Arabidopsis thaliana PGR7 encodes a conserved chloroplast protein that is necessary for efficient photosynthetic electron transport.

Hou-Sung Jung,Krishna K Niyogi,Marinus Pilon,Yuki Okegawa,Elizabeth Kellogg,Kimmen Sjölander,Toshiharu Shikanai,Salah E Abdel-Ghany,Patrick M Shih,Markus Grebe

doi:10.1371/journal.pone.0011688

Abstract

A significant fraction of a plant's nuclear genome encodes chloroplast-targeted proteins, many of which are devoted to the assembly and function of the photosynthetic apparatus. Using digital video imaging of chlorophyll fluorescence, we isolated proton gradient regulation 7 (pgr7) as an Arabidopsis thaliana mutant with low nonphotochemical quenching of chlorophyll fluorescence (NPQ). In pgr7, the xanthophyll cycle and the PSBS gene product, previously identified NPQ factors, were still functional, but the efficiency of photosynthetic electron transport was lower than in the wild type. The pgr7 mutant was also smaller in size and had lower chlorophyll content than the wild type in optimal growth conditions. Positional cloning located the pgr7 mutation in the At3g21200 (PGR7) gene, which was predicted to encode a chloroplast protein of unknown function. Chloroplast targeting of PGR7 was confirmed by transient expression of a GFP fusion protein and by stable expression and subcellular localization of an epitope-tagged version of PGR7. Bioinformatic analyses revealed that the PGR7 protein has two domains that are conserved in plants, algae, and bacteria, and the N-terminal domain is predicted to bind a cofactor such as FMN. Thus, we identified PGR7 as a novel, conserved nuclear gene that is necessary for efficient photosynthetic electron transport in chloroplasts of Arabidopsis.

Highlights

The photosynthetic apparatus of plants consists of several large multiprotein thylakoid membrane complexes that are composed of proteins encoded by both the nuclear and chloroplast genomes
The pgr7 mutant was isolated in a video imaging screen for mutants that are defective in nonphotochemical quenching of chlorophyll fluorescence (NPQ) during illumination with high light, but the primary defect of the mutant appears to be in photosynthetic electron transport rather than in the NPQ machinery
Besides low NPQ, the plant size of pgr7 was smaller than that of the wild type (Figure 2), and the chlorophyll content was lower in pgr7 compared to the wild type (Table 1)

Summary

Introduction

The photosynthetic apparatus of plants consists of several large multiprotein thylakoid membrane complexes that are composed of proteins encoded by both the nuclear and chloroplast genomes. A number of nuclear genes are involved in expression of the chloroplast genome at multiple levels, including transcription, mRNA maturation, translation, targeting, and assembly of complexes that comprise a functional photosynthetic electron transport system. The low pH drives proton binding to photosystem II (PSII) components including the PsbS protein [5,6]. Both protonation and Z binding cause conformational changes that are involved in the thermal dissipation, which can be quantified by measuring nonphotochemical quenching of chlorophyll fluorescence (NPQ) [3]

Methods

Results

Conclusion