Ureide metabolism in response to abiotic stress in plants

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In plants, ureide metabolism is often associated with nitrogen-fixing “tropical” legumes such as soybean and common bean because up to 95% of nitrogen transported from root nodules through the xylem is in the form of two ureide compounds, allantoin and allantoate. Recently, accumulation of allantoin and allantoate has been associated with response to abiotic stresses in other species, including rice and Arabidopsis. Elevated levels of allantoin confers protection from a wide range of abiotic stresses, and decreases stress-induced reactive oxygen species. Our goal is to understand how ureide metabolism and abiotic stress are linked by examining the regulation of ureide synthesis and catabolism, determining the cellular basis of this response, and identifying the signaling pathway(s) involved.In response to abiotic stressors including drought, high light, or treatment with heavy metal such as cadmium, the ureide allantoin accumulates in plant tissues accompanied by up-regulation of genes coding for upstream enzymes such as urate oxidase, and down-regulation of allantoinase gene expression and decreased allantoinase enzyme activity. Allantoin itself confers some degree of resistance or tolerance to abiotic stressors, pointing to a specific physiological role for allantoin accumulation in response to stress. What these treatments have in common is that they all contribute to oxidative stress and generation of reactive oxygen species in plant cells. Our current model proposes that allantoin alleviates oxidative stress through activation of antioxidant enzymes that remove reactive oxygen species such as hydrogen peroxide and superoxide. This decreases the oxidative stress cells are exposed to and allow the plants to tolerate these conditions better when comparing wild-type controls to mutants lacking allantoinse, or when comparing plants that have been treated with allantoin to untreated plants. Data gathered to date by our lab and others suggest that this may involve both ABA-dependent and ABA-independent signaling pathways. We plan to investigate the connection. In understanding this mechanism we seek to answer questions that include, What is the spatial and temporal relationship between oxidative stress and ureide accumulation at the cellular level? What pathways and mechanisms are involved in allantoin-mediated upregulation of antioxidant pathways? What is the metabolic or fitness cost of allantoin-mediated abiotic stress tolerance under ideal and stress conditions? Can this pathway be manipulated in economically crop species to improve stress tolerance? The goal is to contribute to a better understanding of this process, with an ultimate aim of improving plant stress tolerance in plants.