Understanding chloroplast biogenesis using second-site suppressors of immutans and var2

Abstract

Chloroplast biogenesis is an essential light-dependent process involving the differentiation of photosynthetically competent chloroplasts from precursors that include undifferentiated proplastids in leaf meristems, as well as etioplasts in dark-grown seedlings. The mechanisms that govern these developmental processes are poorly understood, but entail the coordinated expression of nuclear and plastid genes. This coordination is achieved, in part, by signals generated in response to the metabolic and developmental state of the plastid that regulate the transcription of nuclear genes for photosynthetic proteins (retrograde signaling). Variegation mutants are powerful tools to understand pathways of chloroplast biogenesis, and over the years our lab has focused on immutans (im) and variegated2 (var2), two nuclear gene-induced variegations of Arabidopsis. im and var2 are among the best-characterized chloroplast biogenesis mutants, and they define the genes for plastid terminal oxidase (PTOX) and the AtFtsH2 subunit of the thylakoid FtsH metalloprotease complex, respectively. To gain insight into the function of these proteins, forward and reverse genetic approaches have been used to identify second-site suppressors of im and var2 that replace or bypass the need for PTOX and AtFtsH2 during chloroplast development. In this review, we provide a brief update of im and var2 and the functions of PTOX and AtFtsH2. We then summarize information about second-site suppressors of im and var2 that have been identified to date, and describe how they have provided insight into mechanisms of photosynthesis and pathways of chloroplast development.

A Putative Chloroplast Thylakoid Metalloprotease VIRESCENT3 Regulates Chloroplast Development in Arabidopsis thaliana

“…The Isolation of Three New Virescent Mutants-We have carried out large scale ethyl methanesulfonate (EMS) and activation tagging mutagenesis looking for genetic suppressors of the Arabidopsis variegation mutant yellow variegated (var2) (44). During the screening process, we isolated a mutant that displayed both a leaf variegation phenotype typical of var2-5 and an additional virescent phenotype, i.e.…”

FtsH facilitates proper biosynthesis of photosystem I in Arabidopsis thaliana

“…Indeed, FtsH2 has been observed to be attached to the thylakoid-associated polysomes, which are responsible for the translation of chloroplast-encoded membrane proteins like PsaA/B (Sirpiö et al, 2007). Furthermore, var2 suppressor screens have shown that the decreased translation rate caused by the secondary mutation in the var2 background leads to proper chloroplast development Putarjunan et al, 2013), which indicates that the functional role of FtsH2 is linked to the chloroplast translation machinery. Moreover, the chloroplast-encoded PSI assembly factor Ycf4 accumulated in high amounts in var1 and var2 plants (Fig.…”

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

“…Although significant progress has been made toward our understanding of molecular mechanisms on chloroplast development, it still remains largely unclear how plastid and nuclear gene expressions are coordinately regulated. Leaf-variegated mutants, which display green sectors and nongreen sectors in the same leaf, are recognized as an ideal system for investigating the mechanisms of chloroplast formation (Liu et al, 2010c;Putarjunan et al, 2013). The prominent characteristic of variegation mutants is that the severity of leaf variegation depends on environmental and developmental cues.…”

“…The prominent characteristic of variegation mutants is that the severity of leaf variegation depends on environmental and developmental cues. To date, a number of variegation mutants have been reported in Arabidopsis (Arabidopsis thaliana; for review, see Yu et al, 2007;Putarjunan et al, 2013). Among these mutants, variegated1 (var1), var2, and immutans (im) are well characterized.…”