State Key Laboratory for Conservation and Utilization of Subtropical Agro-bioresources, South China Agricultural University, Guangzhou, China.
Physiol Plant. 2014 Mar;150(3):463-76. doi: 10.1111/ppl.12104. Epub 2013 Oct 12.
Glycolate oxidase (GLO) is a key enzyme for photorespiration in plants. Previous studies have demonstrated that suppression of GLO causes photosynthetic inhibition, and the accumulated glycolate with the deactivated Rubisco is likely involved in the regulation. Using isolated Rubisco and chloroplasts, it has been found that only glyoxylate can effectively inactivate Rubisco and meanwhile inhibit photosynthesis, but little in vivo evidence has been acquired and reported. In this study, we have generated the transgenic rice (Oryza sativa) plants with GLO being constitutively silenced, and conducted the physiological and biochemical analyses on these plants to explore the regulatory mechanism. When GLO was downregulated, the net photosynthetic rate (Pn) was reduced and the plant growth was correspondingly stunted. Surprisingly, glyoxylate, as a product of the GLO catalysis, was accumulated in response to the GLO suppression, like its substrate glycolate. Furthermore, the glyoxylate content was found to be inversely proportional to the Pn while the Pn is directly proportional to the Rubisco activation state in the GLO-suppressed plants. A mathematical fitting equation using least square method also demonstrated that the Rubisco activation state was inversely proportional to the glyoxylate content. Despite that the further analyses we have conducted failed to reveal how glyoxylate was accumulated in response to the GLO suppression, the current results do strongly suggest that there may exist an unidentified, alternative pathway to produce glyoxylate, and that the accumulated glyoxylate inhibits photosynthesis by deactivating Rubisco, and causes the photorespiratory phenotype in the GLO-suppressed rice plants.
乙醇酸氧化酶（GLO）是植物光合作用中光呼吸的关键酶。先前的研究表明，抑制 GLO 会导致光合作用抑制，而失活的 Rubisco 积累的乙醇酸可能参与了这种调节。使用分离的 Rubisco 和叶绿体，已经发现只有乙醛酸才能有效地使 Rubisco 失活，同时抑制光合作用，但在体内很少获得和报道相关证据。在这项研究中，我们生成了 GLO 持续沉默的转基因水稻（Oryza sativa）植株，并对这些植株进行了生理生化分析，以探索其调控机制。当 GLO 下调时，净光合速率（Pn）降低，植物生长相应受到抑制。令人惊讶的是，作为 GLO 催化产物的乙醛酸，像其底物乙醇酸一样，也会因 GLO 的抑制而积累。此外，发现乙醛酸含量与 Pn 呈反比，而 Pn 与 GLO 抑制植株中 Rubisco 的激活状态呈正比。最小二乘法拟合方程也表明，Rubisco 的激活状态与乙醛酸含量呈反比。尽管我们进一步的分析未能揭示乙醛酸如何在 GLO 抑制时积累，但目前的结果强烈表明，可能存在一种未被识别的替代途径来产生乙醛酸，并且积累的乙醛酸通过使 Rubisco 失活来抑制光合作用，并导致 GLO 抑制的水稻植株出现光呼吸表型。
