ATYCO is a crucial factor for thylakoid formation and photosynthesis regulation in Arabidopsis

Chloroplast development underpins plant growth, by not only enabling photosynthesis but also facilitating other essential biochemical processes. Despite this, the regulatory mechanisms and functional components of chloroplast development remain largely uncharacterized due to their complexity. In this study, we identified a plastid-targeted gene, ATYCO, as a critical factor in early chloroplast development of Arabidopsis. ATYCO knock-out mutant (yco) exhibited a seedling-lethal, albino phenotype, resulting from dysfunctional chloroplasts lacking thylakoid membranes. In contrast, YCO knock-down mutants produced a chlorophyll-deficient cotyledon and normal leaves when supplemented with sucrose, suggesting a differential role of ATYCO in cotyledons and true leaves. Transcription analysis revealed that ATYCO deficiency could be partially compensated by sucrose supplementation. In the YCO mutant, the transcript levels of plastid-encoded polymerase (PEP)-dependent genes and nuclear-photosynthetic genes, as well as the accumulation of photosynthetic proteins, were significantly reduced. Moreover, the chlorophyll-deficient phenotype in YCO knock-down line can be effectively suppressed by inhibition of PSI cyclic electron transport activity, implying an interaction between YCO and PSI cyclic electron transport. Taken together, our findings de underscore the vital role of ATYCO in early chloroplast development and photosynthesis.