Molecular and physiological factors influencing low temperature breakdown in stonefruit

Abstract

The development of CI in peaches and plums presents a challenge in the long-term cool storage of these fruit. In order to be able to store fruit for longer periods of time, and subsequently have the possibility of exporting stonefruit to overseas market via cheap transportation methods, a better understanding of the factors affecting CI needs to be investigated. Therefore, the overall aim of this study was to investigate the molecular events that are responsible for the development of CI in peaches and plums. In addition, this study focused on the molecular events leading to the development CI in peaches of different maturities. Finally, this study also focused on molecular events involved in the development of CI in peaches and plums, and the associated differences between the two species. In this study, I found that treatment with the ethylene antagonist, 1-MCP, increased CI in peaches harvested at an early harvest maturity stage but not in commercially mature fruit. The physiological age of the fruit was defined by measurements of the rates of ethylene production and respiration during ripening. Conducting these experiments with fruit of known physiological age overcame previous uncertainty about the severity of CI in peaches treated with 1-MCP. Peaches harvested at early and commercial maturity were used for molecular analyses of the differential transcript accumulation of between the 1-MCP treated and control fruit following cool storage. This study also confirmed the results of previous studies which showed that CI was more severe in peaches treated with 1-MCP but was reduced in Japanese-type plums. The physiological data showed that 1-MCP prolonged the storage life of plums in cool storage. The contrasting response of peaches and plums to 1-MCP provided a novel way to exaggerate the differences in susceptibility to CI in these two species that was utilised in molecular investigations of differential transcript accumulation between the two species. Molecular analyses were carried out using I PEACH1.0 to identify differences between 1-MCP and control fruit in both early and commercial maturity peaches, at multiple storage times. In this study, I was able to identify 113 and 95 genes differentially expressed in early and commercially mature peaches, respectively, prior to storage. After three weeks cool storage, 78 and 46 differentially expressed genes were identified in the early and commercially mature peaches, respectively. qPCR analysis confirmed the expression of the genes found to be differentially expressed. qPCR analysis showed that genes for 9-cis-epoxycarotenoid dioxygenase and the major cherry allergen Pru av were related to cool storage differences. In addition, the genes, cytochrome b5 domain-containing protein, PIN1-like auxin transport protein, protein disulfide isomerise, expansin and 1-aminocyclopropane-1-carboxylate oxidase were found to be related to the development of CI in relation to the two different maturities. Analysis of the differences seen between the peaches and plums following 1-MCP treatment was also investigated using the I PEACH1.0. Genes examined were in fruit from the 2007/08 and 2008/09 seasons. In 2007/08, the microarray analysis compared plum cultivars with a peach cultivar following 1-MCP treatment. Prior to storage in the 2007/08 season, there were 62, 154 and 54 genes differentially expressed in Zee Lady (ZP) peaches, Ruby Red (RR) and October Sun (OS) plums, respectively. After 3 (in peaches) and 4 (in plums) weeks storage, in the 2007/08 season there were 145, 266 and 36 genes differentially expressed in ZP peaches, RR and OS plums, respectively. In the 2008/09 season, comparisons were made between OS plums and early and commercially mature ZP peaches after 3 (peach) and 4 (plum) weeks cool storage. In this comparison, there were 164, 78 and 53 genes differentially expressed in OS plums, early and commercial maturity ZP peaches, respectively. Genes associated with the onset of CI and differentially expressed in response to 1-MCP treatment between the two species included, omega-6 fatty acid desaturase, an ethylene receptor and endopolygalacturonase. Cool storage-related genes across both species were identified by qPCR as an auxin response transcription factor and auxin/IAA. In addition, genes related to differences between species following 1-MCP treatment and cool storage included ACC synthase and leucoanthocyanidin dioxygenase. Proposals for future research include analysis of differential transcript accumulation in fruit sampled at more frequent intervals during the development of CI in cool storage and on removal to a ripening temperature and further exploration of the differential responses of plums and peaches to 1-MCP.

Date of Award	2013
Original language	English