Ethylene biosynthesis controlled by NON-RIPENING: A regulatory conflict between wounding and ripening

The phytohormone ethylene is involved in multiple aspects of morphological and physiological processes in plants. Tomato rapidly and transiently increases ethylene production during fruit ripening and in plant defense responses. The transcription factor non-ripening (NOR) has significant effects on fruit ripening via regulation of ethylene biosynthesis-related genes. The nor loss-of-function allele produces a basal level of ethylene during ripening, in contrast to the induced ethylene evolution observed upon Agrobacterium tumefaciens infection. The use of ACC deaminase represses ethylene production and significantly improves the efficiency of Agrobacterium-mediated T-DNA transfer in nor plants. Analyses of the transcription levels of the ethylene biosynthesis genes ACC synthase (ACS) and ACC oxidase (ACO) in nor plants revealed that the induced ethylene production was largely due to transcriptional accumulation of ACS2 and ACO1. Accumulation of ACS2 and ACO1 mRNA opposes NOR-mediated regulation in tomato fruit during ripening, and the feedback regulation of NOR is rendered ineffective by defense responses, thereby precluding the control of its own expression. The ethylene synthesis mechanisms respond properly to NOR-mediated transcriptional regulation that is differed through the wound-induced and ripening-induced signaling pathway.