Response of Chloroplast NAD(P)H Dehydrogenase-Mediated Cyclic Electron Flow to a Shortage or Lack in Ferredoxin-Quinone Oxidoreductase-Dependent Pathway in Rice Following Short-Term Heat Stress

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., 04149 with a high capacity (hcef) and C4023 with a low capacity (Icef). The absorbance change at 820 nm (AA820) was used here to assess the charge separation in the PSI reaction center (P-700). The results obtained show that short-term heat stress abolishes the ferredoxin-quinone oxidoreductase (FOR)-dependent CEF in rice and accelerates the initial rate of P-700+ re-reduction. The P-700+ amplitude was slightly increased at a moderate heat-stress (35 degrees C) because of a partial restriction of FOR but it was decreased following high heat-stress (42 degrees C). Assessment of PSI and PSII activities shows that PSI is more susceptible to heat stress than PSII. Under high temperature, FOR dependent CEF was completely removed and NDH dependent CEF was up-regulated and strengthened to a higher extent in Q4023 than in Q4149. Specifically, under normal growth temperature, hcef (04149) was characterized by higher FOR- and chloroplast NAD(P)H dehydrogenase (NDH)-dependent CEF rates than Icef (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., FOR-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 FOR pathway. The potential role of the NDH dependent pathway during the evolution of C-4 photosynthesis is briefly discussed.