Urea, currently the most widely used nitrogen (N) fertilizer worldwide (IFASTAT, https://www.ifast at.org/datab ases), might contain biuret [(CONH 2) 2 NH], as a common impurity. Biuret is formed by the thermal condensation of urea. It has been known since the 1950s that excessive amounts of biuret in urea fertilizers cause injury in crops (Jones, 1954; Sanford et al., 1954). A wide range of crops can be potentially affected by biuret toxicity, which often manifests as leaf chlorosis and stunted growth, especially in the young seedling stage (Mikkelsen, 1990). Earlier studies indicated that biuret inhibited protein synthesis in Xanthium pensylvanicum leaves (Webster et al., 1957) and wheat (Triticum aestivum) germplasms (Ogata & Yamamoto, 1959). The protein content, however, did not so much decrease in biuret-injured orange (Citrus sinensis) leaves (Impey & Jones, 1960). It remains uncertain whether biuret has a direct effect on the protein synthetic machinery. Additionally, ultrastructural analyses showed that changes in chloroplast structure in biuret-injured leaves were similar to those in senescent leaves in grapefruit (Citrus paradise) and orange plants (Achor & Albrigo, 2005). Moreover biuret seems to remain unmetabolized in plants, because it was still detected in orange leaves, eight months after foliar spraying was performed (Impey & Jones, 1960). The exact mechanism underlying biuret toxicity in plants, however, is still far from being understood.
Biuret, a common impurity in urea fertilizers, is toxic to plants, but little is known about the physiological mechanisms underlying its toxicity. Here, we analyzed biuret toxicity in rice (Oryza sativa) plants. We carried out uptake experiments using 15 N-labelled biuret and demonstrated that biuret could reach sub millimolar concentrations in rice plants. We also demonstrated that the hydrolysis of biuret in plant cells could confer biuret tolerance to rice plants. This occurred because transgenic rice plants that overexpressed an exogenous biuret hydrolase cloned from a soil bacterium gained improved tolerance to biuret toxicity. Our results indicate that biuret toxicity is not an indirect toxicity caused by the presence of biuret outside the roots, and that biuret is not quickly metabolized in wild-type rice plants. Additionally, it was suggested that biuret was used as an additional nitrogen source in transgenic rice plants, because biuret hydrolase-overexpressing rice plants accumulated more biuret-derived N, as compared to wild-type rice.
AimsExcess 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. MethodsA previous study had shown that transgenic rice plants overexpressing bacterial biuret hydrolase had improved biuret tolerance. Here, we grew wild-type and bacterial biuret hydrolaseoverexpressing 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. ResultsThe 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. ConclusionThe 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.
Excess biuret, a common impurity in urea fertilizers, is toxic to plants. Little is known about the mechanisms of biuret toxicity in plants. A previous study had shown that transgenic rice ( Oryza sativa) plants overexpressing bacterial biuret hydrolase improved biuret tolerance. Additionally, the biuret hydrolase-overexpressing plants showed a higher N ratio than wild-type plants when the roots were fed N-labeled biuret. Here, we determined biuret accumulation in rice seedlings by directly measuring the biuret. We found that the biuret hydrolase-overexpressing plants did not contain biuret, whereas wild-type plants accumulated biuret in shoots in the order of mmol L tissue water. We also found that the concentration of allantoin, a nitrogen-rich intermediate compound in the purine degradation pathway, in rice shoots 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 putative allantoin transporter genes. Furthermore, another nitrogenous compound citrulline, which is a nonprotein amino acid, accumulated in rice suspension cells under biuret toxicity. The accumulation of these two nitrogen-rich compounds suggests that rice plants subjected to biuret toxicity may need to reduce the generation of surplus ammonium ions.
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