Masuda Shinji, Mizusawa Kazuki, Narisawa Takakuni, Tozawa Yuzuru, Ohta Hiroyuki, Takamiya Ken-Ichiro
Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology, Yokohama, 226-8501 Japan.
Plant Cell Physiol. 2008 Feb;49(2):135-41. doi: 10.1093/pcp/pcm177. Epub 2008 Jan 4.
The chloroplast, an essential organelle for plants, performs a wide variety of metabolic processes for host cells, which include photosynthesis as well as amino acid and fatty acid biosynthesis. The organelle conserves many bacterial systems in its functions, implicating its origin from symbiosis of a photosynthetic bacterium. In bacterial cells, the stringent response acts as a global regulatory system for gene expression mediated by a small nucleotide, guanosine 5'-diphosphate 3'-diphosphate (ppGpp), that is necessary for cell adaptation to diverse environmental stimuli such as amino acid starvation. Recent studies indicated that proteins similar to the bacterial ppGpp synthase/hydrolyase are conserved in plants, although their precise roles are not known. Here we show that the stringent response in chloroplasts is crucial for normal plant fertilization. Specifically, one of the Arabidopsis ppGpp synthase homologs, CRSH (Ca(2+)-activated RelA/SpoT homolog), exhibits calcium-dependent ppGpp synthesis activity in vitro, and is localized in chloroplasts in vivo. A knockdown mutation of CRSH in Arabidopsis results in a significant reduction in silique size and seed production, indicating that plant reproduction is under the control of chloroplast function through a ppGpp-mediated stringent response.
叶绿体是植物的重要细胞器，为宿主细胞执行多种代谢过程，包括光合作用以及氨基酸和脂肪酸的生物合成。该细胞器在功能上保留了许多细菌系统，这暗示其起源于光合细菌的共生。在细菌细胞中，严谨反应作为一种由小核苷酸鸟苷5'-二磷酸3'-二磷酸（ppGpp）介导的基因表达全局调控系统，这对于细胞适应多种环境刺激（如氨基酸饥饿）是必需的。最近的研究表明，植物中存在与细菌ppGpp合酶/水解酶相似的蛋白质，尽管其确切作用尚不清楚。在这里，我们表明叶绿体中的严谨反应对正常植物受精至关重要。具体而言，拟南芥ppGpp合酶同源物之一CRSH（钙激活的RelA/SpoT同源物）在体外表现出钙依赖性ppGpp合成活性，并且在体内定位于叶绿体中。拟南芥中CRSH的敲除突变导致角果大小和种子产量显著降低，表明植物繁殖受叶绿体功能通过ppGpp介导的严谨反应的控制。
