First published online April 1, 2005; 10.1105/tpc.104.030106
The Plant Cell 17:1625-1636 (2005)
© 2005 American Society of Plant Biologists
The Sulfate Transporter SST1 Is Crucial for Symbiotic Nitrogen Fixation in Lotus japonicus Root Nodules
Lene Krusella,
Katja Krausea,
Thomas Otta,
Guilhem Desbrossesa,
Ute Krämera,
Shusei Satob,
Yasukazu Nakamurab,
Satoshi Tabatab,
Euan K. Jamesc,
Niels Sandald,
Jens Stougaardd,
Masayoshi Kawaguchie,
Ai Miyamotof,
Norio Suganumaf and
Michael K. Udvardia,1
a Max Planck Institute of Molecular Plant Physiology, 14476 Golm, Germany
b Kazusa DNA Research Institute, Kisarazu, Chiba 292-0812, Japan
c School of Life Sciences, University of Dundee, Dundee DD1 5EH, United Kingdom
d Department of Molecular Biology, University of Aarhus, DK-8000 Aarhus, Denmark
e Department of Biological Sciences, University of Tokyo, Hongo, Tokyo 113-0033, Japan
f Department of Life Science, Aichi University of Education, Kariya, Aichi 448-8542, Japan
1 To whom correspondence should be addressed. E-mail udvardi{at}mpimp-golm.mpg.de; fax 49-331-567-8250.
Symbiotic nitrogen fixation (SNF) by intracellular rhizobia within legume root nodules requires the exchange of nutrients between host plant cells and their resident bacteria. Little is known at the molecular level about plant transporters that mediate such exchanges. Several mutants of the model legume Lotus japonicus have been identified that develop nodules with metabolic defects that cannot fix nitrogen efficiently and exhibit retarded growth under symbiotic conditions. Map-based cloning of defective genes in two such mutants, sst1-1 and sst1-2 (for symbiotic sulfate transporter), revealed two alleles of the same gene. The gene is expressed in a nodule-specific manner and encodes a protein homologous with eukaryotic sulfate transporters. Full-length cDNA of the gene complemented a yeast mutant defective in sulfate transport. Hence, the gene was named Sst1. The sst1-1 and sst1-2 mutants exhibited normal growth and development under nonsymbiotic growth conditions, a result consistent with the nodule-specific expression of Sst1. Data from a previous proteomic study indicate that SST1 is located on the symbiosome membrane in Lotus nodules. Together, these results suggest that SST1 transports sulfate from the plant cell cytoplasm to the intracellular rhizobia, where the nutrient is essential for protein and cofactor synthesis, including nitrogenase biosynthesis. This work shows the importance of plant sulfate transport in SNF and the specialization of a eukaryotic transporter gene for this purpose.
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