EAGER: Elevated Carbon Dioxide, Nitrogen Metabolism, and Photorespiration

氮是植物最需要的矿物元素。植物氮的主要来源是它们从土壤中吸收的硝酸盐。布鲁姆实验室的研究已经证实，地球大气中二氧化碳浓度的上升抑制了大多数植物枝条中硝酸盐转化为蛋白质的过程。造成这种现象的一种生理机制与光呼吸有关，这是一种植物中的糖与氧气而不是二氧化碳发生反应的过程。光呼吸被错误地描述为一种浪费的过程，但事实上，光呼吸产生将硝酸盐转化为蛋白质所必需的能量。二氧化碳浓度升高会降低光呼吸作用，从而导致植物蛋白浓度下降，植物生长减慢，食物价值降低。拟议的研究将研究光呼吸如何转移能量将硝酸盐转化为蛋白质。它将研究促进这些反应的酶的化学结构，并追踪从一个反应到另一个反应的能量流动。这一信息将为研究光合作用的效率提供一个新的视角，并更深入地了解二氧化碳上升对植物物种分布的影响。大气中CO2浓度的升高抑制了NO3在C3植物枝条中向蛋白质的同化，并在NO3为主要氮源时阻碍了C3植物的生长。将CO2浓度与NO3同化联系起来的一个过程是光呼吸。光呼吸刺激叶绿体输出苹果酸，增加细胞质中NADH的可用性，从而使NO3还原为NO2，这是NO3同化的第一步。CO2富集降低了光呼吸作用，减少了可用于还原NO3的NADH量。拟议的研究将更详细地研究这一机制。几乎所有关于Rubisco的研究都是在镁的存在下进行的，镁有利于羧基化，而锰有利于氧化和电子转移，这可能会刺激NADP+的还原。本研究的目的是：1)利用x射线晶体学确定烟草Rubisco中锰和NADP+的位置；2)通过电子顺磁共振（EPR）鉴定瞬时酶-底物配合物和催化反应中间体在光呼吸过程中的结构变化；3)对Rubisco催化大亚基中参与光呼吸电子转移的残基进行位点定向诱变，提供强大的结构探针。光呼吸和硝酸盐同化之间的能量转移可以解释为什么超过95%的高等植物物种在经历了2000多万年相对较低的大气CO2浓度和多次引入C4途径后仍然完全依赖C3固定：C3固定比以前认为的更有效，因为光呼吸支持硝酸盐同化。此外，这些机制可以解释为什么植物对升高的CO2的反应是高度可变的：在依赖硝酸盐的C3植物中，CO2抑制茎部硝酸盐同化导致有机氮缺乏并减缓生长，而在依赖铵的C3植物中，升高的CO2促进生长。

Nitrogen is the mineral element that plants require in greatest amounts. A major source of nitrogen for plants is the nitrate that they absorb from soils. Research in the Bloom laboratory has established that rising CO2 (carbon dioxide) concentration in Earth's atmosphere inhibits the conversion of nitrate into proteins in the shoots of most plant species. One physiological mechanism responsible for this phenomenon involves photorespiration, a process through which sugars in a plant react with oxygen instead of CO2. Photorespiration has been erroneously portrayed as a wasteful process, but in fact, photorespiration generates energy necessary for converting nitrate into protein. Elevated CO2 decreases photorespiration, which leads to declines in plant protein concentrations, slower plant growth, and lower food value. The proposed research will examine how photorespiration transfers energy to convert nitrate into protein. It will examine the chemical structure of the enzymes that facilitate these reactions and to trace the flows of energy from one reaction to another. This information will provide a new perspective on the efficiency of photosynthesis and greater insight on the influence that rising CO2 will have on the distribution of plant species.Rising CO2 concentration in the atmosphere inhibits NO3 assimilation into proteins in the shoots of C3 plants and impedes their growth when NO3 is the predominant N source. One process that links CO2 concentration to shoot NO3 assimilation is photorespiration. Photorespiration stimulates the export of malic acid from chloroplasts and increases the availability of NADH in the cytoplasm that empowers the reduction of NO3 to NO2, the first step of NO3 assimilation. CO2 enrichment decreases photorespiration, decreasing the amount of NADH available for NO3 reduction. The proposed research will examine this mechanism in greater detail. Nearly all studies of Rubisco are conducted in the presence of magnesium, which favors carboxylation, rather than in the presence of manganese, which favors oxygenation and electron transfers that might stimulate NADP+ reduction. The objectives of this research are: 1) use X-ray crystallography to confirm the locations of manganese and NADP+ within tobacco Rubisco, 2) identify via electron paramagnetic resonance (EPR) the structural changes of transient enzyme-substrate complexes and catalytic reaction intermediates during photorespiration, and 3) conduct site-directed mutagenesis of residues in the catalytic large subunit of Rubisco that are involved with photorespiratory electron transfer to provide powerful structural probes. Energy transfers between photorespiration and nitrate assimilation should explain why over 95% of higher plant species still rely solely on C3 fixation even after more than 20 million years of relatively low atmospheric CO2 concentrations and multiple introductions of the C4 pathway: C3 fixation is more efficient than previously thought because photorespiration supports nitrate assimilation. Moreover, these mechanisms should explain why plant responses to elevated CO2 are highly variable: in C3 plants that are dependent on nitrate, CO2 inhibition of shoot nitrate assimilation causes organic N deficiencies and slows growth, whereas in C3 plants dependent on ammonium, elevated CO2 promotes growth.