Embrapa Soybean, Londrina, Paraná, Brazil, Rodovia Carlos Strass, Distrito da Warta, Londrina PR, Brasil.
BMC Biotechnol. 2011 Nov 21;11:111. doi: 10.1186/1472-6750-11-111.
The photorespiratory nitrogen cycle in C₃ plants involves an extensive diversion of carbon and nitrogen away from the direct pathways of assimilation. The liberated ammonia is re-assimilated, but up to 25% of the carbon may be released into the atmosphere as CO₂. Because of the loss of CO₂ and high energy costs, there has been considerable interest in attempts to decrease the flux through the cycle in C₃ plants. Transgenic tobacco plants were generated that contained the genes gcl and hyi from E. coli encoding glyoxylate carboligase (EC 4.1.1.47) and hydroxypyruvate isomerase (EC 5.3.1.22) respectively, targeted to the peroxisomes. It was presumed that the two enzymes could work together and compete with the aminotransferases that convert glyoxylate to glycine, thus avoiding ammonia production in the photorespiratory nitrogen cycle.
When grown in ambient air, but not in elevated CO₂, the transgenic tobacco lines had a distinctive phenotype of necrotic lesions on the leaves. Three of the six lines chosen for a detailed study contained single copies of the gcl gene, two contained single copies of both the gcl and hyi genes and one line contained multiple copies of both gcl and hyi genes. The gcl protein was detected in the five transgenic lines containing single copies of the gcl gene but hyi protein was not detected in any of the transgenic lines. The content of soluble amino acids including glycine and serine, was generally increased in the transgenic lines growing in air, when compared to the wild type. The content of soluble sugars, glucose, fructose and sucrose in the shoot was decreased in transgenic lines growing in air, consistent with decreased carbon assimilation.
Tobacco plants have been generated that produce bacterial glyoxylate carboligase but not hydroxypyruvate isomerase. The transgenic plants exhibit a stress response when exposed to air, suggesting that some glyoxylate is diverted away from conversion to glycine in a deleterious short-circuit of the photorespiratory nitrogen cycle. This diversion in metabolism gave rise to increased concentrations of amino acids, in particular glutamine and asparagine in the leaves and a decrease of soluble sugars.
C₃ 植物的光呼吸氮循环涉及大量的碳和氮从同化的直接途径转移。释放的氨被重新同化，但多达 25%的碳可能作为 CO₂释放到大气中。由于 CO₂的损失和高能量成本，人们对试图减少 C₃ 植物中循环通量产生了相当大的兴趣。生成了含有来自大肠杆菌的 gcl 和 hyi 基因的转基因烟草植物，分别编码乙醛酸-carboligase（EC 4.1.1.47）和羟丙酮酸异构酶（EC 5.3.1.22），靶向过氧化物酶体。人们推测，这两种酶可以协同作用，并与将乙醛酸转化为甘氨酸的转氨酶竞争，从而避免光呼吸氮循环中氨的产生。
在环境空气中生长，但在高浓度 CO₂中不生长时，转基因烟草品系的叶片上出现坏死病变的独特表型。在选择进行详细研究的 6 条品系中，有 3 条品系含有 gcl 基因的单个拷贝，有 2 条品系含有 gcl 和 hyi 基因的单个拷贝，有 1 条品系含有 gcl 和 hyi 基因的多个拷贝。在含有 gcl 基因单个拷贝的 5 条转基因品系中检测到 gcl 蛋白，但在任何转基因品系中均未检测到 hyi 蛋白。与野生型相比，在空气中生长的转基因品系中，可溶性氨基酸（包括甘氨酸和丝氨酸）的含量通常增加。在空气中生长的转基因品系中，茎中的可溶性糖（葡萄糖、果糖和蔗糖）含量降低，这与碳同化减少一致。
已经生成了能够产生细菌乙醛酸-carboligase 但不能产生羟丙酮酸异构酶的烟草植物。当暴露在空气中时，转基因植物表现出应激反应，表明一些乙醛酸在光呼吸氮循环的有害短路中被转移，远离转化为甘氨酸。这种代谢的转变导致了氨基酸，特别是叶片中谷氨酰胺和天冬酰胺的浓度增加，以及可溶性糖的减少。
