Regulation of pollination-induced ethylene and its role in petal abscission of Pelargonium×hortorum

Pollination in geranium (Pelargonium×hortorum) results in rapid non-autocatalytic ethylene production followed by abscission of turgid petals. In the present work, the regulation of post-pollination ethylene was investigated in various floral organs. Ethylene production and petal abscission increased as the number of stigmatic lobes pollinated increased. Direct application of 1-aminocyclopropane-1-carboxylic acid (ACC) to the stigma induced ethylene production and petal abscission only at high concentrations, which are not naturally present in the pollen. Application of aminoethoxyvinylglycine (AVG), an inhibitor of ACC synthase activity, to the stigma effectively prevented pollination-induced ethylene production and petal abscission. Pollination increased the levels of ACC 46-fold in stigma plus style tissue and 6-fold in the basal portion of petals. Enhanced accumulation of N-malonyl-ACC (MACC), a conjugated form of ACC, was also observed in stigma and style tissues after pollination. Pollination increased the activity of ACC synthase 2.5-fold in stigma and style tissues and 3-fold in the basal portion of petals. At 2 h after pollination, a 4-fold increase in the expression of the ACC synthase gene GAC-1 was detected specifically in the stigma and style. In vivo activity of ACC oxidase was 1.5-fold higher in pollinated than in unpollinated or wounded stigma and style. In the lower part of the pistil, the activity of ACC oxidase was lower than in the upper pistil, regardless of treatment. The activity of ACC oxidase was significantly higher in the basal portion of petals than the upper portion of petals. Pollination had no effect on the activity of ACC oxidase in petals. Treatment of florets with cycloheximide effectively prevented pollination-induced petal abscission, but ethylene production was completely unaffected, providing evidence for the post-transcriptional regulation of ethylene production. Stigma and style tissues accounted for 57% of the total ethylene production in the flower following pollination. When the stigma and style were removed and the top of the sterile ovary pollinated, there was no pollination-induced ethylene production or petal abscission. We propose that rapid petal abscission in response to pollination results from increased ethylene production, which occurs via activation of ACC synthase primarily in the stigma and style. This contrasts with post-pollination ethylene production in some wilting-type flowers in timing, lack of autocatalytic ethylene and predominantly post-transcriptional regulation.