# Molecular and Hormonal Mechanisms Regulating Fleshy Fruit Ripening
## 3 Ripening Is Influenced by Multiple Hormones and Light
### Ethylene Initiates and Promotes Ripening
RIN-deficient fruit undergo partial ripening, however, and if ethylene is added externally, they ripen slightly more, but not fully, whereas if 1-MCP is added just before the onset of ripening, ripening initiation and progression are almost completely inhibited for at least 20 days. This demonstrates that ethylene is sufficient to initiate ripening (Figure 4). This fits with the previous demonstration that RIN activates transcription of _ACS2_ and _ACS4_[129], for system-2 ethylene synthesis. [PERSON] et al. [15] proposed a model (Figure 7) in which ethylene signalling initiates ripening, where RIN is required to upregulate system-2 ethylene synthesis (see Section 6) and also many other ripening-related genes are required for the progression of full ripening. Several other plant hormones (discussed later in Section 3) also upregulate specific _ACS_ and _ACO_ genes and stimulate ripening by promoting ethylene production. At the genetic level, ethylene effects are brought about by ERFs. Tomato has 77 ERFs, which can be divided into nine subclades (A-J), and some are activators, while others are repressors. Members of the ERF F subclade possess the transcriptional repression EAR-motif [130]. A number of different ERFs have been shown to regulate individual genes or processes, contributing to changes in colour, flavour, texture and aroma in different fruits [131]. During ripening, transcripts for 27 ERFs accumulate, while mRNA levels for another 28 decrease [132], which suggests that different _ERFs_ have contrasting roles in fruit development and ripening. Differences between _ERF_ expression in the tomato ripening mutants _ripening inhibitor_ (_rin_), _non-ripening_ (_nor_), _Never-ripe_ (_Nr_) and wild-type (WT) identified ERFs that were both strongly up- and down-regulated during normal ripening. Three _ERFs_, members of sub-class E, were dramatically downregulated in the mutants, indicating that they probably had important roles in controlling ripening events [132]. Several tomato _ERFs_ have been shown to be involved in fruit softening, probably by mediating ethylene production [133], and _ERFs_ have been identified in ripening apple, banana, citrus, grape, ki-wifruit, persimmon and tomato [134; 135; 136; 56]. AP2a, which belongs to the AP2/ERF family, appears to be involved in repressing ethylene production, since reducing _AP2a_ expression results in enhanced ethylene production and softer fruits [133]. One problem associated with the designation of TFs as ERFs is that the attribution is most often made on the basis of computer sequence analysis and homology rather than functional assay. This overlooks the possibility that some so-called 'ERFs' are actually regulated by hormones other than ethylene, such as auxin, ABA, JA, etc., and similar arguments can be advanced for other TF families, such as the auxin response factors (ARFs), discussed in Section 3.2. This could explain why some ERF-type genes are expressed in non-climacter traits, where hormones other than ethylene are important in ripening regulation. Future challenges will involve unravelling the molecular mechanisms underlying the specificity of ethylene responses during plant development and fruit ripening. Deciphering the function of _ERF_ genes in both ethylene-dependent and ethylene-independent processes during ripening and identifying the target genes of individual ERFs will be instrumental in clarifying their specific contribution to fruit ripening [111].