Photorespiration, Nitrate Assimilation, and Climate Change

Rubisco是地球上最常见的蛋白质，占植物氮的50%。这种酶催化糖与CO2或O2的反应，从而分别启动光合CO2同化或光呼吸途径。这两种反应之间的平衡取决于大气中CO2和O2的相对浓度。在目前的大气水平下，C3植物中的光呼吸消耗了在CO2同化过程中合成的25%以上的糖，因此被视为一种浪费的过程。在高CO2条件下，光呼吸被抑制，光合CO2同化占主导地位。以前的研究表明，植物需要光呼吸将硝酸盐转化为蛋白质。因此，高CO2浓度，通过抑制光呼吸，也可能抑制硝酸盐同化。这为植物对大气CO2浓度升高的响应提供了新的解释。例如，糖枫是少数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浓度上升导致这些树木蛋白质缺乏。因此，硝酸盐和铵的平衡使用将影响许多植物物种的初级生产力和分布。拟议的研究将探讨硝酸盐同化与光呼吸之间的生理机制。它将确定Rubisco是否具有另一种功能：促进植物细胞中迄今无法解释的能量转移。它将量化自然界中植物使用硝酸盐和铵作为氮源的程度。它将检查目前跟踪氮在生物体中流动的方法的有效性。这些信息将决定植物科学的未来研究，并指导有关全球气候变化问题的公共政策。该项目将涉及研究生和博士后助理的培训，结果将纳入P.I.。的本科教学。

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.