Introduction
Biuret is a common impurity in urea fertilizers. It is a byproduct of
the urea granulation process and is formed by the thermal condensation
of two urea molecules. When urea fertilizers are added to arable lands,
biuret, a contaminant in these fertilizers, is also applied. Biuret in
the soil is decomposed by microorganisms and eventually produces ammonia
and carbon dioxide, relatively slow but the same as urea’s fate (Aukema
et al., 2020; Cameron et al., 2011; Robinson et al., 2018). Biuret is
not known to be toxic to animals; however, excess biuret can stunt plant
growth and cause chlorosis of leaves (Mikkelsen, 1990). Consequently,
the permissible biuret concentration in many countries is 1.2% for urea
fertilizers.
Previous studies on biuret toxicity have shown that it inhibits protein
synthesis in plants (Ogata T. & Yamamoto M., 1959; Webster et al.,
1957). A recent experiment also found that biuret toxicity can alter the
expression levels of many genes involved in environmental stress
response (Ochiai et al., 2020). However, the mechanisms underlying
biuret toxicity are still not well understood. Clarification of this
mechanism may help prevent potential plant injury.
We previously found that rice plants that were overexpressingbiuret hydrolase from a soil bacterium had an enhanced biuret
tolerance (Ochiai et al., 2020). The experiment used15N-labeled biuret to show that biuret
hydrolase overexpressing plants take up more biuret than wild-type
plants. However, the form of15N in plants after uptake is not known. In this
study, we examined the biuret accumulation, in wild-type andbiuret hydrolase overexpressing rice plants using HPLC-UV,
to understand how the biuret
concentration in plants causes injury.
In addition, we hypothesized that biuret inhibits the metabolism of
compounds with a similar structure, specifically ureido compounds. As an
ureido, we focused on allantoin because of its multiple roles and
importance to plants. Allantoin, a compound composed of a hydantoin ring
and ureido group, is an intermediate in the purine degradation pathway.
It contains four nitrogen (N) atoms per molecule and contributes to N
recycling in plants (Soltabayeva et al., 2018). Allantoin and allantoic
acid are the dominant forms of assimilated-N transported from roots to
shoots through the xylem in tropical leguminous plants (Schubert et al.,
1986). Many plant species accumulate allantoin under abiotic stress such
as salinity, drought, and heavy metal toxicity (Casartelli et al., 2019;
Kaur et al., 2021; Lescano et al., 2016; Nourimand and Todd, 2016;
Watanabe et al., 2013). The accumulated allantoin can enhance the
abiotic stress tolerance of plants (Watanabe et al., 2013). We
investigated the effect of biuret on the accumulation and metabolism of
allantoin in rice seedlings.
Furthermore, we performed a metabolome analysis using rice suspension
cells to examine metabolite
changes under biuret toxicity.