Roles Of Salicylic Acid In Response To Low Temperature Stress In Cucumber (Cucumis Sativus L.) Seedlings

The cucumber（Cucmis Sativus L.）seedling was used as experimental materials to study the effectof low temperature（10℃） on endogenous SA content in leaves and roots, and the roles of SA inresponse to low temperature stress in cucumber seedlings.1. By ultra-high performance liquid chromatography, we found that low temperature stress couldinduce the accumulation of endogenous SA in cucumber leaves and root. The expression of SAmetabolism genes and activitives of the key enzyme in SA synthesis were also influenced by lowtemperature stress: the expression of PR1-1a and PAL genes and the activities of PAL and benzoicacid-2-hydroxylase （BA2H）were induced by low temperature stress. On the other hand, the expressionof SAGT gene was restrained until72hours.2. To determine the roles of endogenous SA in the resistance again low temperature stress, theseedlings were treated with100μmol·L1inhibitors（Pac and AOPP）of SA biosynthesis and exogenousSA（50μmol·L1）. It was found that the accumulation of SAcould be prevented by spraying with Pac orAOPP. What’s more, malondialdehyde（MDA）content, relative electrolyte leakage（REL）and theexpression of PR1-1a, WRKY21, CBF, COR47, RAB18, P5CS, P5CR and HSP70genes at differenttreatment times were measured, the results suggested that low temperature resulted in the increasing inMDA content, REL and the expression of resistance genes. SA decreasing led to reduction of PR1-1a,COR47, CBF, P5CR, P5CS expression, caused a greater elevation of MDA content and REL, andgenerated more necrosis blot under low temperature stress. Furthermore, application of exogenous SAto inhibitor-treated seedlings rescued the relative expression of PR1-1a, WRKY21, CBF, COR47, RAB18,P5CS, P5CR, alleviated the accumulation of MDA and REL, and therefore the necrosis degree.3. To determine the effects of endogenous SA on photosynthetic systems in cucumber seedlingsunder low temperature stress, the chlorophyll fluorescence parameters, and relative expression ofRubisco small subunit（RbcS）and Carbonic anhydrase（CA）genes were determined at differenttreatment times. The results exhibited that low temperature resulted in a reduction in maximumphotochemical efficiency of PSII（Fv/Fm）, effective photochemical quantum yield of PS II（ΦPSII）,potential activities of PSII（Fv/Fo）, and electron transport rate（ETR）. Pac andAOPP spraying causeda greater reduction in PSII efficiency. Low temperature stress led to a decreased allocation of lightabsorbed by PSII antenna to the photochemical reaction and an increased allocation of excessive energy,and SA decreasing caused a much more allocation of light to dissipation as excessive energy. Also, theexpression of RbcS and CA genes was down-regulated by low temperature stress, and the reduction wasgreater in inhibitor-treated seedlings. Furthermore, application of exogenous SA to inhibitor-treatedseedlings alleviated the reduction of photosynthetic efficiency and rescued the repressed geneexpression of RbcS and CA.These results suggested that the low temperature stress could promote the accumulation of SAthrough the PAL/BA2H pathway. Endogenous SA could be involved in the regulation of resistance genes expression. Low temperature-induced SA accumulation was also required for maintenance ofphotosynthetic efficiency and carbon assimilation capacity. Combing these results, we could concludethat endogenous SA was essential for the resistance to low temperature stress in cucumber seedlings.