Allantoin involved in species interactions with rice and other organisms in paddy soil

Allantoin (5-ureidohydantoin) plays an essential role in the assimilation, metabolism, transport, and storage of nitrogen in numerous higher plants, but its ecological implications are largely unknown. In this study allantoin was found in tissues of 11 rice (Oryza sativa) varieties tested, and its structure was characterised by X-ray diffraction analysis to confirm the fact that allantoin was actually obtained from the rice plants. Furthermore, the endogenous allantoin was exuded from the rice roots into the rhizosphere soils and had a great diversity of biological effects on associated weeds and microbes by soil interactions once released. However, allantoin levels in tissues or soils could not be distinguished between the allelopathic and non-allelopathic rice varieties. Field experiments showed that levels of allantoin released from rice varieties varied with their growth stages and reached the maximal levels at the stem elongation or panicle initiation to booting stages and then decreased dramatically. Allantoin could significantly stimulate the germination and growth of Echinochloa crus-galli and populations of soil bacteria and actinomycetes at selected test concentrations (30–500 μg/g), but had no effect on soil fungi. The half-life (t _1/2 ) of allantoin in autoclaved soil (20.2 ± 2.5 h, r ² = 0.95) was almost three-times longer than in non-autoclaved soil (7.3 ± 1.9 h, r ² = 0.92), indicating that rapid biodegradation or transformation of allantoin occurs in paddy soil. The results suggest that not only may allantoin play a role in the transport and storage of nitrogen in rice tissues but it may also participate in species interactions between rice and other organisms in paddy soil.