Ethylene is not the only regulator of fruit ripening. A cold treatment can trigger ripening in detached apple and kiwifruit, acting either independently of ethylene or by increasing sensitivity to existing very low levels of ethylene (Tacken et al. 2010; Mworia et al. 2012). Other hormones also appear to play important roles; particularly, declining levels of auxin and increasing levels of abscisic acid may control the onset of ripening in non-climacteric species such as grape and strawberry. Abscisic acid may also play a role in controlling the onset of ripening of climacteric species. Uncovering the interaction between auxin, abscisic acid and ethylene in ripening regulation is an emerging area of research (McAtee et al. 2013).
While most ripening-associated traits appear to be regulated by hormonal changes, there are also a number of genes that control the developmental switch to ripening (Klee and Giovannoni 2011). Some of these were identified in tomato by the study of ripening mutants that arose spontaneously during commercial tomato production. The ripening-inhibitor (rin), Colorless non-ripening (Cnr) and non-ripening (nor) mutants all have fruit that fail to ripen, though with different characteristics. Although these fruit do not produce elevated levels of ethylene and will not ripen in response to exogenous ethylene, they are not completely insensitive and some ethylene-responsive genes (but not the whole ripening process) can be induced by ethylene treatment. The products of these three genes are transcription factors and are thought to be key developmental genes that control ripening progression, apparently acting upstream of the ethylene production pathway.
RIN encodes a MADS-box gene that clusters in the SEPALLATA clade. CNR encodes a SQUAMOSA promoter binding protein. Both proteins are necessary for the induction of ripening-related increases in respiration and ethylene biosynthesis, although since they are important in the ripening of both climacteric and non-climacteric fruit they appear to be more global regulators of ripening with some functions that are ethylene independent. Transgenic tomato fruit that had been suppressed in the ethylene signalling pathway and treated with 1-MCP showed an ethylene-independent increase in the expression of ripening-related ACS genes and ethylene production (Yokotani et al. 2009). This is apparently controlled by developmental factors, and would be sufficient to induce the autocatalytic increase in ACS expression and ethylene production typical of tomato ripening.