Diversification of NOR-like genes resulted in functional similarity in tomato

Fruit shelf life is an important tomato breeding trait. Ethylene production greatly affects shelf life and is genetically regulated. Tomato ripening is associated with increased ethylene production, and tomato is used as a model for studying fruit ripening, particularly the ethylene biosynthesis and signaling pathways. The key ripening regulator NOR encodes a NAC domain transcription factor which is part of a large plant-specific gene family. Both CRES-T and RNAi gene suppression transgenic lines revealed delayed ripening signs. Further screening of the Micro-Tom ethyl methanesulfonate (EMS)-mutagenized population enabled the selection of alleles responsible for phenotype alterations. Analyses of the transcription levels of ethylene biosynthesis genes ACC synthase (ACS) and ACC-oxidase (ACO) revealed that reduced ethylene production was largely due to transcriptional suppression of ACO1 and ACO3. ACO failed to oxidize the ethylene precursor, thus producing a non-climacteric phenotype in nor. Evidence indicated possible feedback regulation of NOR and cross-regulation of NOR-like genes, and functional conservation suggests that NAC coding genes may undergo functional differentiation. NOR plays a major role in ripening control, whereas NOR-like genes have a minor role; however, a better understanding of NOR-like genes could provide insights into the complex transcriptional regulation of fruit ripening.