphorylation, major components of the cellular signal transduction pathway, in the regulation of pollination-induced ethylene sensivity in Phalaenopsis petals.

The transmissible “sensitivity factor” is neither ethylene or ACC, nor auxin, abscisic acid or jasmonic acid. The most likely candidates are short chain saturated fatty acids (C₇ to C₁₀). The

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