Evolutionary origin and functional mechanism of Lhcx in the diatom photoprotection
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Diatoms, a group of red-lineage algae that possess plastids derived from red algae through secondary endosymbiosis, utilize the light-harvesting complex (LHC) subfamily Lhcx for photoprotection; however, its evolutionary origin and functional mechanism remain unclear.
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To address these questions, we performed molecular phylogenetic analysis of Lhcx/Lhcsr subfamilies and produced the Lhcx1 knockout mutants of the diatom Chaetoceros gracilis , which has low Lhcx redundancy.
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Phylogenetic analysis revealed that diatom Lhcxs and the green algal Lhcsrs evolved from a common ancestor, with green plants acquiring them through horizontal gene transfer. The Lhcx1 mutants of C. gracilis mostly abolished non-photochemical quenching (NPQ), and the time-resolved fluorescence measurements clarified that Lhcx1-mediated quenching occurs in the energetically detached antenna complexes. Consistently, clear-native PAGE using Amphipol suggested that CgLhcx1 would interact with FCP L-dimer, functioning as a peripheral antenna for C 2 S 2 M 2 PSII–FCPII. Notably, the lhcx1 mutant exhibited a higher PSII effective quantum yield than wild-type (WT) under high-light acclimation, attributed to reduced antenna size and enhanced carbon fixation capacity. The lack of NPQ accelerated high-light acclimation, with increased xanthophylls in lhcx1 , indicating that compensatory mechanisms can lead to enhanced photosynthetic efficiency.
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Our findings provide novel insights into the origin and molecular mechanisms of photoprotection in diatoms.
