Abstract
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 thf1 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 thf1 variegation was suppressed by a mutation in σ-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, thf1 at the early stage of chloroplast development. Interestingly, mutations in SIG2 but not in other σ-factors also suppressed thf1 leaf variegation. Furthermore, we found that leaf variegation of thf1 and var2 could be suppressed by several virescent mutations, including yellow seedling1, brz-insensitive-pale green2, and nitric oxide-associated protein1, indicating that virescent mutations suppress leaf variegation. Taken together, our results provide unique insights into thf1-mediated leaf variegation, which might be triggered by defects in plastid gene transcription.