Enhanced expression of genes for ACC synthase, ACC oxidase, and NAC protein during high-temperature-induced necrosis of young inflorescences of Cymbidium

Abstract

Growing Cymbidium under high-temperature conditions (25-30°C) results in the necrosis of young inflorescences. An increase in the evolution of ethylene was correlated with the necrosis. To study the molecular aspects of high-temperature-induced necrosis of Cymbidium floral buds, we isolated complementary DNA (cDNA) clones for proteins that are likely to be involved in the biosynthesis of ethylene during high-temperature-induced necrosis of young inflorescences, namely, 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (CyACS1) and ACC oxidase (CyACO1). In addition, a cDNA (CyNAC1) encoding an NAC protein whose expression is modulated during high-temperature treatment was isolated by differential display. High levels of expression of CyACS1, CyACO1 and CyNAC1 were observed in the necrotic inflorescences of wild-type Cymbidium at high temperatures. Bud necrosis was not observed in the mericlone mutant (nhn, non-high-temperature-induced necrosis) of Cymbidium. Ethylene evolution was lower in nhn than in wild-type, but application of exogenous ACC or ethephon to the young inflorescences of nhn restored the high-temperature necrosis response. Expression of CyACS1, CyACO1 and CyNAC1 did not increase with high-temperature treatment in the nhn mutant. Expression levels of CyACS1, CyACO1 and CyNAC1 in necrotic inflorescences of nhn treated with 5.0 mM ACC were much lower than in necrotic inflorescences of wild-type at high temperatures, but CyACS1 and CyNAC1 were stimulated by ACC treatment. These results suggest that ethylene is involved in high-temperature necrosis of young inflorescences of Cymbidium and that an NAC protein may be involved in the regulatory mechanisms of genes that are regulated during necrosis. © Physiologia Plantarum 2006.