Chloroplast structure and photosynthetic acclimation in shade-adapted lycophyte Selaginella: a window into photosynthesis of ancient vascular plants?

Selaginella is a diverse genus of lycophytes, an ancient group of vascular plants that is sister to all other vascular plants (euphyllophytes, including angiosperms). The majority of Selaginella species thrive in environments with humid, shaded conditions. This review examines current knowledge and unresolved issues concerning the distinctive photosynthetic adaptations of shade-adapted Selaginella species, addressing aspects from cell morphology to thylakoid organisation. Unique is their giant chloroplast, in which the thylakoid system can differentiate into lamellar and granal (bizonoplast). Optimisation of light harvesting and photoprotection is supported by distinctive thylakoid membrane composition and supramolecular organisation of LHCII with PSII and PSI, involving the role of LHCB6 phosphorylation and an outstanding capacity to keep PSI oxidised. Selaginella appears to combine ancestral characteristics with highly specialised adaptations to deep shade, offering insight into the evolutionary trajectory of photosynthesis in ancient vascular plants.