Lhcb9-dependent photosystem I structure in moss reveals evolutionary adaptation to changing light conditions during aquatic-terrestrial transition

In plants and green algae, light-harvesting complexes I and II (LHCI and LHCII) constitute the antennae of photosystem I (PSI), thus effectively increasing the cross-section of the PSI core. The moss Physcomitrium patens (P. patens) represents a well-studied evolutionary intermediate between green algae and flowering plants. P. patens possesses at least three types of PSI with different antenna sizes. The largest PSI form (PpPSI-L) exhibits a unique organization found neither in flowering plants nor in algae. Its formation is mediated by the P. patens-specific LHC protein, Lhcb9. While previous studies revealed the overall architecture of the PpPSI-L, its assembly details and the relationship between different PpPSI types remain unclear. Here, we report a high-resolution structure of the PpPSI-L. We identified 14 PSI core subunits, one Lhcb9, one phosphorylated LHCII trimer, and eight LHCI monomers arranged as two belts. Our structural analysis established the essential role of Lhcb9 and the phosphorylated LHCII in stabilizing the complex. In addition, our results suggest that PpPSI switches between three different types, which share identical modules. This feature may contribute to the quick and dynamic adjustment of the light-harvesting capability of PSI under different light conditions.