Photorespiration, Nitrate Assimilation, and Climate Change

地球上最普遍的蛋白质是Rubisco，它占植物氮素的50%。这种酶催化糖与二氧化碳或氧气的反应，从而分别启动光合作用的二氧化碳同化或光呼吸途径。这两个反应之间的平衡取决于大气中二氧化碳和氧气的相对浓度。在目前的大气水平下，C3植物的光呼吸消耗了二氧化碳同化过程中合成的超过25%的糖，因此，一直被视为一种浪费过程。在高CO2条件下，光呼吸受到抑制，光合作用以CO2同化为主，硝酸盐和氨氮是植物氮素的主要来源。以前的研究表明，植物需要光呼吸来将硝酸盐转化为蛋白质。因此，高浓度的二氧化碳通过抑制光呼吸，也可能抑制硝酸盐的同化。这为植物对大气中二氧化碳浓度上升的反应提供了新的解释。例如，糖枫是少数几种高度依赖硝酸盐作为氮源的硬木之一。美国东北部糖枫的减少可能是由于二氧化碳上升导致这些树的蛋白质缺乏所致。因此，硝酸盐和铵的利用之间的平衡将影响许多植物的初级生产力和分布。拟议的研究将探讨将硝酸盐同化与光呼吸联系起来的生理机制。它将决定Rubisco是否具有另一种功能：促进植物细胞中迄今未知的能量转移。它将量化自然界中植物利用硝酸盐和铵作为氮源的程度。它将检查目前追踪氮在生物体中流动的方法的有效性。这些信息将决定未来植物科学的研究，并指导有关全球气候变化问题的公共政策。该项目将包括培养研究生和博士后助理，并将结果纳入P.I.S本科教学。

The most prevalent protein on Earth, one that comprises as much as 50% of the nitrogen in plants, is Rubisco. This enzyme catalyzes the reaction of a sugar with either CO2 or O2 and thereby initiates, respectively, the photosynthetic CO2 assimilatory or photorespiratory pathways. The balance between the two reactions depends on the relative concentrations of CO2 and O2 in the atmosphere. At current atmospheric levels, photorespiration in C3 plants dissipates over 25% of the sugars synthesized during CO2 assimilation and, thus, has been viewed as a wasteful process. Under high CO2 conditions, photorespiration is inhibited and photosynthetic CO2 assimilation dominates.Nitrate and ammonium are the major sources of nitrogen for plants. Previous work showed that plants require photorespiration to convert nitrate into proteins. Thus high CO2 concentrations, by inhibiting photorespiration, may also inhibit nitrate assimilation. This provides a new interpretation for plant responses to rising atmospheric CO2 concentrations. For example, sugar maple is one of the few hardwoods that is highly dependent on nitrate as a nitrogen source. The decline of sugar maples in the Northeast U.S. may derive from protein deprivation of these trees under rising CO2. Therefore, the balance between nitrate and ammonium usage will influence the primary productivity and distribution of many plant species.The proposed research would examine the physiological mechanisms that link nitrate assimilation to photorespiration. It would determine if Rubisco serves another function: facilitating heretofore unexplained energy transfers in plant cells. It would quantify to what degree plants in nature use nitrate and ammonium as nitrogen sources. It would check the validity of current methods for following the flow of nitrogen through organisms. Such information will determine future research in the plant sciences as well as guide public policy on global climate change issues. The project will involve training of graduate students and a post-doctoral associate and results will be incorporated into the P.I.'s undergraduate teaching.