Abstract:
Fruit are important sources of food that can contribute a large portion of the fibre, water and vitamins required for a healthy human diet. The process of fruit ripening is a critical phase in the development of fruit as it is this process that renders fruit attractive and palatable for consumption. A better understanding of the mechanisms that regulate fruit ripening is likely to enhance our ability to predict and/or control the ripening process in fruit. This thesis investigated the molecular control of fruit ripening in the non-‐model crop plant Actinidia chinensis cv. ‘Hort16A’ that produces climacteric fleshy fruit similar to tomato. This crop species has particular relevance to the New Zealand economy, being its largest horticultural industry. Results in this thesis show that, at the physiological level, the ripening associated attributes of pericarp colour change, sugar accumulation, and pericarp softening initiate at distinctly different points in Actinidia chinensis ‘Hort16A’ fruit. The progression of each of these ripening attributes also occurred in multiple phases and at different rates. Analysis of the expression of genes associated with the progression of each ripening attribute demonstrated that multiple genes are likely to be required for each attribute to occur. These genes displayed complex expression patterns during the ripening of this fruit indicating that complex regulatory networks are likely to be required to regulate ripening in this fruit. Developmental and tissue specific regulation was observed in some genes involved in the progression of ripening attributes. This demonstrates a further level of complexity of ripening that occurs in a spatial and temporal manner. Treatment of ethylene resulted in the enhanced progression of pericarp softening and sugar accumulation but not pericarp colour change. The expression of genes involved in the progression of each of these attributes however all showed significant changes in response to ethylene treatment. Interestingly ethylene biosynthesis was not observed during the on-‐vine ripening of Actinidia chinensis ‘Hort16A’ fruit but this fruit can respond to ethylene treatment. This observation suggests that Actinidia chinensis ‘Hort16A’ fruit may have evolved a mechanism that involves two ripening phases that target different vectors for seed dissemination. It is however also possible that this fruit may respond to ethylene in this manner to enhance the progression of ripening in unfavourable abiotic conditions, thus ensuring some seed dispersal is achieved each year. Two kiwifruit NOR-‐like (AcNOL1 and AcNOL3) genes were stably expressed in non-‐ripening mutant tomato fruit. The fruit of these transgenic lines subsequently displayed an increased progression of ripening compared to non-‐ ripening mutant fruit. The progression of ripening in these fruit corresponded with complex changes to the expression of distinctly different genes at different points of ripening. This shows that AcNOL1 and AcNOL3 regulate different ripening related aspects when expressed in tomato fruit. Transient expression of AcNOL1 and AcNOL3 in kiwifruit resulted in increased biosynthesis of ethylene, a key positive regulator of ripening. This indicates that these genes both have roles in the progression of ripening in kiwifruit. AcNOL1 and AcNOL3 display different expression profiles at distinct stages of ripening suggesting that they are likely to differentially contribute to the regulation of ripening in kiwifruit.