RNA editing implicated in chloroplast-to-nucleus communication

Abstract

Photosynthesis and respiration provide the chemical energy that sustains life on Earth. In eukaryotes, this essential metabolism is performed by chloroplasts and mitochondria. Although both of these organelles have small genomes that are remnants of their endosymbiotic origins, most of the proteins in these organelles are encoded by nuclear genes. One consequence of this distribution of genetic material is that many of the multisubunit protein complexes that are central to energy metabolism are composed of proteins encoded by nuclear genes and either chloroplast genes (photosynthesis) or mitochondrial genes (respiration). Thus, the coordination of nuclear and chloroplast (or mitochondrial) activities requires the anterograde flow of information from the nucleus and retrograde signaling back to the nucleus. Although we know that retrograde signals from chloroplasts (i.e., chloroplast signals) influence numerous chloroplastic and extrachloroplastic processes, we still have major gaps in our knowledge of this type of signaling (1, 2). RNA editing refers to processes that change the identities of nucleotides and processes that add or delete nucleotides from RNAs. RNA editing has been reported in viruses and diverse eukaryotes. In flowering plants, RNA editing converts cytidines to uridines in the RNAs transcribed from the chloroplast and mitochondrial genomes (3). In PNAS, Zhao et al. (4) make a strong case that RNA editing in chloroplasts contributes to a type of chloroplast-to-nucleus signaling defined by the genomes uncoupled ( gun ) mutants in Arabidopsis . The gun mutant screen was the first forward genetic screen developed that specifically interrogates chloroplast-to-nucleus signaling. The mutant alleles from this screen uncouple the expression of photosynthesis-associated nuclear genes (PhANGs) from chloroplast function. Chloroplasts develop from nonphotosynthetic proplastids during the development of photosynthetic organs such as cotyledons and leaves. Light is a major positive regulator of PhANG expression. When light-grown plants are treated with inhibitors or harbor mutant alleles that … [↵][1]1Email: larkin{at}mail.hzau.edu.cn. [1]: #xref-corresp-1-1