Aerenchymatous phellem (AP) is important for internal aeration and adaptation to waterlogging in plants. Herein, the extensive accumulation of triterpenoids such as lupeol and betulinic acid was identified in AP. However, the biological and physiological roles of these triterpenoids in plants are largely unknown. Lupeol is converted from 2,3-oxidosqualene by lupeol synthase (LUS) and oxidized to betulinic acid. Functional analysis of LUS genes in soybean revealed that GmLUS1 is crucial for triterpenoid biosynthesis in AP. Lupeol and betulinic acid were found to be the major components of epicuticular wax on the surface of AP cells, and they contributed to tissue hydrophobicity and oxygen transport to roots. Additionally, the lus1 mutant produced a shallow root system due to less oxygen transport via AP under waterlogged conditions. In conclusion, triterpenoid accumulation in AP aids internal aeration and root development for adaptation to waterlogging.

Summary Soybeans (Glycine max) develop newly differentiated aerenchymatous phellem (AP) in response to waterlogging stress. AP is formed in the hypocotyl and root, thus contributing to internal aeration and adaptation to waterlogging for several legumes. Extensive accumulation of triterpenoids – lupeol and betulinic acid – has been identified in AP. However, their physiological roles in plants remain unclarified. Lupeol is converted from 2,3‐oxidosqualene by lupeol synthase (LUS) and oxidized to betulinic acid. Notably, soybeans have two LUS genes (GmLUS1 and GmLUS2). Functional analysis was performed to reveal the biological and physiological functions of triterpenoids in AP using lus mutants. The AP cells of lus1 mutant lacked triterpenoid accumulation and epicuticular wax. Lupeol and betulinic acid were the major components of epicuticular wax and contributed to tissue hydrophobicity and oxygen transport to the roots. Tissue porosity in AP was lower in the lus1 mutant than in the wild‐type, which resulted in reduced oxygen transport to the roots via AP. This reduction in oxygen transport resulted in shallow root systems under waterlogged conditions. Triterpenoid accumulation in AP contributes to effective internal aeration and root development for adaptation to waterlogging, suggesting the significance of triterpenoids in improving waterlogging tolerance.