The Sulfate Transporter SST1 Is Crucial for Symbiotic Nitrogen Fixation in Lotus japonicus Root Nodules

Abstract

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. Mapbased 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.

Tissue-specific down-regulation of LjAMT1;1 compromises nodule function and enhances nodulation in Lotus japonicus

“…5a) as well as their nodulation kinetics (Fig. 3d-f) are reminiscent of the phenotype exhibited by many of the nitrogen fixation-impaired plant mutants, which exhibit normal nodulation kinetics up to the time when nitrogen fixation usually begins but enhanced nodulation compared to the wild-type afterwards (Cordoba et al 2003;Gordon et al 1999;Suganuma et al 2003;Krusell et al 2005).…”

How rhizobial symbionts invade plants: the Sinorhizobium–Medicago model

“…Contrary to its name, sucrose synthase in pea nodules catabolizes sucrose to UDP-glucose and fructose, which is ultimately metabolized to malate and transported to bacteroids 101,105,115 . The L. japonicus sulphate transporter Sst1 is also required for nitrogen fixation and has been detected in symbiosome membranes 117,118 . Sulphate is required to form iron-sulphur clusters in nitrogenase subunits 119 .…”

Proteomic Analysis of the Soybean Symbiosome Identifies New Symbiotic Proteins*

“…Sulfur is a component of the metallo-clusters of nitrogenase, essential for the reduction of nitrogen, and must be actively transported across membranes (23). LjSST1 was identified from a fix Ϫ mutant in L. japonicus and complemented a yeast strain deficient in sulfate transport (23). LjSST1 is also one of the few transporters that has been previously identified on the SM through proteomic analysis (34).…”

“…Several additional transport processes on the SM have been identified, including proteins for transport of iron, zinc, calcium, and sulfate (22)(23)(24)(25)(26)(27). In addition, the movement of hydrogen ions has been reported through the activity of an H ϩ -ATPase (28 -30).…”