Takabayashi, A. | Search bioRxiv

bioRxiv2025

Evolutionary origin and functional mechanism of Lhcx in the diatom photoprotection

Diatoms are red-lineage algae that utilize the light-harvesting complex (LHC) subfamily Lhcx for photoprotection via non-photochemical quenching (NPQ); however, its evolutionary origin and molecular mechanism remain poorly understood. Through molecular phylogenetic analysis, we show that diatom Lhcxs and green algal Lhcsrs evolved from a common ancestor, with green plants subsequently acquiring them via horizontal gene transfer. To investigate the functional role of Lhcx1, we generated knockout mutants of Chaetoceros gracilis, a diatom with low Lhcx redundancy. The lhcx1 mutants nearly abolished NPQ, and time-resolved fluorescence measurements revealed that Lhcx1-mediated quenching occurs in energetically detached antenna complexes. Clear-native PAGE with Amphipol further indicated that CgLhcx1 interacts with the FCP L-dimer, functioning as a peripheral antenna for the C2S2M2 PSII-FCPII supercomplex. Notably, under high-light acclimation, lhcx1 mutants exhibited higher PSII effective quantum yields than wild type, attributable to reduced antenna size and enhanced carbon fixation capacity. The absence of NPQ accelerated high-light acclimation and was accompanied by increased xanthophyll accumulation, indicating that compensatory mechanisms can enhance overall photosynthetic efficiency. Together, these findings reveal the evolutionary origin of Lhcx/Lhcsr proteins and define the molecular basis of Lhcx1-mediated photoprotection in diatoms, providing fundamental insights into LHC-based photoprotection across photosynthetic lineages.

plant biology↗

bioRxiv2024

Arabidopsis SUFB and CLP protease regulate SUFBC2D to manipulate iron-sulfur cluster biosynthesis in chloroplast

Iron-sulfur (Fe-S) clusters are essential cofactors for Fe-S proteins. SUFBC2D complex is the scaffold responsible for Fe-S cluster assembly in chloroplasts. However, the regulatory mechanism on SUFBC2D remains elusive. In this study, we report that the transcription of SUFB responds rapidly to leaf senescence, whereas the transcription of SUFC and SUFD does not. Intriguingly, their protein contents remain stable during leaf senescence. We further found that leaf death was occurred only when SUFB RNAi was induced, and SUFB and SUFC contents decreased much faster in the SUFB-RNAi lines than in the SUFC-RNAi lines, indicating that SUFB had a faster turnover rate than SUFC. Moreover, overexpressing SUFB increased the contents of SUFC and SUFD, and SUFBC2D, whereas overexpressing SUFC did not increase SUFB and SUFD. Our findings reveal that SUFB stabilizes SUFC and SUFD via forming SUFBC2D, whereas SUFC lacks this function. Furthermore, SUFB expression was sharply downregulated when the plants were subjected to iron deficiency, whereas SUFC and SUFD expression was not. Interestingly, the contents of all three SUF members decreased, indicating that plants degrade SUFBC2D in response to iron deficiency by downregulating SUFB transcription. We subsequently studied the degradation mechanism of SUFBC2D. Our results indicated that all SUFs are substrates of the caseinolytic protease (CLP) because they all accumulated in the CLP impaired mutant, and they physically interact with CLPS1, the substrate recognition adaptor of CLP. Collectively, our findings provide novel insights into how plants regulate SUFBC2D complex via SUFB to adapt to leaf senescence and iron deficiency.

plant biology↗