Abstract
Cyclic electron flow (CEF) around photosystem I (PSI) can protect photosynthetic electron carriers under conditions of stromal over-reduction. The goal of the research reported in this paper was to investigate the responses of both PSI and photosystem II (PSII) to a short-term heat stress in two rice lines with different capacities of cyclic electron transfer, i.e., Q4149 with a high capacity (hcef) and C4023 with a low capacity (lcef). The absorbance change at 820 nm (ΔA820) was used here to assess the charge separation in the PSI reaction center (P700). The results obtained show that short-term heat stress abolishes the ferredoxin-quinone oxidoreductase (FQR)-dependent CEF in rice and accelerates the initial rate of P700+ re-reduction. The P700+ amplitude was slightly increased at a moderate heat-stress (35°C) because of a partial restriction of FQR but it was decreased following high heat-stress (42°C). Assessment of PSI and PSII activities shows that PSI is more susceptible to heat stress than PSII. Under high temperature, FQR-dependent CEF was completely removed and NDH-dependent CEF was up-regulated and strengthened to a higher extent in C4023 than in Q4149. Specifically, under normal growth temperature, hcef (Q4149) was characterized by higher FQR- and chloroplast NAD(P)H dehydrogenase (NDH)-dependent CEF rates than lcef (C4023). Following thermal stress, the activation of NDH-pathway was 130 and 10% for C4023 and Q4149, respectively. Thus, the NDH-dependent CEF may constitute the second layer of plant protection and defense against heat stress after the main route, i.e., FQR-dependent CEF, reaches its capacity. We discuss the possibility that under high heat stress, the NDH pathway serves as a safety valve to dissipate excess energy by cyclic photophosphorylation and overcome the stroma over-reduction following inhibition of CO2 assimilation and any shortage or lack in the FQR pathway. The potential role of the NDH-dependent pathway during the evolution of C4 photosynthesis is briefly discussed.
Highlights
Light energy is captured by plants through the LHCII and LHCI, which is converted into chemical energy by the function of the two photosystems I and II (PSI and photosystem II (PSII))
This study shows that there are natural variations of Cyclic electron flow (CEF) capacities between rice lines studied here; PSI and PSII activities in those lines with higher CEF are more tolerant to heat stress
We show that the NDHdependent CEF may compensate for the shortage or lack in the ferredoxin-quinone oxidoreductase (FQR)-dependent CEF under heat stress
Summary
Light energy is captured by plants through the LHCII and LHCI, which is converted into chemical energy by the function of the two photosystems I and II (PSI and PSII). The two photosystems operate in tandem to drive the LEF to reduce NADP+, thereby forming the reducing power in the forms of reduced ferredoxin or NADPH in the stroma. The e− transport through the Cytb6/f, an intermediate complex between the two photosystems, generates a proton gradient across the thylakoid membrane ( pH) that is subsequently used by the ATP-synthase pump to synthesize ATP. Besides the LEF, ATP can be produced from CEF around PSI; therein electrons are recycled from reduced ferredoxin or NADPH to the PQ pool operating in the e− transport from the PSII to the Cytb6/f (Bendall and Manasse, 1995). CEF can generate a proton gradient ( pH) and drives ATP synthesis by ATP synthase without simultaneous generation of NADPH (Heber et al, 1978; Heber and Walker, 1992)
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