Center for Integrative Genomics, Faculty of Biology and Medicine, University of Lausanne, Genopode Building, 1015 Lausanne, Switzerland.
Plant Mol Biol. 2010 Aug;73(6):687-95. doi: 10.1007/s11103-010-9649-9. Epub 2010 May 15.
Higher plants possess multiple members of the phytochrome family of red, far-red light sensors to modulate plant growth and development according to competition from neighbors. The phytochrome family is composed of the light-labile phyA and several light-stable members (phyB-phyE in Arabidopsis). phyA accumulates to high levels in etiolated seedlings and is essential for young seedling establishment under a dense canopy. In photosynthetically active seedlings high levels of phyA counteract the shade avoidance response. phyA levels are maintained low in light-grown plants by a combination of light-dependent repression of PHYA transcription and light-induced proteasome-mediated degradation of the activated photoreceptor. Light-activated phyA is transported from the cytoplasm where it resides in darkness to the nucleus where it is needed for most phytochrome-induced responses. Here we show that phyA is degraded by a proteasome-dependent mechanism both in the cytoplasm and the nucleus. However, phyA degradation is significantly slower in the cytoplasm than in the nucleus. In the nucleus phyA is degraded in a proteasome-dependent mechanism even in its inactive Pr (red light absorbing) form, preventing the accumulation of high levels of nuclear phyA in darkness. Thus, light-induced degradation of phyA is in part controlled by a light-regulated import into the nucleus where the turnover is faster. Although most phyA responses require nuclear phyA it might be useful to maintain phyA in the cytoplasm in its inactive form to allow accumulation of high levels of the light sensor in etiolated seedlings.
高等植物拥有多种光敏色素家族成员，这些成员作为红光和远红光光感受器，根据来自邻居的竞争来调节植物的生长和发育。光敏色素家族由光不稳定的 phyA 和几个光稳定的成员（拟南芥中的 phyB-phyE）组成。phyA 在黄化幼苗中积累到高水平，对于密集冠层下幼苗的建立是必需的。在光合活性的幼苗中，高水平的 phyA 抵消了避荫反应。在光生长的植物中，phyA 水平通过光依赖性抑制 PHYA 转录和光诱导的蛋白酶体介导的激活光受体降解的组合来维持低水平。光激活的 phyA 从细胞质中运输，在黑暗中存在于细胞质中，在细胞核中需要进行大多数光敏色素诱导的反应。在这里，我们表明 phyA 通过依赖蛋白酶体的机制在细胞质和细胞核中降解。然而，细胞质中 phyA 的降解速度明显慢于细胞核中。在细胞核中，phyA 在依赖蛋白酶体的机制中降解，即使处于其非活性 Pr（红光吸收）形式，也可防止黑暗中细胞核中积累高水平的 phyA。因此，phyA 的光诱导降解部分受光调节的核内导入控制，在核内，周转率更快。尽管大多数 phyA 反应需要核内 phyA，但将 phyA 以非活性形式保持在细胞质中可能有助于在黄化幼苗中积累高水平的光传感器。
