4.3 PTOX and NDH-mediated cyclic electron transfer
Under stress conditions, the cyclic electron transfer rate usually increases (Brayton et al., 2006; Shikanai 2007; Takahashi et al., 2013; Strend et al., 2015). In contrast, here we show that in SA , which has a great capacity of channeling electrons to PTOX, the rate of cyclic electron transfer rate decreased (Figure 6). This is clearly shown by data from the post-illumination Fo rise (PIR) signal, which was used here to assay NDH (Burrows et al., 1998). Using this method, we found a stark contrast in the responses of NDH-dependent CEF and PTOX to salt stress between species (SA and SV ). In SV , the strong stimulation of NDH-dependent CEF following salt stress (236%) was concurrent to a nearly stable PTOX level (Figs. 6 and 8). However, in SA , we observed a decline in the NDH-dependent CEF (Figure 6) together with an increase of PTOX expression levels, which was up-regulated by up to 4 times compared to the control as assessed by both RNA-expression abundance and protein abundance (Figure 8).
Our finding of this negative relationship between PTOX and NDH-CEF is in line with a number of earlier reports, e.g. Ahmad et al. (2012) demonstrated a dramatic decline in NDH activity in tobacco expressing PTOX from green algae (Cr-PTOX1); PTOX competes efficiently with CEF for plastoquinol (PQH2) in the CRTI-expressing (carotene desaturase) lines (Galzerano et al., 2014); Joët et al. (2002) also showed a decrease in NDH-dependent CEF in tobacco transgenic lines expressing PTOX from Arabidopsis. The activity of cyclic electron transfer is regulated by an array of mechanisms, including redox status (Lasano et al., 2001; Breyton et al., 2006; Takahashi et al., 2013), H2O2 (Strand et al., 2015), metabolite levels (Livingston et al., 2010), Ca signaling (Terashima et al., 2012; Lascano et al., 2003), and even phosphorylation of NDH components (Lascano et al., 2003). It is likely that NaCl induced differential changes in the NDH and PTOX, though mechanism is complexly unknown. It is possible that some internal signals from chloroplast, such as redox status of chloroplast electron transfer chain, or particular compound in the photosynthetic carbon metabolism, or even H2O2, might differentially regulate PTOX and NDH-CET. Mechanisms how PTOX and NDH-CET were differentially regulated under NaCl needs further elucidation. It is worth mentioning here that SA has been used as a model halophyte grass species to study adaptation to plants to salt stress and to mine salt stress-responsive genes (Subudhi & Baisakh, 2011). Several ealier studies have demonstrated the utility of genes from this halophyte to improve crop salt tolerance (Baisakh et al. 2012; Karan & Subudhi, 2012a, b). Therefore, elucidation of how PTOX and NDH-CET respond under NaCl to protect photosystem and leaf functioning can help develop new strategy to protect photosystems under salt stress.
Abbreviations : A , photosynthetic rate; CAT, catalase; CEF, cyclic electron flow; Chl, chlorophyll; DAB, 3, 3’ diaminobenzidine; DBMIB, 2,5-dibromo-3-methyl-6-isopropyl-p- benzoquinone; ETC, electron transport chain; FI, fluorescence induction;gs , stomatal conductance; Fo, minimum fluorescence (PSII RCs open); Fm, maximum fluorescence (PSII RCs close); gETC, conductance of the electron transfer chain; n -PG, n -propyl gallate (n -PG, 3,4,5-trihydroxy-benzoic acid-n-propyl ester); NBT, nitroblue tetrazolium; NDH, NAD(P)H dehydrogenase; NPQ, non-photochemical quenching; PQ, plastoquinone; PQH2, plastoquinol; PTOX, plastid terminal oxidase; P700, photosystem I reaction center; ROS, reactive oxygen species; SP, saturating pulse; SOD, superoxide dismutase; SV , Setaria viridis ; SA ,Spartina alterniflora .