Abstract
Chloroplast-localized sigma factor (SIG) proteins promote specificity of the plastid-encoded RNA polymerase. SIG2 function appears to be necessary for light-grown Arabidopsis thaliana plants. Specific photoreceptors or light-dependent factors that impact the light-induced accumulation of SIG2 have not been reported. A molecular link between phytochromes and nuclear-encoded SIG2, which impacts photomorphogenesis specifically under red (R) and far-red (FR) light, is described here. Both phyA and phyB promote SIG2 transcript accumulation. Disruption of SIG2 results in R- and FR-specific defects in the inhibition of hypocotyl elongation and cotyledon expansion, although no impairments in these responses are detected for sig2 mutants under blue (B) or white (W) light. SIG2 also impacts root elongation under W and R, and the R-dependent expression of PIF4, encoding a phytochrome-interacting factor, and HY2, which encodes a phytochrome chromophore biosynthetic enzyme. Whereas SIG2 apparently impacts the accumulation of the phytochromobilin (PΦB) phytochrome chromophore, sig2 mutants differ significantly from PΦB mutants, primarily due to wavelength-specific defects in photomorphogenesis and disruption of a distinct subset of phytochrome-dependent responses. The molecular link between phytochromes and SIG2 is likely to be an important part of the co-ordination of gene expression to maintain stoichiometry between the nuclear-encoded phytochrome apoprotein and plastid-derived PΦB, which combine to form photoactive phytochromes, and/or light-dependent SIG2 accumulation is involved in an inductive light signalling pathway co-ordinating components between nucleus and plastids.
Highlights
The establishment of photosynthesis during plant development requires the assembly of functional chloroplasts, which is a light-regulated process requiring fine co-ordination of the expression of nuclear- and chloroplast-encoded genes
The role of SIG2 in the regulation of plastid gene expression and retrograde regulation of nuclear-encoded photosynthesis-associated genes is not light dependent, i.e. the expression of Photosynthetic Associated Nuclear Genes (PhANGs) and plastid-encoded transcripts was downregulated in dark-grown sig2 mutant seedlings (Woodson et al, 2013), expression of SIG2 is impacted by phytochromes (Fig. 1) and light (Fig. 2)
SIG2 impacts hypocotyl elongation (Fig. 4) and root elongation (Fig. 6) in a lightdependent manner, i.e. the lengths of hypocotyls or roots of sig2 mutant seedlings did not differ from WT in complete darkness
Summary
The establishment of photosynthesis during plant development requires the assembly of functional chloroplasts, which is a light-regulated process requiring fine co-ordination of the expression of nuclear- and chloroplast-encoded genes. In recent studies investigating the function of mesophyll-localized phytochromes, it was noted that both RbcL and RbcS protein levels were reduced in lines depleted of phytochromes, which. Phytochromes are photoreceptors that consist of two distinct components, i.e. a nuclear-encoded apoprotein and a plastid-synthesized linear tetrapyrrole chromophore. A single chromophore exists, i.e. phytochromobilin (PΦB), which is synthesized fully in the plastid and exported to the cytosol, where it becomes covalently attached to all of the apoproteins to produce photoactive holophytochromes (Terry et al, 1993)
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