Department of Energy-Plant Research Laboratory, Michigan State University, East Lansing, MI 48824, USA.
J Exp Bot. 2013 Dec;64(18):5457-72. doi: 10.1093/jxb/ert308. Epub 2013 Sep 27.
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.
质体定位的西格玛因子（SIG）蛋白促进质体编码 RNA 聚合酶的特异性。SIG2 功能似乎是光培养拟南芥植物所必需的。尚未报道影响 SIG2 光诱导积累的特定光受体或依赖光的因素。这里描述了一种phytochromes 和核编码 SIG2 之间的分子联系，该联系特异性地影响红光（R）和远红光（FR）下的光形态建成。phyA 和 phyB 都促进 SIG2 转录物的积累。SIG2 的破坏导致在抑制下胚轴伸长和子叶扩张方面出现 R 和 FR 特异性缺陷，尽管在 B 或 W 光下，sig2 突变体没有检测到这些反应的损伤。SIG2 还影响 W 和 R 下的根伸长，以及编码phytochrome-interacting factor 的 PIF4 和编码phytochrome 发色团生物合成酶的 HY2 的 R 依赖性表达。虽然 SIG2 显然影响 phytochromobilin（PΦB）phytochrome 发色团的积累，但 sig2 突变体与 PΦB 突变体有很大的不同，主要是由于光形态建成中的波长特异性缺陷和破坏了一组不同的依赖 phytochrome 的反应。phytochromes 和 SIG2 之间的分子联系可能是协调基因表达以维持核编码 phytochrome 脱辅基蛋白和质体衍生的 PΦB 之间的化学计量的重要部分，phytochrome 发色团结合形成光活性 phytochrome，和/或依赖光的 SIG2 积累涉及协调核和质体之间成分的诱导光信号通路。
