Abstract
Light perception by photoreceptors impacts plastid transcription, development, and differentiation. This photoreceptor-dependent activity suggests a mechanism for photoregulation of gene expression in the nucleus and plastid that serves to coordinate expression of critical genes of these two organelles. This coordinate expression is required for proper stoichiometric accumulation of components needed for assembly of plastids, photosynthetic light-harvesting complexes and components such as phytochromes. Chloroplast-targeted sigma factors, which function together with the plastid-encoded RNA polymerase to regulate expression of plastid-encoded genes, and nuclear-encoded plastid development factors, such as GLK1 and GLK2, are targets of phytochrome regulation. Such phytochrome-dependent functions are hypothesized to allow light-dependent regulation, and feasibly tuning, of plastid components and function in response to changes in the external environment, which directly affects photosynthesis and the potential for light-induced damage. When the size and protein composition of the light-harvesting complexes are not tuned to the external environment, imbalances in electron transport can impact the cellular redox state and cause cellular damage. We show that phytochromes specifically regulate the expression of multiple factors that function to modulate plastid transcription and, thus, provide a paradigm for coordinate expression of the nuclear and plastid genomes in response to changes in external light conditions. As phytochromes respond to changes in the prevalent wavelengths of light and light intensity, we propose that specific phytochrome-dependent molecular mechanisms are used during light-dependent signaling between the nucleus and chloroplast during photomorphogenesis to coordinate chloroplast development with plant developmental stage and the external environment.
Highlights
The involvement of regulatory factors or transcriptional regulators in organismal responses to environmental signals is well known
PHYTOCHROMES IMPACT THE EXPRESSION OF MULTIPLE NUCLEAR-ENCODED GENES ENCODING CHLOROPLAST-TARGETED SIGMA FACTORS We previously demonstrated that the expression of chloroplasttargeted transcriptional regulator SIG2 is regulated by phytochromes (Oh and Montgomery, 2013)
The level of SIG6 mRNA is phytochrome dependent, as its accumulation was significantly lower in a transgenic Arabidopsis line depleted of mesophyll-localized phytochromes through CAB3-promoter-driven expression of a gene encoding a phytochrome chromophore-degrading enzyme biliverdin reductase (BVR) (Figure 1A)
Summary
The involvement of regulatory factors or transcriptional regulators in organismal responses to environmental signals is well known. Additional studies demonstrated that both blue light-responsive cryptochrome and red/far-red light-responsive phytochrome photoreceptors perceive light and stimulate plastid development (Thum et al, 2001), and phytochromes are involved in regulating chloroplast gene transcription in mature leaves (Chun et al, 2001). Consistent with these observations, phytochrome-deficient mutants, including phyB (Reed et al, 1994) and chromophore-deficient hy and hy mutants (Chory et al, 1989), exhibit defects in chloroplast development and/or differentiation. Insights into the identity and functions of specific photoreceptor-dependent effectors that impact chloroplast development and function are limited
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