A comprehensive model of tomato fruit ripening regulation by the transcription factors NOR-like1, NAC-NOR, and MADS-RIN

Tomato (Solanum lycopersicum) fruit ripening involves climacteric ethylene production, lycopene accumulation, texture softening, and flavor enhancement, a highly coordinated process accompanied by profound gene expression changes. To construct a comprehensive model of ripening regulation, we studied the effects on ripening phenotypes and underlying gene expression changes in combinations of knockout alleles of NON-RIPENING-like1 (NL1), NON-RIPENING (NAC-NOR, NOR), and RIPENING INHIBITOR (MADS-RIN). Thus, we demonstrated that the products of the putative paralogous transcription factor genes NL1 and NOR together orchestrate ripening initiation and progression through ethylene production. NL1, or the ethylene production that it induces, together with NOR, stimulates the gene expression of transcription factor MADS-RIN, which then becomes the major driver of all ripening processes studied here. NOR and, particularly, NL1 have relatively minor but discernable and clearly different quantitative contributions to the ripening progression after initiation. Thus, the comprehensive model establishes a hierarchy of gene expression events regulating the start and progression of fruit ripening.