Dynamic regulation of the chloroplast SUFBC2D complex by SUFB expression and CLP protease in Arabidopsis

Chloroplast iron-sulfur (Fe-S) clusters, essential cofactors for Fe-S proteins, are assembled by the sulfur utilization factor (SUF) BC2D complex, but regulation of their biogenesis remains unclear. We show that during leaf senescence, SUFB transcription rapidly increases while SUFC/SUFD transcripts and the abundance of all three SUF proteins remain unchanged. Leaf necrosis with green color occurs solely when SUFB RNA interference (RNAi) is induced, and the decrease of SUFs is obviously faster in SUFB-RNAi than SUFC-RNAi lines. Furthermore, SUFB exhibits a higher turnover rate than SUFC/SUFD, which accelerates during senescence. Overexpressing SUFB elevates SUFC/SUFD/SUFBC2D levels, revealing that SUFB stabilizes SUFC/SUFD via SUFBC2D complex formation, a function SUFC lacks. Under iron deficiency, SUFB transcript level declines, whereas SUFC/SUFD transcript levels remain unchanged. Nevertheless, abundance of SUFs decreases, suggesting that the reduced amount of SUFBC2D is mediated by the down-regulation of SUFB transcription. We subsequently found that SUFs accumulated in a caseinolytic protease (CLP)-impaired mutant and exhibited resistance to degradation when cytoplasmic translation was inhibited by cycloheximide, revealing that their turnover is mediated by CLP. Moreover, SUFB, SUFC, and SUFD physically interact with CLPS1, a substrate adaptor of the CLP complex. Collectively, SUFB expression and CLP coordinately regulate Fe-S biogenesis by controlling SUFBC2D abundance, providing critical insights into plant adaption to leaf senescence and iron deficiency.