Re-evaluation of the rin mutation and the role of RIN in the induction of tomato ripening

Tomato (Solanum lycopersicum) rin mutants completely fail to ripen: they do not produce red pigmentation, soften or induce an ethylene burst. Therefore, RIN has long been believed to function as a major regulator that is essential for the induction of ripening. Here, we provide evidence contradicting this concept of RIN function, showing induction of fruit ripening in the absence of RIN. A CRISPR/Cas9-mediated RIN-knockout mutation did not repress initiation of ripening and the mutant fruits showed moderate red colouring. Moreover, inactivation of the rin mutant allele partially restored the induction of ripening. Therefore, RIN is not required for the initiation of ripening and rin is not a null mutation, but rather is a gain-of-function mutation that produces a protein that actively represses ripening. Since the discovery of the rin mutant a half-century ago, many models have depicted RIN as indispensable for the induction of ripening; these models should be reconsidered in light of these results.