Curled root hairs are a site of entry for Bradyrhizobium infecting hydroponically grown soybean plants with mature root systems

Abstract

Early emergent nodules on roots of hydroponically grown soybean plants (Glycine max (L.) Merr.) were sectioned serially to locate the site of infection by Bradyrhizobium japonicum. The plants had been inoculated only after their root systems had produced numerous higher order branches. The hydroponic solutions contained all required nutrients, including either a suboptimal concentration of nitrate (0.5 mM) or an excess of nitrate (4.0 mM). In all six nodules examined, three with suboptimal nitrate and three with excess nitrate, we found a centrally located root hair containing an infection thread. We conclude that mature root systems of soybean grown in aqueous culture can undergo infection through root hairs in the way that is typical of young seedlings grown either in pots of vermiculite or pouches.