Triterpenoids in aerenchymatous phellem contribute to internal root aeration and waterlogging adaptability in soybean

Abstract 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.