Signal transduction in non-climacteric fruit ripening

Abstract

Fleshy fruit ripening involves changes in numerous cellular processes and metabolic pathways, resulting from the coordinated actions of diverse classes of structural and regulatory proteins. These include enzymes, transporters and complex signal transduction systems. Many aspects of the signaling machinery that orchestrates the ripening of climacteric fruits, such as tomato (Solanum lycopersicum), have been elucidated, but less is known about analogous processes in non-climacteric fruits. The latter include strawberry (Fragaria x ananassa) and grape (Vitis vinifera), both of which are used as non-climacteric fruit experimental model systems, although they originate from different organs: the grape berry is a true fruit derived from the ovary, while strawberry is an accessory fruit that is derived from the floral receptacle. In this article, we summarize insights into the signal transduction events involved in strawberry and grape berry ripening. We highlight the mechanisms underlying non-climacteric fruit ripening, the multiple primary signals and their integrated action, individual signaling components, pathways and their crosstalk, as well as the associated transcription factors and their signaling output.

Figure 1

Primary signals implicated in the regulation of non-climacteric (NC) fruit ripening. NC fruit ripening is regulated by both internal and external cues, e.g. plant hormones and environmental signals, respectively. During fruit growth and development, there is a decrease of osmotic potential resulting from the accumulation of soluble solids and intercellular separation resulting from degradation of the cell wall and middle lamella. Cell separation can start from the very early stage of fruit set, and as such the two events may potentially serve as early signals, contributing to the initiation of the hormone signals and their signal amplification. Primary signals trigger cellular signal transduction, thereby controlling the changing pattern of fruit ripening-associated metabolism.

Figure 2

Fruit ripening-associated signal transduction. Perception of the primary internal signals (IAA, ABA, and JA) as well as environmental signals act to initiate an intermediate signaling relay, which leads to regulation of downstream transcription factors (TFs), and finally, the regulation of a diversity of ripening-associated metabolic pathways. Circled are the types of signaling components TFs implicated in the regulation of non-climacteric (NC) fruit ripening. The names within the ovals refer to the specific signaling components or TFs that have already been identified and characterized in strawberry or grape berry. Question marks represent putative events that have not yet been characterized.