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 hydrolaseimproved biuret tolerance. Additionally, the biuret
hydrolase -overexpressing plants showed a higher 15N
ratio than wild-type plants when the roots were fed15N-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.