Involvement of polyamines in the root nodule regulation of soybeans (Glycine max)

In leguminous plants, infection with rhizobia leads to the formation of root nodules that fix atmospheric nitrogen and supply it as ammonium to the host plant cells. The formation of nitrogen-fixing nodules is nutritionally beneficial, but excessive nodule production appears to be detrimental to the host legumes because of the resultant over-consumption of photosynthetic products. Therefore, the number of root nodules in leguminous plants is tightly regulated by shoot-root signaling known as a feedback or autoregulation of nodulation. Recently, we found that foliar application of polyamines and an effective inhibitor of polyamine biosynthesis regulates the number of nodules in soybean plants. In this review, we discuss the possible function of polyamines as a systemic regulator of nodule formation.