The atypical kinase ABC1K1 interacts with the EXECUTER pathway to promote chloroplast biogenesis.

Multiple chloroplast-to-nucleus signaling pathways contribute to the regulation of chloroplast biogenesis during plant greening. Here, we provide evidence for the direct implication of the atypical kinase ABC1K1. ABC1K1 is required for sufficient plastoquinone (PQ) allocation to the photosynthetic electron transport chain. Unexpectedly, mutation of abc1k1 suppresses greening and results in pale cotyledons under red light. This phenotype was not observed in other photosynthetic mutants and points to a specific signaling defect. Under red light, abc1k1 accumulated EXECUTER1 (EX1), a trigger of singlet oxygen (1O2) signaling. Consistent with the role of the FTSH metalloprotease in chloroplast biogenesis and EX1 degradation, the ftsh2 mutant var2, mimicked the greening defect of abc1k1 and accumulated EX1 under red light. We propose that this novel ABC1K1-dependent signal is required for chloroplast biogenesis to progress in challenging light conditions.