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Plant and Cell Physiology Advance Access originally published online on January 4, 2008
Plant and Cell Physiology 2008 49(2):135-141; doi:10.1093/pcp/pcm177
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© The Author 2008. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

Rapid Paper

The Bacterial Stringent Response, Conserved in Chloroplasts, Controls Plant Fertilization

Shinji Masuda1,*, Kazuki Mizusawa1, Takakuni Narisawa2, Yuzuru Tozawa2,3, Hiroyuki Ohta4,5 and Ken-ichiro Takamiya1,5

1Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology, Yokohama, 226-8501 Japan
2Graduate School of Science and Engineering, Ehime University, Ehime, 890-8577 Japan
3Cell-Free Science and Technology Research Center, Ehime University, Ehime, 890-8577 Japan
4Center for Biological Resources and Informatics, Tokyo Institute of Technology, Yokohama, 226-8501 Japan
5Research Center for the Evolving Earth and Planets, Tokyo Institute of Technology, Yokohama, 226-8501 Japan

*Corresponding author: E-mail, shmasuda{at}bio.titech.ac.jp; Fax, +81-45-924-5823.


  Abstract

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 (Ca2+-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.

Keywords: Arabidopsis - Chloroplast - ppGpp - RelA - SpoT - Stringent response

Abbreviations: BAPTA, 1,2-bis(o-aminophenoxy)ethane-N,N,N,'N'-tetraacetic acid; CaMV, cauliflower mosaic virus; CRSH, Ca2+-activated RelA/SpoT homolog; GFP, green fluorescent protein; GUS, β-glucuronidase; IPTG, isopropyl-β-D-thiogalactopyranoside; LHCP, light-harvesting chlorophyll-binding protein; LSU, RUBISCO large subunit; (p)ppGpp, guanosine 5'-di(tri)phosphate 3'-diphosphate; RSH, RelA/SpoT homolog.

(Received November 30, 2007; Accepted December 18, 2007)
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