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. The chlorophyll (Chl) content was markedly reduced by heat stress, inhibiting photochemical activity. However, larger decreases in growth and photosynthetic parameters [net photosynthetic rate (PN), stomatal conductance (GS), intercellular CO2 concentration (Ci), and leaf transpiration rate (E)] 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 (TR/CSM), electron transport flux from QA to QB per cross section of PSII (ET/ CSM) and performance index on absorption basis (PIABS) 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°C 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 Chl 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 Chl a/b-binding protein), energy metabolism (ATPase), and proteins of PSII reaction center (D2, CP43) under heat stress.