Increased hydrolase gene expression and hydrolase activity in the abscission zone involved in chilling-induced abscission of Dendrobium flowers

Abstract After 4 days exposure to15 °C and 5 °C temperature, cut cv. Ear Sakul Dendrobium inflorescences were returned to 25 °C.The exposure of open flowers and floral buds to 5 °C induced more abscission than exposure to 15 °C after transfer to 25 °C. Ethylene production rates of floral buds and open flowers with and without 1-MCP treatment were comparatively low during exposure to to15 °C and 5 °C. Flower buds had a greater increase of ethylene production than open flowers after upon transfer to 25 °C and flower buds stored at 5 °C had a greater increase than that of flower buds stored at 15 °C. 1-MCP treatment reduced significantly more ethylene production rates of floral buds stored at 5 °C than those stored at 15 °C compared with open flowers upon transfer to 25 °C on the last 3 days. The abscission zone (AZ) activities of pectin methylesterase (PME), polygalacturonase (PG), β-1,4 glucanase (cellulase, CEL) and β-galactosidase (β-GAL) increased after exposure to5 °C, while β-GAL also increased after 15 °C exposure. Treatment with 100 nl L −1 1-MCP for 4 h at 25 °C, lower temperature exposure reduced abscission in both open flowers and floral buds. 1-MCP also prevented an increase in the activities of PME, PG, CEL and β-GAL in the AZ. Genes encoding a PME, a CEL, and a β-GAL were isolated from the AZ. In the absence of 1-MCP an early increase was found in the expression of the β-GAL and PME genes in the AZ, that was correlated with the abscission rates. An increase in the expression of the CEL gene was found later in abscission. 1-MCP prevented the increase in expression of the β-GAL, PME and CEL genes in the AZ. It was concluded that chilling-induced abscission was mediated by ethylene, that induced gene expression and increased the activity of cell wall hydrolases. The high correlation between abscission and gene expression of β-GAL and CEL suggests that they were involved relatively early and again in late stages of abscission. The role of PME and PG was less clear, but they were possibly involved in the abscission after the 5 °C treatment.