Xu Huawei, Zhang Jianjun, Zeng Jiwu, Jiang Linrong, Liu Ee, Peng Changlian, He Zhenghui, Peng Xinxiang
Laboratory of Molecular Plant Physiology, College of Life Sciences, South China Agricultural University, Guangzhou 510642, China.
J Exp Bot. 2009;60(6):1799-809. doi: 10.1093/jxb/erp056. Epub 2009 Mar 5.
Photorespiration is one of the most intensively studied topics in plant biology. While a number of mutants deficient in photorespiratory enzymes have been identified and characterized for their physiological functions, efforts on glycolate oxidase (GLO; EC 1.1.3.15) have not been so successful. This is a report about the generation of transgenic rice (Oryza sativa L.) plants carrying a GLO antisense gene driven by an estradiol-inducible promoter, which allowed for controllable suppressions of GLO and its detailed functional analyses. The GLO-suppressed plants showed typical photorespiration-deficient phenotypes. More intriguingly, it was found that a positive and linear correlation existed between GLO activities and the net photosynthetic rates (P(N)), and photoinhibition subsequently occurred once P(N) reduction surpassed 60%, indicating GLO can exert a strong regulation over photosynthesis. Various expression analyses identified that Rubisco activase was transcriptionally suppressed in the GLO-suppressed plants, consistent with the decreased Rubisco activation states. While the substrate glycolate accumulated substantially, few changes were observed for the product glyoxylate, and for some other downstream metabolites or genes as well in the transgenic plants. Further analyses revealed that isocitrate lyase and malate synthase, two key enzymes in the glyoxylate cycle, were highly up-regulated under GLO deficiency. Taken together, the results suggest that GLO is a typical photorespiratory enzyme and that it can exert a strong regulation over photosynthesis, possibly through a feed-back inhibition on Rubisco activase, and that the glyoxylate cycle may be partially activated to compensate for the photorespiratory glyoxylate when GLO is suppressed in rice.
光呼吸作用是植物生物学中研究最为深入的课题之一。虽然已经鉴定出许多光呼吸酶缺陷型突变体，并对其生理功能进行了表征，但对乙醇酸氧化酶（GLO；EC 1.1.3.15）的研究尚未取得如此成功。本文报道了携带由雌二醇诱导型启动子驱动的GLO反义基因的转基因水稻（Oryza sativa L.）植株的产生，该基因可实现对GLO的可控抑制及其详细的功能分析。GLO抑制型植株表现出典型的光呼吸缺陷表型。更有趣的是，发现GLO活性与净光合速率（P(N)）之间存在正线性相关，并且一旦P(N)降低超过60%，随后就会发生光抑制，这表明GLO可以对光合作用施加强有力的调节。各种表达分析表明，在GLO抑制型植株中，核酮糖-1,5-二磷酸羧化酶/加氧酶活化酶在转录水平上受到抑制，这与核酮糖-1,5-二磷酸羧化酶/加氧酶活化状态的降低一致。虽然底物乙醇酸大量积累，但在转基因植株中，产物乙醛酸以及一些其他下游代谢物或基因几乎没有变化。进一步分析表明，乙醛酸循环中的两个关键酶异柠檬酸裂解酶和苹果酸合酶在GLO缺陷条件下高度上调。综上所述，这些结果表明GLO是一种典型的光呼吸酶，它可以对光合作用施加强有力的调节，可能是通过对核酮糖-1,5-二磷酸羧化酶/加氧酶活化酶的反馈抑制，并且当水稻中GLO受到抑制时，乙醛酸循环可能会部分激活以补偿光呼吸乙醛酸。
