Understanding the physiological mechanisms of ripening in capsicum.

Capsicum (Capsicum annuum L.) is considered a non-climacteric fruit, exhibiting limited respiration and ethylene levels. The physiological mechanisms of ripening in capsicum have not been fully understood to date, especially the probable reason behind the non-climacteric behaviour. In this thesis, the protein (Chapter 3) and metabolite (Chapter 4) profiles of capsicum at different ripening stages have been reported. Several proteomic and metabolic candidates, for example ACC oxidase (ACO) enzyme, sugars (glucose, fructose, sucrose) and malate were chosen and analysed in different tissue types (peel, pulp and seeds/placenta) and cultivars with different ripening times (Chapter 5). The results suggested that some of these candidates were differentially present in different tissues and cultivars which implied that ripening could be regulated spatially and temporally. Furthermore, proteomic analysis also identified an ACO isoform 4 (CaACO4) which was found during capsicum ripening onset and corresponded to the increase in the overall ACO activity (Chapter 3). The expression of several ACO isoforms including CaACO4, and other identified ACC synthase (ACS) and Ethylene receptor (ETR) isoforms were therefore characterised to shed some light on their roles in this non-climacteric fruit (Chapter 6). CaACO4 was the only ACO isoform expressed significantly higher during capsicum ripening onset, confirming the earlier proteomic results. The expression of several ACS and ETR isoforms, normally associated with the climacteric increase of ethylene in tomato (a close relative of capsicum), was also limited as was ACS activity and ACC content. The production of ACC, as an ethylene precursor, may therefore be the rate-limiting step for ethylene production in non-climacteric capsicum. The postharvest application of ethylene did not promote capsicum ripening or induce the expression of most ACO, ACS and ETR isoforms, suggesting they are not regulated by ethylene as usually observed in climacteric fruit such as tomato. However, 1-methylcyclopropene treatment significantly delayed capsicum ripening postharvest, particularly when applied at Breaker stage (the onset of ripening), suggesting that blocking ethylene perception could affect ripening and that the basal level of ethylene normally produced in non-climacteric fruit may be (partially) required for ripening (Chapter 6). Other proteomic candidates such as Copper chaperone, TCP chaperone, Cysteine synthase and Spermidine synthase were also isolated and investigated due to their possible roles in capsicum ripening. However, unlike CaACO4, the RNA expression of these candidates did not follow their respective proteomic trends, suggesting a regulation at the post-translational level (Chapter 7). The identified candidates including CaACO4 are now a resource for further investigation to identify factors that may be involved in capsicum ripening.Thesis (Ph.D.) -- University of Adelaide, School of Agriculture, Food and Wine, 201