Alternative electron pathways in photosynthesis: strength in numbers

Electrons supplied by photosystem I (PSI) can have various fates, serving to regenerate NADPH during linear electron flow (LEF), or entering any of several minor ‘alternative electron pathways’ (AEPs). AEPs are thought to play a role in the regulation of photosynthesis and the alleviation of PSI photoinhibition (reviewed in Allahverdiyeva et al., 2015; Yamori & Shikanai, 2016; Alboresi et al., 2019). Several AEPs have been described, including two modes of cyclic electron flow (CEF), one involving the NADH dehydrogenase-like (NDH) complex (NDH-CEF), the other employing the two proteins PGR5 and PGRL1 (PGR5-CEF) (Fig. 1). Pseudo-cyclic electron flow (PCEF) results in the reduction of oxygen to water – by the Mehler reaction (Mehler-PCEF) during which reactive oxygen species (ROS) are generated and scavenged (reviewed in Leister, 2019), or by the direct reduction of oxygen to water catalysed by flavodiiron proteins (FLVs) (FLV-PCEF) (reviewed in Alboresi et al., 2019) (Fig. 1). These four AEPs are not uniformly distributed across all phylogenetic groups of photosynthetic organisms, but the moss Physcomitrella patens harbours all of them, and is, therefore, ideally suited for investigations of their functional overlaps. In this issue of New Phytologist, Storti et al. (2020; pp. 1316–1326) have extended their previous study on AEPs in P. patens and combined and analysed mutations in three AEPs – the two CEF pathways and FLV-CEF. They found that P. patens tolerates the simultaneous loss of both CEF pathways, but becomes severely impaired even under very low light intensities if FLV-PCEF is inactivated as well. This demonstrates that any one of the three AEPs is dispensable, but each one makes a contribution to the maintenance of PSI function even under nonstressful conditions