Abstract
Pyropia yezoensis can survive the severe water loss that occurs during low tide, making it an ideal species to investigate the acclimation mechanism of intertidal seaweed to special extreme environments. In this study, we determined the effects of high salinity on photosynthesis using increasing salinity around algal tissues. Both electron transport rates, ETR (I) and ETR (II), showed continuous decreases as the salinity increased. However, the difference between these factors remained relatively stable, similar to the control. Inhibitor experiments illustrated that there were at least three different cyclic electron transport pathways. Under conditions of severe salinity, NAD(P)H could be exploited as an endogenous electron donor to reduce the plastoquinone pool in Py. yezoensis. Based on these findings, we next examined how these different cyclic electron transport (CETs) pathways were coordinated by cloning the gene (HM370553) for ferredoxin-NADP+ oxidoreductase (FNR). A phylogenetic tree was constructed, and the evolutionary relationships among different FNRs were evaluated. The results indicated that the Py. yezoensis FNR showed a closer relationship with cyanobacterial FNR. The results of both real-time polymerase chain reaction and western blotting showed that the enzyme was upregulated under 90–120‰ salinity. Due to the structure-function correlations in organism, Py. yezoensis FNR was proposed to be involved in NAD(P)H-dependent Fd+ reduction under severe salinity conditions. Thus, through the connection between different donors bridged by FNR, electrons were channeled toward distinct routes according to the different metabolic demands. This was expected to make the electron transfer in the chloroplasts become more flexible and to contribute greatly to acclimation of Py. yezoensis to the extreme variable environments in the intertidal zone.
Highlights
Photosynthetic organisms do not always grow under optimal conditions
The PGR5-PGRL1 protein-dependent pathway, which shows antimycin A (AA) sensitivity (Shikanai, 2014), and NDH-dependent cyclic flow, which is mediated with the chloroplastic NAD(P)H dehydrogenase complex (NDHcomplex), whose activity is inhibited by rotenone (Ro) (Endo et al, 2008), have been extensively studied
The activity of PS II became negligible when the samples were subjected to treatment with 150 salinity
Summary
Photosynthetic organisms do not always grow under optimal conditions. higher plants, eukaryotic algae, and cyanobacteria usually suffer from drought or desiccation stress. Many studies have shown that dehydration leads to a decrease in PS II activity and the activation of stress responding process (Baker, 1991; Golding and Johnson, 2003) Under these conditions, the cyclic electron transport (CET) around PS I was enhanced (Müller et al, 2001). The common factor of these CETs involves reduction of the plastoquinone (PQ) pool by either ferredoxin (Fd) or NAD(P)H/NADH, followed by re-reduction of P700+ by electrons through the cytochrome b6/f complex and plastocyanin. The difference among these CET pathways is usually attributed to the enzyme that mediates reduction of PQ from the PS I acceptor side (Bukhov and Carpentier, 2004). Photoproduction of O2− around PS I can consume the extra energy absorbed through water-water cycle (Asada, 1999), which constitutes another pseudo-CET and complements the function of PS I-driven electron transport (Shikanai and Yamamoto, 2017)
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