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

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.

Sucrose supply from leaves is required for aerenchymatous phellem formation in hypocotyl of soybean under waterlogged conditions.

Background and Aims: Soil waterlogging often causes oxygen deficiency in the root systems of plants and severely inhibits plant growth. Formation of aerenchyma - interconnected spaces that facilitate the movement of gases between and within the aerial and submerged parts of plants - is an adaptive trait for coping with waterlogged conditions. Soybean (Glycine max) forms porous secondary tissues known as aerenchymatous phellem (AP), which are derived from the outermost cell layer of phellogen. To understand what factors other than waterlogging are involved in phellogen and AP formation, we examined how their formation in soybean seedlings was affected by darkness, CO2 deficiency and blockage of phloem transport. Methods: Aerenchymatous phellem and phellogen formation were expressed as area ratios in cross-sections of hypocotyl. CO2 was depleted by use of calcium oxide and sodium hydroxide. Phloem transport was blocked by heat-girdling of hypocotyls. Sucrose levels were measured by spectrophotometry. Key Results: Under light conditions, waterlogging induced the accumulation of high concentrations of sucrose in hypocotyls, followed by phellogen and AP formation in hypocotyls. Phellogen formation and AP formation were inhibited by darkness, CO2 deficiency and blockage of phloem transport. Phellogen formation and AP formation were also inhibited by excision of shoots above the epicotyl, but they recovered following application of sucrose (but not glucose or fructose application) to the cut surface. Conclusions: The results demonstrate that sucrose derived from leaves is essential for AP and phellogen formation in soybean hypocotyls under waterlogged soil conditions. Maintenance of a high sucrose concentration is thus essential for the development of phellogen and AP and the differentiation of phellogen to AP.