Biuret Toxicity Induces Accumulation of Nitrogen-Rich Compounds in Rice Plants

Excess biuret, a common impurity in urea fertilizers, is toxic to plants. Little is known about the mechanisms of biuret toxicity in plants. This study aimed to investigate the accumulation of biuret and the changes in metabolites in rice (Oryza sativa) plants under biuret toxicity. A previous study had shown that transgenic rice plants overexpressing bacterial biuret hydrolase had improved biuret tolerance. Here, we grew wild-type and bacterial biuret hydrolase-overexpressing rice plants in hydroponics at different biuret levels. Concentrations of biuret and allantoin, a nitrogenous intermediate in the purine degradation pathway, in the plants were determined. The expression levels of genes related to purine degradation and ureide metabolisms were analyzed using wild-type plants. Additionally, we performed a metabolome analysis using rice suspension cells. The biuret hydrolase-overexpressing plants did not contain biuret, whereas wild-type plants accumulated biuret in shoots in the order of mmol L−1 tissue water. The concentration of allantoin in shoots of wild-type plants under biuret toxicity was higher than those in control conditions. Inhibition of allantoinase activity by biuret was not detected, and allantoin accumulation appeared to be associated with changes in the expression of allantoinase, allantoate amidohydrolase and putative allantoin transporter genes. Furthermore, another nitrogenous compound citrulline, which is a non-protein amino acid, accumulated in rice suspension cells under biuret toxicity. The accumulation of these compounds suggests that rice plants subjected to biuret toxicity need to reduce the concentration of surplus ammonium ions via synthesizing nitrogen-rich compounds.