Structural insights into light harvesting and photosystem II evolution in the basal streptophyte Mesostigma viride

The basal streptophyte Mesostigma viride represents a critical evolutionary transition toward land plants, yet the molecular basis of its ancestral light-harvesting strategy remains elusive. Here, we present a 2.25-angstrom resolution cryo-EM structure of the M. viride PSII-LHCII-LHCP supercomplex, revealing a unique C 2 S 2 M p2 L p2 -type architecture. Our structure reveals a nearly complete set of conserved PSII core subunits, indicating that much of the PSII structural framework found in terrestrial plants was established early in the streptophyte lineage. At the periphery, the complex features a specialized antenna module in which two ancestral LHCP monomers flank CP29. These ancestral antenna proteins bind an unusually high number of carotenoids, with seven carotenoids per subunit, and extensively incorporate marine-type keto carotenoids, siphonaxanthin, and siphonein, throughout the complex, thereby broadening absorption in the blue-green region. The canonical LHCII trimer exhibits a distinctive chlorophyll b macrocluster and a 96-femtosecond ultrafast energy transfer component. Together, these findings provide a high-resolution molecular framework for an evolutionary intermediate that combines high-capacity aquatic light harvesting with structural features potentially relevant to photoprotection during the transition toward terrestrial life.