Polyamines as potential regulators of nutrient exchange across the peribacteroid membrane in soybean root nodules

The effect of cytoplasmic polyamines on peribacteroid membrane transport processes in soybean (Glycine max L.) was investigated. The concentration of free polyamines in soybean nodule cytoplasm has been estimated by others to be in the micromolar range. The H⁺-ATPase was inhibited by 37 and 54% by 200 μM spermidine and putrescine, respectively. Spermine applied to the cytoplasmic face of the peribacteroid membrane was found to inhibit both inward and outward currents through a non-selective cation channel permeable to ammonium (K_d 2.1 μM at - 100 mV). Malate transport into intact symbiosomes was reduced by 15-30% by 15 mM spermidine, cadaverine and putrescine. A non-specific stimulation of malate transport by polycations was found to occur at concentrations in the micromolar range. The results suggest that polyamines can affect all the peribacteroid membrane transport processes tested. In particular, we conclude that the combined inhibitory effects of polyamines on the ATPase and the ammonium channel have the potential to reduce nitrogen supply to the plant in vivo. The possibility of competing polyamine and ureide synthesis in the nodule is discussed.