Soyasaponins, a new class of root exudates in soybean (Glycine max)

Root exudates are plant metabolites secreted from the roots into the soil. These exudates are involved in many important biological processes, including acquisition of nutrients, defense, and signaling to rhizosphere bacteria, such as isoflavones of soybean crucial for the symbiosis with rhizobium. Less is known, however, about other types of root exudates. This study shows that soybean roots secrete large amounts of soyasaponins (triterpenoid glycosides) as root exudates. The soyasaponins are classified into four groups, with group A being the most secreted of these compounds, whereas DDMP soyasaponins being the group showing greatest accumulation in root tissues, suggesting a selection system for secreted compounds. Time-course experiments showed that the soyasaponin secretion peaked during early vegetative stages. Particularly, soyasaponin Ah was the major compound secreted by soybean roots, whereas deacetylated derivative Af was the major compound secreted specifically during VE stage. The secretion of soyasaponins containing glycosyl moieties is an apparent loss of photosynthates. This phenomenon has been also observed in other legume species, although the composition of secreted soyasaponins is plant species-dependent. The identification of triterpenoid saponins as major metabolites in legume root exudates will provide novel insights into chemical signaling in the rhizosphere between plants and other organisms.