Upregulation Of Photosynthetic Capacity And Thylakoid Membrane Protein Enhanced Tolerance To Heat Stress In Wucai(Brassica Campestris L.)

The hot climate of southern China from late summer to early fall is one of the major factors limiting the yield and quality of wucai(Brassica campestris L.).Under high temperature stress,heat-tolerant cultivars presented moderate injury to the photosynthesic apparatus,less inhibition of photochemical activity,better osmotic adjustment and antioxidant defences capacity compared to heat-sensitive cultivars.To study the effects of high temperature on the growth and development of wucai,plants of WS-1(heat-tolerant)and WS-6(heat-sensitive)were exposed to four heat stress treatments in growth chambers for 3 days.Chloroplasts of two cultivars evaluated for photosynthetic characteristics,fatty acid composition and differentially expressed proteins of thylakoid membrane.Larger decreases in growth and photosynthetic parameters occurred under heat stress in WS-6,compared with WS-1.In addition,WS-6 showed an obviouse K point in o-J-I-P steps under extremely high temperature,which indicated OEC had been damaged.WS-1 showed higher of maximum quantum yield of primary PSⅡ photochemistry(FV/FM),number of active reaction centres per cross section of PSⅡ(RC/CSM),average absorbed photon flux per cross section of PSⅡ(ABS/CSm),maximum trapped exciton flux per cross section of PSⅡ(TRO/CSM),electron transport flux from Qa to Qb per cross section of PSⅡ(ETO/CSM)and performance index on absorption basis(PlABS)which indicated greater heat stability in terms of PSⅡ function under higher temperature.Compared to WS-6,WS-1 showed higher membrane stability and photochemical efficiency,and greater increase of saturated fatty acids(SFA),especially palmitic acid under heat stress.WS-1 had higher recovery rate compared to WS-6 after 41℃ heat stress treatment.Additionally,two-dimensional blue native/SDS-PAGE analysis of chloroplast was carried out to compare the differentially expressed proteins between two cultivars.We obtained seven major protein complexes included supercomplexes,PSI-LHCⅡ/PSⅡ monomer,PSⅡ monomer,CP43 less PSⅡ/ATP synthase,LHCⅡ trimer,LHCⅡ monomer and ATP synthase after first dimentional seperation in both cultivars after the first dimensional separation.Then ten differential membrane proteins included light-harvesting Chi a/b-binding(LHC)protein,ATP synthase subunit alpha,ATP synthase subunit beta,photosystem I P700 chlorophyll a apoprotein A2,photosystem Ⅱ CP43 reaction center protein,photosystem Ⅱ D2 protein and photosystem Ⅱ OS have been found between WS-1 and WS-6.These differentially proteins in cellular membranes could contribute to the differential level of heat tolerance between two wucai cultivars.Our results demonstrated that the heat-tolerant cultivar WS-1 had a greater capacity for photosynthesis and membrane stability by upregulating proteins abundance including light harvesting(light-harvesting Chi ajb-binding protein),energy metabolism(ATPase),and proteins of PSⅡ reaction center(D2,CP43)under heat stress.