Nitrate transport pathway into soybean nodules traced by tungstate and¹⁵NO₃^-

Abstract

Tungstate (WO42-) 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 1~NO3-for 24 h, a significantly large amount of NO3-and lSN 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 decreased 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 O~ 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 C1 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.It is well known that combined N especially the NO3-form inhibits all phases of nodulation and the N2 fixation process, although the mechanisms have not been fully elucidated (Streeter 1988). NO3-inhibits the early steps of infection initiation, such as initial cortical cell divisions and infection thread formation, also retards the nodule growth,

Abscisic acid relations of plants grown on tungsten enriched substrates

“…This agrees with findings of Mizukoshi et al (1995) who observed that after a 3 days treatment of soybean plants most of the tungsten taken up is deposited in the root cortex. Taking that in mind one should expect the most distinct tungsten effects in the roots.…”

Involvement of Ureides in Nitrogen Fixation Inhibition in Soybean1

“…Therefore, it seems more likely that a catabolic product of ureide resulting from the high concentrations of ureides in the shoot (Table I) might be more closely associated with the triggering mechanism for the observed feedback response. Such a conclusion is consistent with the suggestion of Mizukoshi et al (1995) and Bacanamwo and Harper (1997) that Asn might be closely linked to a feedback inhibition of nitrogenase activity. The possibility of a shoot-derived signal on nitrogenase activity has also been indicated in other experiments.…”

“…Neo and Layzell (1997) suggested that Gln was the most likely candidate for the feedback signal compound. However, nitrate inhibition studies with soybean implicated changes in Asn concentration in the regulation of N 2 -fixation activity (Mizukoshi et al, 1995;Bacanamwo and Harper, 1997).…”

Nitrate inhibits soybean nodulation by regulating expression of CLE genes

“…Nodule growth and N 2 fixation activity were suppressed only in the nodules formed on the roots treated with nitrate. This might be due to a high accumulation of nitrate in those roots and nodules [24], especially in a cortical part [25]. In contrast to local regulation, some studies have revealed that nitrate systemically controls nodulation and N 2 fixation under the presence of a high concentration of nitrate [26] or the continuous supply of low concentration (1 mM) of nitrate [24].…”