The Mechanism Of Gibberellins In Regulation Of Chilling Tolerance Of Postharvest Tomato Fruit

Chilling injury(CI) is a primary postharvest problem for cold sensitive fruits and vegetables during storage. There is a theoretical and practical need to understand the mechanisms of CI and to develop safe, effective and economic cold-resistant inducing methods. Gibberellins(GAs) are important plant hormones that play crucial roles in modulating multiple developmental processes and cellular responses to biotic and abiotic stresses. Many researches indicated that salicylic acid(SA) as a signal molecule plays an important role in fruit anti-cold process, and there is a cross talk between GA and SA. However, little information is available concerning the mechanism of synergism between SA and GA in cold resistance inducement of postharvest fruits.In order to investigate the effect of GA on chilling stress tolerance of tomato fruit, and further explore the interaction relationship between GA and SA, tomato(Solanum lycopersicum L. cv. Zhefen No. 702) at the mature green stage treated with different concentration of exogenous gibberellic acid(GA3) were stored at 4 °C for 28 days. The most suitable concentration of GA3 was selected by CI index. Cell microstructure, malondialdehyde(MDA) content, electrolyte leakage, the activities of phospholipase C(PLC), phospholipase D(PLD), lipoxidase(LOX), polygalacturonase(PG) and superoxidase(SOD), catalase(CAT), the expression of GA signaling pathway and GA metabolism genes, the C-repeat/dehydration-responsive element-binding factor(CBF) genes and the SA biosynthesis gene(ICS1) were also evaluated in this study. In addtion, harvested fruit of wild-type tomato(Solanum lycopersicum L. cv. Moneymaker) and GA-deficient tomato(gib-3) at mature green stage were used for further study to investigate the role of GA in fruit tolerance to chilling stress.The main results are as follows.(1) Compared with the control, 0.5 mM GA3 treatment effectively reduced the CI index of tomato fruit, maintained the integrity of cell microstructure, significantly slowed down the increase of MDA content and electrolyte leakage, inhibited the activities of PLC, PLD, LOX and PG, and elevated the activities of SOD and CAT. Thus, cold resistance in GA3-treated fruit was enhanced.(2) Cold inhibited the increase of GA3 levels, While fruits treated with 0.5 mM GA3 exhibited higher tissue GA3 levels and lower expression of the gene encoding a crucial GA signaling component and growth repressor known as DELLA protein(GAI) and key GA biosynthetic genes(GA20ox1 and GA3ox1), whereas the expression of GA catabolic gene(GA2ox1) was up-regulated. Furthermore, GA3 treatment also increased the expression of the key regulator of cold response gene CBF1 and elevated the level of endogenous SA in tomato fruit via isochorismate synthase(ICS) pathway.(3) The CI index, MDA content, electrolyte leakage and activities of PLC and PLD in gib-3 tomato fruit were higher than those in wild-type tomato fruit during cold storage, while the activities of SOD and CAT were lower than those in the wild-type. Compared with the wild-type tomato fruit, the expression of the GAI and GA3ox1 genes was elevated in gib-3 tomato fruit, whereas the expression of GA catabolic gene(GA2ox1) and the key regulator of cold response gene CBF1 were reduced. Moreover, the lack of GA also inhibited the expression of SA biosynthesis gene ICS1.In conclusion, GA plays an important role in regulation of chilling tolerance of postharvest tomato fruit. First of all, exogenous application of GA3 can maintain the integrity of cell membrane and enhance chilling stress tolerance in postharvest tomato fruit during cold storage. Secondly, GA3 treatment elevates GA3 levels, and stimulates the feedback regulation and the expression of the key regulator of cold response gene CBF1. Thirdly, GA3 treatment induces the biosynthesis of SA, a signaling molecule that has been involved in plant responses to abiotic stresses. Finally, gib-3 tomato fruit seem to be more sensitive to chilling injury than the wild-type, and SA biosynthesis is also inhibited in chilled gib-3 tomato fruit. These findings are comprehensive insights into the integration mechanisms of cold-resistance in postharvest fruit, and might be possible to devise effective approaches for postharvest CI management.