The Role of Brassinosteroids in the Regulation of Senescence in Lilium Orientalis

Abstract

Fresh flowers of cut lily plants often have a long postharvest life, but this changes after cold storage, which is often an essential process in the horticultural industry. In many cut lilies, a relatively short period of cold storage of 1 week often leads to early leaf and flower senescence. Early leaf and flower senescence are likely ethylene-dependent, either due to a higher level of ethylene production or a change in ethylene sensitivity. Such changes could be due to alteration in the expression of ethylene biosynthesis, perception, and signalling genes. Brassinosteroids (BRs) can delay the onset of senescence and chilling injury in crops but their effects on chilling tolerance in cut flowers have not been investigated. In this thesis, I investigated the role of BR and cold storage on cut lilies (Lilium orientalis) and the role of ethylene in cold-related injuries. More specifically, I investigated: i) the effects of cold storage and BR on postharvest function and ethylene production in a range of cut lily cultivars (Chapter II); ii) the effects of cold storage and BRs + cold storage on the expression pattern of ethylene related genes and a BRs-related transcription factor gene in cut lilies ‘Premium Blonde’ and ‘Marlon’ (Chapter III); and iii) the effects of ethylene on senescence processes and the expression pattern of its biosynthesis, receptor, and signal transduction pathway genes during senescence with and without cold storage in cut lily ‘Marlon’ (Chapter IV). The results of chapters 2 and 3 indicated that: 1) the cut lily cultivars had different levels of sensitivity to cold storage; 2) BR decreased ethylene production and the detrimental effects of cold storage; 3) BR treatment resulted in a greater expression LoBZR1 and a lower expression of most ethylene-related genes compared to cold storage treatment in ‘Premium Blonde’ and ‘Marlon’ lilies. Together, these findings suggest that the chilling injuries in cut lilies are ethylene mediated which could be due to either higher ethylene biosynthesis or higher ethylene sensitivity or a combination of both as a result of alteration in the expression of the related genes. Thus, delayed chilling injuries by application of BR is likely caused by the suppression of the over-expression of ethylene-related genes which results in less ethylene biosynthesis and possibly sensitivity. To understand whether chilling injuries are ethylene-dependent cut lily ‘Marlon’ plants in chapter IV were subjected to ethylene and 1-Methylcyclopropene (1-MCP; an ethylene action inhibitor) before and after cold storage. In non-cold-stored plants, ethylene and 1-MCP treatments did not affect postharvest quality, ethylene production, and the expression pattern of the investigated genes. Cold storage led to much earlier leaf yellowing and while bud opening was not affected, flowers senesced and abscised three days earlier. In cold-stored plants, ethylene and 1-MCP had no effects on bud opening and leaf yellowing but flower senescence and abscission were hastened by ethylene and delayed by 1-MCP. Similarly, the expression of all the genes examined and ethylene production were significantly increased by ethylene but significantly decreased by 1-MCP in cold-stored plants, although not at all time points. These results indicate that there is no sensitivity to ethylene in fresh-cut lily ‘Marlon’. But the response significantly differs following cold storage, in a tissue-dependant manner as flower senescence and abscission were delayed by 1-MCP but leaf senescence was not. Hence, ethylene does influence flower senescence following cold storage but leaf senescence is under the control of other currently unknown factors. Together this research indicates that focus on ethylene blockers to prolong postharvest life in lilies will be effective for flowers but not leaves. However, BR application was not seen to be beneficial at a practical level in all cultivars particularly in cultivars with a low level of sensitivity as the delays of senescence were very short.