Mechanisms Of Luffa Rootstock-induced Drought Tolerance In Cucumber

As one of the major vegetable crops, cucumber (Cucumis sativus) plays an important role in greenhouse cultivation and agricultural production. Water is a critical factor that affects the vegetative and reproductive growth of cucumber. Cucumber always faces water stress during its growth and development especially in arid and semi-arid areas. Grafting is an effective method to improve tolerance to environmental stress in horticultural plants and has been widely used in crop production. However, our knowledge about the mechanisms involved in the perception of the signal of different environmental adversities by rootstocks and subsequent communication with scions is fragmentary. We explored the mechanisms of luffa rootstock-induced drought tolerance in cucumber plants from aspects of photosynthesis, water loss, stomatal movement, effects of ABA on stomatal movement, stomatal sensitivity to ABA and water use efficiency(WUE). Cucumis sativus L. and Luffa cylindrica L. M. Roem. were two plant materials used in the current study.We grafted cucumbmer onto rootstocks of cucumber or luffa that resulted in two groups of grafted plants namely Cs/Cs and Cs/Lc plants, respectively. The results are as follows:1. Effects of luffa rootstock on drought tolerance of cucumber. After sufficient watering, natural drought was imposed by withholding water for both Cs/Cs and Cs/Lc plants. We investigated the phenotype, oxidative stress, dry matter accumulation, integrated water use efficiency(WUE) and activity of antioxidant enzymes at the ninth day and observed the net photosynthetic rate, transpiration rate, instantaneous water use efficiency, leaf area and changes in water loss. We found that the Cs/Cs plants were obviously wilted while Cs/Lc plants were still relatively strong after 9 days, and the leaf area, dry weight accumulation, PSII Fv/Fm, Pn, WUE, activity of antioxidant enzymes of Cs/Lc plants were much higher while the oxidative stress and water loss of Cs/Lc plants were significantly lower than Cs/Cs plants. All these results suggested that cucumber plants showed an enhanced drought tolerance when grafted onto rootstock of luffa.2. We investigated the effects of ABA on luffa rootstock-induced drought tolerance of cucumber plants. We compared the difference of stomatal aperture, H2O2 accumulation in stoma, ABA accumulation in roots, xylem sap and leaves as well as the transcript level of 9-cis- epoxycarotenoid dioxygenase2 (NCED2), a key rate-limiting gene for ABA biosynthesis, in the leaves of plants after exposure to water deficit and the stomatal density of new leaves developed under drought stress. In order to further verify the role of ABA in this process, we inhibited ABA biosynthesis in roots or leaves of two grafted plants, while added ABA to roots of Cs/Cs plants. We found that there was an earlier decline in stomatal aperture and H2O2 accumulation in stoma of Cs/Lc plants than Cs/Cs plants. In addition to early up-regulation of NCED2 expression in leaves, rapid ABA accumulation in roots and transport through xylem to leaves of Cs/Lc plants were observed that eventually resulted in an earlier accumulation of ABA in leaves of Cs/Lc plants too. Moreover, time-course analysis showed that leaves from Cs/Cs plants had a more significant water loss over time than those from Cs/Lc plants. Meanwhile, stoma of Cs/Lc plants was more sensitive to exogenous ABA. Interestingly, the stomatal density of new leaves of Cs/Lc plants developed under drought stress was much lower than that in Cs/Cs plants. Furthermore, irrigation with AbamineSG, which inhibits activity of NCED, compromised luffa rootstock-induced drought tolerance whilst drought tolerance was only partially compromised by foliar application of AbamineSG in Cs/Lc plants. On the other hand, Cs/Cs plants irrigated with ABA showed increased drought tollerance. In conclusion, on one hand, Cs/Lc plants could reduce stomatal aperture and water loss by early inducing ABA accumulation in roots and increasing stomatal sensitivity to ABA which may be mediated by H2O2, and on the other hand, the decreased density of stoma in new leaves of Cs/Lc plants developed under drought process may also contribute to luffa rootstock-induced drought tolerance.3. We studied the effects of luffa rootstock on drought tolerance of cucumber under different degree of drought stress. Pn, Gs, Tr, iWUE, stomatal aperture, water potential (ψ), leaf temperature and transcript level of genes in ABA biosynthesis and signaling were detected in Cs/Cs and Cs/Lc plants growned in SWC of 50%、40%、 30%.15% and 7% for 7 days. We found that Cs/Lc plants all showed stronger drought tolerance under different SWC with higher Pn, iWUE,ψ and leaf temperature and lower Gs, Tr, stomatal aperture and water loss compared with Cs/Cs plants. In addition, the positive regulator genes (PYL1, PYL2, PYL5, PYL8, SnRK2.1, SnRK2.2, ABCG22, RAB18, RD29B) in ABA biosynthesis and signaling were sharply up-regulated whilst the negative regulator gene PP2C1 was more significantly down-regulated in Cs/Lc plants than that in Cs/Cs plants in response to water deficit. To sum up, Cs/Lc plants showed enhanced drought tolerance by decreasing water loss and increasing WUE which were also associated with an improved sensitivity to ABA and reduced stomatal aperture under different degree of drought stress.