Abstract
Sigma factor (SIG) proteins contribute to promoter specificity of the plastid‐encoded RNA polymerase during chloroplast genome transcription. All six members of the SIG family, that is, SIG1–SIG6, are nuclear‐encoded proteins targeted to chloroplasts. Sigma factor 2 (SIG2) is a phytochrome‐regulated protein important for stoichiometric control of the expression of plastid‐ and nuclear‐encoded genes that impact plastid development and plant growth and development. Among SIG factors, SIG2 is required not only for transcription of chloroplast genes (i.e., anterograde signaling), but also impacts nuclear‐encoded, photosynthesis‐related, and light signaling‐related genes (i.e., retrograde signaling) in response to plastid functional status. Although SIG2 is involved in photomorphogenesis in Arabidopsis, the molecular bases for its role in light signaling that impacts photomorphogenesis and aspects of photosynthesis have only recently begun to be investigated. Previously, we reported that SIG2 is necessary for phytochrome‐mediated photomorphogenesis specifically under red (R) and far‐red light, thereby suggesting a link between phytochromes and nuclear‐encoded SIG2 in light signaling. To explore transcriptional roles of SIG2 in R‐dependent growth and development, we performed RNA sequencing analysis to compare gene expression in sig2‐2 mutant and Col‐0 wild‐type seedlings at two developmental stages (1‐ and 7‐day). We identified a subset of misregulated genes involved in growth, hormonal cross talk, stress responses, and photosynthesis. To investigate the functional relevance of these gene expression analyses, we performed several comparative phenotyping tests. In these analyses, strong sig2 mutants showed insensitivity to bioactive GA 3, high intracellular levels of hydrogen peroxide (H2O2) indicative of a stress response, and specific defects in photosynthesis, including elevated levels of cyclic electron flow (CEF) and nonphotochemical quenching (NPQ). We demonstrated that SIG2 regulates a broader range of physiological responses at the molecular level than previously reported, with specific roles in red‐light‐mediated photomorphogenesis.
Highlights
Functional chloroplasts contain both nuclear- and chloroplastencoded proteins
To investigate the functional relevance of these gene expression analyses to the observed red-light, Sigma factor protein 2 (SIG2)-dependent aspects of growth and development, we conducted a battery of phenotyping tests and determined that sig2 mutants show insensitivity to gibberellic acid (GA), high intracellular levels of hydrogen peroxide (H2O2) that are indicative of stress, and specific defects in photosynthetic performance, such as elevated levels of cyclic electron flow (CEF) and nonphotochemical quenching (NPQ)
We hypothesized that SIG2 is required for phytochrome-mediated photomorphogenesis and growth, in addition to known roles of SIG2 in plastid development (Oh & Montgomery, 2013)
Summary
Functional chloroplasts contain both nuclear- and chloroplastencoded proteins. light-dependent plastid development depends on coordinate control and stoichiometric expression of nuclear and plastid genes. To investigate the functional relevance of these gene expression analyses to the observed red-light, SIG2-dependent aspects of growth and development, we conducted a battery of phenotyping tests and determined that sig mutants show insensitivity to GA, high intracellular levels of hydrogen peroxide (H2O2) that are indicative of stress, and specific defects in photosynthetic performance, such as elevated levels of cyclic electron flow (CEF) and NPQ. To the NCBI Gene Expression Omnibus database (submission number GSE104518)
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