# Molecular and Hormonal Mechanisms Regulating Fleshy Fruit Ripening
## 7 Conclusions and Models for the Control of Ripening  
At the molecular level, there are variations in ripening regulatory circuits that operate in different fruits, although many of the genetic components appear to be similar. Recently, [PERSON] et al. [2] and [PERSON] et al. [16] proposed that there are three types of circuits controlling ethylene synthesis and climacteric fleshy fruit ripening that utilise either MADS, NACs, or both types of TFs together. Each model involves the ethylene transcription factor EIN3. It is proposed that in tomato, EIN3 activates the MADS transcription factor RIN, which forms a complex with TAGL, and activates ethylene biosynthesis genes, forming a positive feedback circuit that generates autocatalytic ethylene during ripening, with RIN also regulating downstream ripening genes. In peach, ripening regulation utilizes a NAC instead of a MADS transcription factor, and in banana, an additional loop involving both _NAC_ and _MADS_ genes enables the banana fruit to synthesise ethylene in the presence of ethylene inhibitor 1-MCP after ripening initiation [2]. Several important conclusions can be drawn from studies on the roles of hormones and TFs in regulating fruit ripening:  
* Addition and removal of repressive and promotive histone marks and DNA methylation are associated with the activation or repression of ripening genes and are an integral part of fruit maturation and preparation for ripening initiation and progression. * Ethylene is responsible for the initiation of ripening in tomato and other climacteric fruits. Several other plant hormones, including ABA, JA and BR, promote ripening by upregulating ethylene biosynthesis genes. It is not yet clear whether these hormones also affect expression of other ripening genes. * Ethylene biosynthesis and signalling genes and homologs of _MADS_ and _NAC_ genes are expressed in ripening of both climacteric and non-climacteric fruits. * In tomato, auxin content and signalling decline during fruit maturation and auxin and SA tend to oppose the action of ethylene and inhibit ripening. * MADS-RIN is required for the upregulation of many ripening genes, and progression and full ripening do not occur without MADS-RIN. * _NAC-NOR_ and several other _NAC_ genes promote the expression of ripening genes involved in production of ethylene and change in colour, flavour and texture. Nor-like1 is important for the full expression of genes involved in ethylene biosynthesis (_ACS2_, _ACS4_), colour formation (_GGPPS2_, _SGR1_) and cell wall metabolism (_PG2a_, _PL_, _CEL2_ and _EXP1_). * Ethylene, acting through ERFs, enhances accumulation of tomato mRNAs for _MADS-RIN_ and _NAC-NOR_. Conversely, NAC-NOR and Nor-like1 enhance ethylene biosynthesis by modulating the expression of specific _ACS_ and _ACO_ genes. * Light, detected by different photoreceptors and operating through HY5, can also modulate ripening behaviour. Figure 7 outlines the main TFs likely to interact with and modulate transcription from the promoters of genes encoding enzymes involved in fruit softening. Similar diagrams can be draw for promoter/TF interactions for a range of other ripening genes. A model outlining the main hormonal and genetic regulatory interactions governing the ripening of tomato fruit is shown in Figure 7. This model takes into consideration the role of histone marks, ethylene and other hormones which modulate ethylene biosynthesis genes, and MADS-RIN, NAC-NOR and other TFs that have been shown to be involved in the regulatory circuits. It should not be considered as a universal model for all fruits, but it  
serves to illustrate the complexity of ripening control mechanisms and may focus ideas to guide future experiments. Figure 6: The range of TFs interacting with the promoters of genes encoding fruit ripening enzymes. Redrawn and modified from [PERSON] et al. [15]. (**a**) Model outlining the role of ethylene and RIN in initiation and progression of climacter ripening in tomato fruit.