EAGER: Elevated Carbon Dioxide, Nitrogen Metabolism, and Photorespiration

氮是植物所需的最大量的矿物质元素。植物氮素的一个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证实，地球大气中二氧化碳浓度的上升会抑制大多数植物物种的幼苗将硝酸盐转化为蛋白质。造成这种现象的一个生理机制涉及光呼吸，这是植物中的糖与氧气而不是二氧化碳反应的过程。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需的能量。二氧化碳浓度升高会降低光呼吸，从而导致植物蛋白质浓度下降，植物生长缓慢，食物价值降低。这项拟议的研究将研究光呼吸如何传递能量，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新的视角，并更好地了解二氧化碳浓度上升对植物物种分布的影响。大气中二氧化碳浓度的上升抑制了C3植物地上部NO3同化为蛋白质的作用，并阻碍了C3植物的生长。将二氧化碳浓度与地上部N3同化联系起来的一个过程是光呼吸。光呼吸作用促进叶绿体苹果酸的输出，增加细胞质中NADH的利用率，从而促进NO2还原为NO2，这是NO3同化的第一步。CO2浓度升高降低了光呼吸作用，减少了NADH可用于NO3还原的量。拟议的研究将更详细地研究这一机制。几乎所有的Rubisco研究都是在镁的存在下进行的，而不是在锰的存在下进行的，镁的存在有利于羧化，而锰的存在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)用X射线结晶学确定锰和NaDP+在烟草Rubisco中的位置；2)通过电子顺磁共振(EPR)鉴定光呼吸过程中瞬时酶-底物复合体和催化反应中间产物的结构变化；3)对与光呼吸电子转移有关的Rubisco催化大亚基中的残基进行定点突变，以提供强大的结构探针。光呼吸和硝酸盐同化之间的能量传递应该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二氧化碳浓度和C4途径的多次引入后，仍然只依赖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，这些机制应该解释为什么植物对高CO2的反应是高度不同的：在依赖于硝酸盐的C3植物中，CO2抑制地上部硝酸盐的同化会导致有机N的缺乏并减缓生长，而在依赖于铵的C3植物中，高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.