Abstract
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 15N
ratio than wild-type plants when the roots were fed
15N-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 -1 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.