The electron-transport machinery in photosynthetic membranes is known to be very sensitive to heat. In this study, the rate of electron transport (ETR) driven by photosystem I (PSI) and photosystem II (PSII) during heat stress in the wild-type Synechocystis sp. strain PCC 6803 (WT) and its ndh gene inactiva-tion mutants △ndhB (M55) and △ndhD1/ndhD2 (D1/D2) was simultaneously assessed by using the novel Dual-PAM-100 measuring system. The rate of electron transport driven by the photosystems (ETRPSs) in the WT, M55, and D1/D2 cells incubated at 30℃ and at 55℃ for 10 min was compared. Incubation at 55 ℃ for 10 min significantly inhibited PSII-driven ETR (ETRPSII) in the WT, M55 and D1/D2 cells, and the ex-tent of inhibition in both the M55 and D1/D2 cells was greater than that in the WT cells. Further, PSI-driven ETR (ETRPSI) was stimulated in both the WT and D1/D2 cells, and this rate was increased to a greater extent in the D1/D2 than in the WT cells. However, ETRPSI was considerably inhibited in the M55 cells. Analysis of the effect of heat stress on ETRPSs with regard to the alterations in the 2 active NDH-1 complexes in the WT, M55, and D1/D2 cells indicated that the active NDH-1 supercomplex and medi-umcomplex are essential for alleviating the heat-induced inhibition of ETRPSII and for accelerating the heat-induced stimulation of ETRPSI, respectively. Further, it is believed that these effects are most likely brought about by the electron transport mediated by each of these 2 active NDH-1 complexes.
MA WeiMin, WEI LanZhen & WANG QuanXi College of Life and Environment Sciences, Shanghai Normal University, Shanghai 200234, China
Phycobilisomes (PBSs) are the main accessory light-harvesting complexes in cyanobacteria and their movement between photosystems (PSs) affects cyclic and respiratory electron transport.However,it remains unclear whether the movement of PBSs between PSs also affects the transthylakoid proton gradient (ΔpH).We investigated the effect of PBS movement on ΔpH levels in a unicellular cyanobacterium Synechocystis sp.strain PCC 6803,using glycinebetaine to immobilize and couple PBSs to photosystem II (PSII) or photosystem I (PSI) by applying under far-red or green light,respectively.The immobilization of PBSs at PSII inhibited decreases in ΔpH,as reflected by the slow phase of millisecond-delayed light emission (ms-DLE) that occurs during the movement of PBSs from PSII to PSI.By contrast,the immobilization of PBSs at PSI inhibited the increase in ΔpH that occurs when PBSs move from PSI to PSII.Comparison of the changes in ΔpH and electron transport caused by the movement of PBSs between PSs indicated that the changes in ΔpH were most likely caused by respiratory electron transport.This will further improve our understanding of the physiological role of PBS movement in cyanobacteria.
