Ureide metabolism in response to cadmium treatment in Arabidopsis

Abstract

Ureides are nitrogenous compounds derived from oxidative degradation of purines. Due to their high nitrogen content, ureides play an important role in nitrogen metabolism, storage, and transport in plants. Among ureides, allantoin has recently been the subject of investigation, not only because it is a nitrogen-rich compound, but also due to its increased concentration in response to different abiotic stresses, and its contribution to stress tolerance in plants. Although a reactive oxygen species (ROS) scavenging property has been proposed for allantoin, the mechanism(s) underlying its protective role is still under debate. This research was carried out to evaluate the effect of cadmium (Cd) on ureide metabolism, clarifying the potential link between allantoin content and Cd tolerance in Arabidopsis thaliana. Molecular and biochemical analyses showed that in wild-type (Col-0) Arabidopsis, allantoin accumulates following Cd exposure because of enhanced transcript level of uricase (UO) and decreased mRNA abundance and enzyme activity of allantoinase (ALN), which are required for allantoin production and degradation, respectively. Interestingly, allantoinase-negative (aln-3) mutants, containing increased amount of allantoin, are relatively tolerant to Cd due to decreased ROS accumulation in response to Cd toxicity. Activity of three antioxidant enzymes, superoxide dismutase (SOD), ascorbate peroxidase (APX) and catalase (CAT), demonstrated higher SOD and APX activity in aln-3 leaves, together with greater activity of SOD and CAT in roots, support better aln-3 growth and confer Cd resistance to these plants. In contrast, allantoinase-overexpressed (ALNox) lines, containing lower allantoin levels, and respond to Cd in the opposite manner than aln-3 mutants, showing a Cd-sensitive phenotype. Differential and opposite responses of aln-3 mutants and ALNox lines to Cd treatment suggests that there is a positive correlation between allantoin content and resistance to Cd treatment. Additionally, ABA-insensitive (abi) mutants were used to investigate the possible cross-talk between allantoin and ABA signalling pathway, indicating that regulatory function of allantoin may be mediated through both ABA-dependent and -independent signalling pathways. These data contribute to our understanding of ureides regulation and function in response to abiotic stresses, setting the stage for future research on ureide metabolism with the purpose of introducing stress-resistant plants.