Genetic Interactions Reveal that Specific Defects of Chloroplast Translation are Associated with the Suppression of var2‐Mediated Leaf Variegation

Abstract

Arabidopsis thaliana L. yellow variegated (var2) mutant is defective in a chloroplast FtsH family metalloprotease, AtFtsH2/VAR2, and displays an intriguing green and white leaf variegation. This unique var2-mediated leaf variegation offers a simple yet powerful tool for dissecting the genetic regulation of chloroplast development. Here, we report the isolation and characterization of a new var2 suppressor gene, SUPPRESSOR OF VARIEGATION8 (SVR8), which encodes a putative chloroplast ribosomal large subunit protein, L24. Mutations in SVR8 suppress var2 leaf variegation at ambient temperature and partially suppress the cold-induced chlorosis phenotype of var2. Loss of SVR8 causes unique chloroplast rRNA processing defects, particularly the 23S-4.5S dicistronic precursor. The recovery of the major abnormal processing site in svr8 23S-4.5S precursor indicate that it does not lie in the same position where SVR8/L24 binds on the ribosome. Surprisingly, we found that the loss of a chloroplast ribosomal small subunit protein, S21, results in aberrant chloroplast rRNA processing but not suppression of var2 variegation. These findings suggest that the disruption of specific aspects of chloroplast translation, rather than a general impairment in chloroplast translation, suppress var2 variegation and the existence of complex genetic interactions in chloroplast development.

Down-regulation of specific plastid ribosomal proteins suppresses thf1 leaf variegation, implying a role of THF1 in plastid gene expression

“…Many suppressor genes identified for leaf variegation of thf1 and var2 function directly in plastid gene expression, suggesting that plastid gene expression is somehow disturbed in the mutants. At a posttranscriptional level, suppressor genes such as FUG1, SCO1, SVR1, SVR8, and PRPL11 function in rRNA processing and translation, which are two-coupled events in chloroplasts (Miura et al 2007;Yu et al 2008;Liu et al 2013;Wu et al 2013). Generally, the levels of mature rRNAs decrease, while their precursors over-accumulate in the suppressor lines of thf1 and var2.…”

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).…”

Balance between Cytosolic and Chloroplast Translation Affects Leaf Variegation

“…THF1 has been suggested to be a subunit of PSII and also involved in the plasma membrane G-protein-mediated sugar signaling (Keren et al, 2005;Huang et al, 2006). Interestingly, var2 and thf1 share many similar genetic suppressor loci, including genes for chloroplast Clp protease complexes and chloroplast ribosomal proteins, suggesting a functional relationship between these two proteins (Yu et al, 2008;Liu et al, 2013;Wu et al, 2013;Ma et al, 2015).…”