Lipid metabolism and autophagy maintain cellular homeostasis under chloroplast dysfunction in Chlamydomonas reinhardtii

Lipid rearrangement is a fundamental adaptive response to cellular stress. This study elucidates the global lipid metabolic adjustments in response to chloroplast dysfunction following the depletion of Orf2971, a plastid-encoded protein essential for protein import in Chlamydomonas reinhardtii. We utilized an integrated approach, combining lipidomics, fluorescence/electron microscopy, flow cytometry, and biochemical assays to comprehensively profile lipid dynamics, organelle ultrastructure, and gene and protein expression after Orf2971 depletion in Chlamydomonas reinhardtii. Orf2971 depletion triggered biphasic triacylglycerol (TAG) dynamics: initial accumulation via de novo synthesis followed by autophagic degradation (lipophagy). Concurrently, elevated cardiolipin drove mitochondrial remodeling with associated morphological changes, enhanced respiratory capacity, and upregulated cardiolipin-translocating genes, indicating the potential induction of mitophagy. Our study reveals a coordinated inter-organellar adaptation to chloroplast stress, where rapid TAG turnover via lipophagy and cardiolipin-mediated mitochondrial enhancement collectively compensate for compromised chloroplast function to maintain cellular homeostasis.