An unrecognized and crucial role of chloroplast division in leaf variegation in Arabidopsis thaliana

Leaf variegation provides a valuable model for elucidating the molecular mechanisms that underlie chloroplast biogenesis, including both chloroplast development and division. While there has been notable advancement in comprehending the role of the Arabidopsis Yellow Variegated2 (VAR2), which encodes the AtFtsH2 subunit of the thylakoid FtsH metalloprotease complex, the exact mechanism governing leaf variegation formation remains elusive. In this study, time-course microscopy analyses of chloroplast number per cell from the first leaf pair revealed that most var2 cells lack chloroplasts. Interestingly, chloroplast biogenesis in the green sector of var2 leaves is significantly delayed but prolonged compared to the wild type (WT), leading to increased heterogeneity in chloroplast number and size among cells. These results suggest that chloroplast biogenesis in the green sector can partially compensate for the defect in the white sector. Additionally, approximately 15% of cells in var2 white sectors are devoid of plastids, and chloroplast number in guard cells is significantly lower in var2 than in WT, indicating that VAR2 mutations impair chloroplast division. Consistently, the var2 phenotype is exacerbated by knocking out key plastid division genes, including Paralog of Accumulation and Replication of Chloroplasts6 (PARC6) and Plastid Division1 (PDV1), but is fully rescued by overexpressing PDV1 or PDV2. Furthermore, VAR2 mutations inhibit etioplast development, while accelerating chloroplast biogenesis by knocking out Constitutively Photomorphogenic1 (COP1) rescues the var2 phenotype. Together, our findings reveal a novel role for VAR2 in plastid division and its essential function in chloroplast biogenesis, providing new insights into the mechanism underlying variegated leaf formation.