GENETIC REGULATION OF BUDBREAK IN KIWIFRUIT

Kiwifruit vines require a period of winter chilling to break bud and flower adequately the following spring. The effects of insufficient winter chilling can be reduced by an application of the dormancy-breaking chemical hydrogen cyanamide (HC). In addition, HC application also increases the number of flowers per shoot. We are investigating the molecular changes occurring during budbreak and in response to HC application. Three microarray experiments were set up to compare the effects of HC application, bud position on a cane in spring, and shoot tipping in summer. It is expected that a combined analysis of these three datasets will identify genes that are associated with budbreak. Here, the data from the first experiment are described, where the effects of HC on dormancy release were investigated. For this experiment, meristems were dissected from buds one day before HC application, and from HC-treated and non-treated vines one day, three days and six days after application. This short time frame, based on previous experimental knowledge, was before increases in bud proline levels and morphological changes. Each microarray slide contained 17,524 oligos representing approximately 17,000 unigenes from HortResearch’s proprietary kiwifruit EST database. This analysis identified 561 ESTs (approximately 3%) with greater than two-fold changes in expression. Of those up-regulated early in the time course, there was a marked increase in genes associated with stress responses. During the qPCR validation process, two transcript accumulation profiles were observed: Profile 1: A single early peak in buds collected from vines treated with HC but no accumulation in buds collected from untreated vines. Profile 2: A biphasic response in buds collected from HC vines (early and late peaks) and a single late peak in untreated buds, later than that seen in treated vines. The early peak appears to be in direct response to HC application and the late peaks appear to be associated with budbreak and shoot outgrowth.