The transcription factors RIN and NOR have both redundant and specific roles in the initiation and progression of fruit ripening.

Abstract

The transcription factors (TFs) RIPENING-INHIBITOR (RIN) and NON-RIPENING (NOR) are key regulators of fruit ripening in tomato (Solanum lycopersicum) fruit ripening. However, the spontaneous rin and nor alleles that were first described were demonstrated to be gain-of-function mutants, prompting a re-evaluation of the roles of RIN and NOR in tomato fruit ripening. Here, we show that the slnor slrin double mutant (a double homozygous loss-of-function mutant of NOR and RIN) exhibits a complete cessation of fruit ripening, revealing that NOR and RIN redundantly but differentially regulate fruit ripening. Besides serving as activators, NOR and RIN are essential in suppressing genes related to photosynthesis. Additionally, at the initiation of ripening, NOR activates RIN expression by binding to its promoter. Following climacteric ethylene production, RIN represses NOR expression. This temporal interaction is crucial for the regulation of abscisic acid (ABA) and ethylene biosynthesis during fruit ripening. Interestingly, NOR and RIN do not form a transcriptional complex. Collectively, our findings provide insights into the regulatory network involving NOR and RIN in fruit ripening and uncover their roles in the crosstalk between the ripening hormones ethylene and ABA.