Biogenesis of Peribacteroid Membrane (PBM) Forming a Subcellular Compartment Essential for Symbiotic Nitrogen Fixation

Concomitant with the release of rhizobia from infection thread, rapid proliferation of endomembrane system occurs, which encloses the invading bacteria in a membrane envelope, peribacteroid membrane (PBM). Although derived from the plasma membrane, the PBM shares features common to both plasma membrane and vacuolar membrane and appears to be a mosaic membrane. We have initiated studies to search for plant genes involved in vectorial transport of proteins to PBM and the peribacteroid space. In yeast, several small GTP-binding proteins mediate protein transfer from the ER to the Golgi. We have isolated soybean homologs of yeast YPT1 and sarl genes and mammalian rab7 genes. The soybean YPT1 homolog (SOYPT1) was shown to complement a yeast ypt1 mutant. Transgenic nodules containing antisense SOYPT1 under a leghemoglobin (Lbc3) promoter were developed on soybean hairy roots. These nodules showed retarded growth and reduced nitrogen fixation activity. Early senesence of PBM occurs in these nodules releasing bacteria inside the vacuole. We have determined topology of a major PBM nodulin, nodulin-26, and have established minimum sequences required for targeting this protein to PBM. We have shown that expression of nodulin-26 gene is negatively controlled in soybean roots and this control is not operative in heterologous transgenic plants.