Leaf variegation of Thylakoid Formationl is suppressed by mutations of specific σ-factors in Arabidopsis

Thylakoid Formation1 (THF1) has been shown to play roles in chloroplast development, resistance to excessive light, and chlorophyll degradation in Arabidopsis (Arabidopsis thaliana). To elucidate mechanisms underlying THF1-regulated chloroplast development, we mutagenized thfl seeds with ethyl methanesulfonate and screened second-site recessive mutations that suppress its leaf variegation phenotype. Here, we characterized a unique suppressor line, 42-6, which displays a leaf virescent phenotype. Map-based cloning and genetic complementation results showed that thfl variegation was suppressed by a mutation in s-FACTOR6 (SIG6), which is a plastid transcription factor specifically controlling gene expression through the plastid-encoded RNA polymerase. Northern-blot analysis revealed that plastid gene expression was down-regulated in not only 42-6 and sig6 but also, thfl at the early stage of chloroplast development. Interestingly, mutations in SIG2 but not in other s-factors also suppressed thfl leaf variegation. Furthermore, we found that leaf variegation of thfl and var2 could be suppressed by several virescent mutations, including yellow seedlingl, brz-insensitive-pale green2,and nitric oxide-associated proteinl, indicating that virescent mutations suppress leaf variegation. Taken together, our results provide unique insights into thfl-mediated leaf variegation, which might be triggered by defects in plastid gene transcription.