Ureide metabolism in response to abiotic stress in plants

在植物中，尿素代谢通常与大豆和菜豆等固氮的热带豆科植物有关，因为从根瘤通过木质部运输的氮有高达95%是以两种脲化合物的形式存在的，即尿囊素和尿囊酸。最近，尿囊素和尿囊酸的积累与其他物种对非生物胁迫的反应有关，包括水稻和拟南芥。尿囊素水平的升高提供了对广泛非生物胁迫的保护，并减少了应激诱导的活性氧物种。我们的目标是通过研究尿素合成和分解代谢的调节，确定这种反应的细胞基础，并确定涉及的信号通路(S)，了解尿素代谢和非生物胁迫是如何联系在一起的。 在干旱、强光或镉等重金属处理等非生物胁迫下，尿囊素在植物组织中积累，同时尿酸氧化酶等上游酶基因表达上调，尿囊酶基因表达下调，尿囊素酶活性降低。尿囊素本身对非生物应激源具有一定程度的抗性或耐受性，表明尿囊素在应对胁迫时积累具有特定的生理作用。这些处理的共同点是，它们都有助于植物细胞中的氧化应激和活性氧的产生。我们目前的模型提出，尿囊素通过激活抗氧化酶来消除过氧化氢和超氧化物等活性氧物种，从而减轻氧化应激。当将野生型对照与缺乏尿囊素的突变体进行比较时，或者将经过尿囊素处理的植物与未经处理的植物进行比较时，这可以减少细胞面临的氧化应激，并使植物更好地耐受这些条件。到目前为止，我们实验室和其他实验室收集的数据表明，这可能涉及ABA依赖和ABA独立的信号通路。我们计划调查两者之间的联系。在理解这一机制时，我们试图回答的问题包括，氧化应激和细胞水平上的脲积累之间的空间和时间关系是什么？尿囊素介导的抗氧化途径上调涉及哪些途径和机制？在理想和应激条件下，尿囊素介导的非生物应激耐受的代谢或适应成本是多少？这条途径能否在经济作物物种中被操纵，以提高抗逆性？目的是为了更好地理解这一过程，最终目的是提高植物的逆境耐受性。

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.