# Molecular and Hormonal Mechanisms Regulating Fleshy Fruit Ripening
## 1 Introduction  
Fruits can develop from several different parts of a flower and possess different characteristics. In this review, we shall consider the biochemical, genetic and hormonal mechanisms involved in the ripening of fleshy fruits, which makes them attractive to frugivores (animals that consume raw fruits, succulent fruit-like vegetables, roots, shoots, nuts and seeds). When the seeds are mature, the cells in the fruit flesh undergo dramatic changes in colour, flavour, texture and aroma that attract frugivores to aid in seed dispersal [1]. This ripening process involves a coordinated set of physiological and biochemical changes involving many cell compartments, including the nucleus (Sections 4-6), plastids and vacuole (Section 2.1) and cell wall (Section 2.2). Ripening is under genetic control and has evolved several times during the course of evolution [2]. The ripening process depends upon the expression of new 'ripening genes,' and the enzymes they encode catalyse a range of biochemical changes that render the fruits attractive to humans and other seed-dispersing animals. Different structural genes and control mechanisms have been recruited to the ripening program in different fruit during the course of their evolution. Fleshy fruit ripening is of great scientific interest because fruits make an important contribution to animal and human diets, provide several health-promoting benefits [3] and fruit production underpins major economic activity worldwide. It is generally accepted that fleshy fruits can be divided into two classes, the climacteric fruits (i.e., apple, apricot, avocado, banana, tomato, etc.) and non-climacteric fruits (i.e., citrus, grape, orange, lemon, raspberry, strawberry, etc.). The main differences between these two groups are the characteristic burst of respiration and ethylene production that occurs at the onset of ripening in climacteric fruits [3; 4; 5]. Non-climacter fruits respire and produce low levels of ethylene, and they can also sometimes respond to this hormone, which promotes, for example, de-greening of citrus. Non-climacteric fruits do not, however, show a burst of CO\({}_{2}\) or ethylene production, which is the classic sign of ripening onset in climacteric fruit. This distinction between two types of fruits is somewhat arbitrary, however, and ripening of both types of fruits involves hormones and similar changes in gene expression, often regulated by structurally related transcription factors (TFs). Hormone signalling operates primarily by modulating expression or action of TFs plus genes involved in hormone biosynthesis, perception and signalling. TFs can also directly and indirectly affect expression of genes required for hormone synthesis, and expression of some ripening TFs is regulated developmentally or by environmental stimuli. Consequently, there is a balance between developmental, environmental, hormonal and genetic cues that govern the processes of fruit development and ripening. The tomato (_Solanum Lycopersicum_) was selected as a model for fruit ripening and the complete genome sequence was determined in 2012 [6]. The tomato genome contains 58 TF families, including a total of 1845 putative TFs [7]. For a complete understanding of the ripening process, it is necessary to know how these TFs and different hormones and environmental factors operate and interact at the molecular level to coordinate the myriad of ripening reactions. Although the genetic and biochemical changes that occur during ripening are important determinants of fruit quality, if some aspects of ripening, such as softening, proceed too far, the fruit become susceptible to infection, spoilage and rotting. Thus, understanding how the different facets of ripening are regulated can help improve fruit quality and quantity, prevent waste and contribute to improved nutrition and food security.