Role of Abscisic Acid in Flood-Induced Secondary Aerenchyma Formation in Soybean (Glycine max) Hypocotyls

Phellogen (cork cambium) usually produces cork tissue, but when flooded it produces secondary aerenchyma, comprising living cells with non-suberized walls in the stems, roots, and root nodules of some Fabaceae. In the cell walls of cork tissues, the plant hormone abscisic acid (ABA), promotes suberin deposition. Thus, ABA may decrease in flooded tissues, where secondary aerenchyma cells are developing. Here, we investigated whether ABA is involved in the formation of aerenchyma in soybean (Glycine max) hypocotyls when flooded. Hypocotyls flooded with water produced a large amount of secondary aerenchyma, and were highly porous. On the other hand, application of 1.0 μM ABA suppressed the enlargement of phellogen-derived cells, thereby suppressing subsequent gas space formation, and then almost completely inhibited aerenchyma development. Berberine-aniline blue staining indicated that not only elongated cells in the secondary aerenchyma but also packed cells, which were formed under flooding with ABA, contained no suberized cell walls. Compared to non-flooded plants, the endogenous ABA concentration in the flooded hypocotyls was decreased to 50% within 24 hr, and the low level was maintained for at least 72 hr. In addition, phellogen developed at 48 hr after flooding and secondary aerenchyma was observed at 72 hr. These results indicate that secondary aerenchyma formation requires a decrease in negative regulator ABA in soybean plants, that is, ABA inhibits elongation of cells derived from phellogen in secondary aerenchyma formation such as internodal cell elongation of floating rice stems