Leaf Variegation of Thylakoid Formation1 Is Suppressed by Mutations of Specific σ-Factors in Arabidopsis

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 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, thf1 at the early stage of chloroplast development. Interestingly, mutations in SIG2 but not in other s-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.

Disruption of carotene biosynthesis leads to abnormal plastids and variegated leaves in Brassica napus

“…PRPS9 and PSRP5 are plastid ribosomal proteins that are second-site mutations and suppress leaf variegation of thf1-1. SIG6 is a plastid transcription factor specifically controlling gene expression through the plastid-encoded RNA polymerase and the mutation of SIG6 suppresses thf1 variegation (Hu et al 2015). Genetic screening for the second-site suppressor lines of thf1 and var2 suggested that the reduced rate of plastid protein biosynthesis is important for chloroplast development in variegated leaves (Ma et al 2015b).…”

Chloroplast Proteases: Updates on Proteolysis within and across Suborganellar Compartments

“…2), are detected in the green but not the white leaf areas, specifically in the chloroplasts, of the var2 mutant grown even under normal light conditions, indicative of its persistent photooxidative stress (Kato et al, 2009). A series of genetic studies in Arabidopsis identified trans-acting factors suppressing var1/var2 leaf variegation; these included Clp subunits, translation factors, a pentatricopeptide repeat protein, a pseudouridine synthase homolog, a plastid transcriptionally active chromosome component, a prokaryotic-like peptide deformylase, ribosomal proteins, circularly permuted GTPase family proteins, and a sigma factor Miura et al, 2007;Yu et al, 2008Yu et al, , 2011Liu et al, 2010aLiu et al, , 2010bLiu et al, , 2013Adam et al, 2011;Wu et al, 2013;Powikrowska et al, 2014;Hu et al, 2015;Ma et al, 2015;Qi et al, 2016). Several models have been proposed to explain leaf variegation suppression, but the precise mechanism remains elusive (Miura et al, 2007;Yu et al, 2008).…”

Essentials of Proteolytic Machineries in Chloroplasts

“…The absence of thylakoid FtsH2/5 (but not FtsH1/8) induces variegated leaf phenotype (Sakamoto et al, 2003;Zhang et al, 2010), probably independent of D1 degradation . The leaf variegation is suppressed by defects in the gene expression apparatus, translation machineries, or other protease components in the chloroplast (Park and Rodermel, 2004;Miura et al, 2007;Yu et al, 2008;Liu et al, 2010aLiu et al, , 2010bAdam et al, 2011;Yu et al, 2011;Liu et al, 2013;Wu et al, 2013;Powikrowska et al, 2014;Hu et al, 2015;Ma et al, 2015;Qi et al, 2015). Constitutive activation of a G-protein a subunit also can suppress the var phenotype (Zhang et al, 2009).…”