Ripening Inhibitor (RIN): A master switch in the molecular regulation of ethylene-dependent climacteric fruit ripening

Abstract

Fruits are an essential source of vitamins and minerals in our diet and constitute a major part of essential plant commodities. Fruit ripening is a complex developmental process, responsible for the transformation of the seed-containing organ into a tissue, attractive to the agents causing seed dispersal as well as fruit consumers. The co-ordinated regulation of different biochemical pathways, involving the transcription of a wide array of genes, brings about the characteristic phenotype that define climacteric fruit ripening, viz., autocatalytic synthesis of the phytohormone ethylene, cell wall softening, cell expansion, colored pigment formation, sugar accumulation, and aroma and volatile production. Ripening Inhibitor (RIN), belonging to the family of SEPALLATA (SEP) group of MADS box transcription factor, is regarded as the nodal player and master switch in dictating the transition of fruit ripening from stage 1 to stage 2, accompanied with ethylene burst. This factor regulates the continuation of ripening process by binding to the CArG box, denoted by (T/A) (T/A) DCCA (A/T) (A/T) (A/T), present in the upstream region of different genes concerned with ethylene production, carotenoid biosynthesis, cell wall modification, volatile and flavor production and other ripening-related  transcription factors like NAC-NOR, CNR, FUL1, etc. RIN can auto-regulate its synthesis by binding to the ethylene responsive element (ERE) of its own promoter as well as its target genes, thereby trans-activating the latter. RIN mutant, that constitute a fusion protein of the DNA-binding domain of MADS-box and repressor domain of the adjacent protein, MACROCALYX, behaves as a transcriptional repressor, instead of an activator. RIN mutants are overall not able to induce the autocatalytic system 2 of ethylene synthesis, causing a drop in the phytohormone level to be able to continue with the ripening process, thereby failing to produce completely ripened fruits. Therefore, silencing of RIN expression, via mutant generation or genetic engineering approaches like RNA interference and genome editing, holds promise in delaying the ripening process and enhancing the shelf life of fruits in order to avoid post-harvest losses. The present review highlights the regulatory mechanism of climacteric fruit ripening, mediated by RIN and the activation of diverse groups of downstream ripening-associated genes.