Plant Sigma Factors

Abstract

Abstract Photosynthesis within chloroplasts is crucial for ecosystem function and all agricultural productivity. Chloroplasts are a type of plastid and contain a small circular genome of prokaryotic origin. This genome encodes proteins crucial for chloroplast function and requires correct regulation of gene expression. One cellular mechanism regulating plastid gene transcription involves sigma factors, which in plants are nuclear‐encoded proteins forming part of the chloroplast transcriptional system. These are required for chloroplast gene promoter recognition and transcription initiation, with specific sigma factors thought to recognise specific chloroplast promoters. Plant sigma factors participate in the adjustment of chloroplast transcription in response to environmental fluctuations and during development. They appear to be ancient, originating from photosynthetic bacteria that were chloroplast ancestors, with an increase in sigma factor copy number during plant evolution providing an example of the evolution of cell signalling. We examine the origins, structure, function and environmental signalling by plant sigma factors. Key Concepts Chloroplasts contain a small circular genome that encodes essential components of the photosynthetic apparatus and machinery for plastid transcription/translation. Some higher plant sigma factors participate in the integration of environmental signals that regulate chloroplast gene transcription. Sigma factors are found in bacteria to higher plants. In plants, they are important regulators of chloroplast transcription. Higher plant sigma factors allow the nuclear control of chloroplast transcription, forming a signalling pathway from the nucleus to chloroplasts. Higher plant sigma factors are thought to have evolved from sigma factors of photosynthetic bacteria that were engulfed by eukaryotic cells during the evolution of chloroplasts. During plant evolution, they transferred to the nuclear genome.