The alternative of ABA degradation pathways as a key process on adaptation of hygrophytes to anaerobic environments

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Echinochloa plants have differentiated upland and paddy ecotypes. In paddy ecotype seeds, abscisic acid (ABA) decreased under both anaerobic and aerobic conditions, in association with their germination. ABA is degraded via an oxidation pathway and/or a glucose conjugation pathway in which oxygen is not required for its reaction. The ABA glucose conjugates increased in the germinating paddy ecotype seeds. In contrast, dihydrophaseic acid, a major catabolite in the oxidation pathway, accumulated in the upland ecotype seeds. Conclusively, it appeared that seeds of the upland ecotype decrease endogenous ABA by the oxidative degradation and those of the paddy ecotype by the conjugative degradation. Similar phenomena, shown in the Echinochloa ecotypes, were observed in all hygrophytes and mesophytes used in this study. We thus propose that the alternative of ABA degradation pathways is a key factor with which hygrophytes adapted to anaerobic environments, which resulted from flooding.