The Efficacy Of ABA In Controlling Pineapple Blackheart And The Antagonism To GAs And Ethylene

Pineapple is one of the most important fruits for export from south China. Blackheart is one of the major diseases that cause losses of fresh pineapple fruits during storage and transportation. Some research showed that preharvest application of GA for increasing fruit size or ethylene for enhancing ripening are among the important reasons leading to pineapple blackheart. In this paper, we studied the efficacy and mechanism of ABA in controlling postharvest pineapple blackheart, and the antagonism of ABA to GA and ethylene, and investigated role of Ac GA2 ox gene in regulating endogenous GA levels by means of ectopic expression. The main results were as follows:1. Postharvest application of 200 mg·L-1(ABA)reduced the internal browning(IB)index and incidence, and maintained quality in pineapple fruit. ABA treatment reduced phenolic contents and polyphenol oxidase and phenylalanine ammonia lyase activities, increased catalase and peroxidase activities, and decreased O2·-, H2O2 levels. This suggests ABA could control IB through inhibiting phenolics biosynthesis and oxidation and enhancing antioxidant capability. Exogenous ABA down-regulated the expression of GA2ox?PPO and PAL, reduced the content of endogenous gibberellins, and increased endogenous ABA content. ABA application can be used as a technique to commercially control pineapple blackheart.2. Exogenous 200 mg·L-1 gibberellin(GA4+7)treatment increased pineapple blackheart, however GA4+7+ABA treatment reduced blackheart. GA treatment reduced endogenous ABA content, up-regulated the expression of GA2 ox, increased endogenous gibberellins content(GA1 and GA4). Compared with GA, GA+ABA treatment increased endogenous ABA content, down-regulated the expression of GA2 ox, reduced GA1 and GA4 content. This suggests the antagonism between the ABA and GA, and shows that exogenous ABA can offset the effects of exogenous GAs in enhancing endogenous GAs biosynthesis and the inhibiting endogenous ABA by synthesis. Therefore, the negative effects of preharvest application of GAs on postharvest pineapple can be minimized through the application of ABA.3 ? Application of 200mg·L-1 ethephon(Eth) significantly increased pineapple blackheart, however, Eth+ABA treatment significantly reduced blackheart. Eth treatment up-regulated the expression of GA2 ox, Eth+ABA showed the opposite effect. This suggests that the antagonism between ethylene and ABA in regulating the expression of GA2 ox. Eth treatment and Eth+ABA treatment reduced GA1 and GA4 content, suggesting Eth treatment aggravated postharvest pineapple blackheart not by increasing the endogenous gibberellins. Eth and Eth+ABA treatment increased the ABA content, but only Eth + ABA treatment significantly controlled blackheart, suggesting existence of certain antagonism between ethylene and ABA in regulating blackheart development. Eth treatment showed higher expression of PPO?PAL than the control, while Eth+ABA treatment lower than the control. This suggests that the former promotes the phenols synthesis and oxidation, while the latter inhibits them in pineapple tissue. All of these results indicate the antagonism between ethylene and abscisic acid in regulation of key genes related to browning.4?The 981 bp c DNA of Ac GA2 ox was cloned into p CAMBIA2300 vector, and expression under the control of the Ca MV 35 S promoter, the resulting vector was introduced into Agrobacterium tumefaciens strain LBA4404, and then transformed Arabidopsis plants(accession Col-0) by floral-dipping method. The stable transgenic lines were obtained. Compared with the wild-type Arabidopsis plant, the transgenic plants expressing Ac GA2 ox had significantly lower seed germination rate?height? root length and hypocotyl length and lower GA content. When cultured on GA4+7-containing medium, the transgenic seeds showed an elevated germination rate, the transgenic seedlings showed increased height, root length and hypocotyl length, in other words, exogenous GA4+7 treatment can reverse the dwarfing traits of Ac GA2 ox overexpression Arabidopsis. This suggests an important role of Ac GA2 ox in regulation endogenous GA levels in pineapple fruits.In conclusion,exogenous ABA can effectively inhibit the occurrence of pineapple blackheart. GA and ethephon aggravated blackheart severity, while ABA treatment can effectively reduce the negative effect of GA and ethylene on pineapple quality. Pineapple Ac GA2 ox gene was involved in control of GA biosynthesis and that the regulation of expression of Ac GA2 ox could influence the occurrence of blackheart of pineapple fruits.