The Regulatory Mechanism Of Exogenous Putrescine On LHC? Aggregation And Its Dissipation Of Excess Excitation Energy In Salt-stressed Cucumber Seedlings

In recent years,secondary salinization has become a major environmental factors that limiting production and quality of vegetable crop in protected cultivation in China.Polyamines,as new plant growth regulators,can relieve the inhibition of the photosynthetic efficiency in salt-stressed plants.It was found that Put mainly reduced the photochemical efficiency of cucumber plants under salt stress by adjusting the dissipation of excess excitation energy in cucumber leaves and avoiding the accumulation of excess excitation energy.However,the way of how Put adjusts excess excitation energy dissipation by regulating the aggregation of LHCII in cucumber seedlings under salt stress remains unclear.In this study,the salt sensitive type of cucumber variety "Jinyou 4" was used as experimental material,we investigated the effects of exogenous Put on NPQ components,spectral characteristics of thylakoid membranes,chlorophyll metabolism pathway,the expression of LHCII protein and transcriptional level,LHCII-associated endogenous PAs levels in cucumber leaves under salt stress,to elucidate Put-mediated protection mechanism of photochemical efficiency through LHCII.The main results were as follows:1.Exogenous Put alleviated the decrease of the maximum quantum yield of photosystem II(Fv/Fm)induced by salt stress,and greatly increased the value of NPQ,which mainly depends on the thylakoid membrane proton gradient(qE)and state transitions of energy dissipation(qT)in salt-stressed cucumber leaves.In addition,Put alleviated the reduction of chlorophyll light harvesting ability and excitation ability of salt-stressed cucumber seedlings.These results indicate that exogenous Put effectively promotes the excess energy dissipation of leaves in cucumber seedlings under salt stress,thus improving the photochemical efficiency and salt stress resistance of cucumber seedlings under salt stress.2.Exogenous Put promoted the conversion of uroporhyrinogen ? to protoporphyrin IX and alleviated decreases in Chl contents under salt stress.Moreover,the Put treatment reduced the activities of uroporphyrinogen ? synthase,chlorophyllase,and Mg-dechelatase and downregulated the transcriptional levels of glutamyl-tRNA reductase,5-aminolevulinate dehydratase,uroporphyrinogen ? synthase,uroporphyrinogen ?decarboxylase,and chlorophyllide a oxygenase,but significantly increased the expression levels of non-yellow coloring 1-like,pheide a oxygenase,and red chlorophyll catabolite reductase.Taken together,these results suggest that Put might improve Chl metabolism by regulating enzyme activities and mRNA transcription levels in a way that improved the photochemical efficiency of cucumber plants.3.Salt stress significantly reduced the type and content of thylakoid protein complexes and complex subunit proteins.Exogenous Put effectively alleviated the salt stress induced-degradation of LHCII monomer and trimer and enhanced the stability of thylakoid supercomplexes.Western blot analysis revealed that Put increased the expression levels of D1,D2 and Lhcb4 and reduced Lhcb3 content in thylakoid membrane of salt-stresed cucumber seedlings.Meanwhile,exogenous Put significantly up-regulated expression of lhcb1-6 gene in the early treatment,then down-regulated the transcriptional levels of these genes in salt-stressed cucumber seedlings.The results showed that exogenous Put could alleviate the degradation of thylakoid protein complex,and stabilize the structure of thylakoid membrane.4.After 3 days of treatment,salt stress enhanced the transglutaminase(TGase)activity of cucumber seedling leaves.Put treatment significantly increased the endogenous total free and conjugated polyamines levels bound to monomeric and trimeric LHCII and further up-regulated the activities of transglutaminases(TGase)in leaves under salt stress.Taken together,these results suggest that exogenous Put regulate LHCII aggregation involved in an alteration in the levels of endogenous bound polyamines,which playing a crucial role in adjusting the photo-adaptation and dissipating excess excitation energy.