Nitrate transport pathway into soybean [Glycine max] nodules traced by tungstate and 15NO3(-)

Tungstate (WO4(2-)) was used as an anion tracer in soybean plants, and the distribution of tungsten (W) in roots and nodules was examined by electron probe X-ray microanalysis (EPMA) and chemical analysis. At 3 d after treatment with 1 mM tungstate, a large amount of W accumulated in root cortex, while the movement of W into the nodules was limited. It is suggested that external anions cannot be readily transported into the nodule cortical region via the apoplasm. In contrast, after treatment with 1.7 mM 15NO3(-) for 24 h, a significantly large amount of NO3(-) and 15N was distributed in the nodule cortical region, although the amount was negligible in the infected region. These results suggest that a low concentration of nitrate can be absorbed from the nodule surface into the cytoplasm, by transport from cell to cell via symplastic pathway through plasmodesmata, and can accumulate in the nodule cortical region. By NO3(-) addition, the ureide concentration in the infected region decreased to one-third of the value of the control nodules which were not treated with NO3(-), the NO3(-) treatment also decrease the glutamine level both in the infected and cortical regions, while the concentrations of glutamic acid and asparagine increased. The changes in the concentration of the N constituents in the infected region may be caused by nitrate accumulation in the cortical region, possibly through the restriction of O2 supply. NO3(-) application increased the asparagine level in the roots. The distribution of the minerals in the nodule was examined by EPMA. The concentrations of N and P were higher but those of K and Cl were lower in the infected region compared to the nodule cortical region. Ca was locally distributed in the surface, sclerenchyma cells, and inner cortex, but the content was low in the infected region, Mg specifically accumulated in the inner and outer cortex inside the sclerenchyma cells but not outside them