光呼吸在线粒体AOX途径介导的光破坏防御中的作用及机制

Mitochondrial alternative oxidase (AOX) pathway is a well known respiration pathway. However, the photo-protective function of the AOX pathway has been just noticed recently. In our previous studies, we found that the AOX pathway can consume excess reducing equivalents (NADPH) formed in chloroplasts transported into mitochondria via the Malate-Oxaloacetate (Mal-OAA) shuttle, which plays an essential role in protecting plants against photodamage. It has puzzled us that the electron transported by AOX pathway is only about 1-3% of that transported by photosynthetic electron transport, but the AOX pathway plays an essential role in photo-protection. Furthermore, we found that the AOX pathway only plays photo-protective role in C3 plants but not in C4 plants. The above facts indicate that there must have been another mechanism which mediates the photo-protection of AOX pathway. The results of our previous studies indicate that the unknown pathway has something to do with photorespiration. So in this study, we are going to explore the intrinsic reason that causes the difference of the photo-protection mediated by the AOX pathway between C3 and C4 plants. And we are going to construct single/double mutants of the genes about the key enzymes in AOX pathway and photorespiration in C3 plant. By interfering the AOX pathway and photorespiration at the level of gene expression,activity of enzymes and metabolites, we are going to explore the effect of inhibition of photorespiration and deletion of key enzymes in photorespiration on the photoprotective function of AOX pathway, and to clarify the interaction between the photorespiration and AOX pathway as well as the regulation of AOX activity by photorespiration metabolites. Through this research ,we plan to elucidate the functional mechanism of respiration in the photoprotection mediated via AOX pathway. The results of this study will help in understanding the mechanism of photoprotection mediated via the AOX pathway and provide a new approach to improve photoprotection ,light use efficiency and yield of crop.

我们近期发现，线粒体AOX途径可以通过消耗由Mal-OAA穿梭进入线粒体的过剩还原力，对植物的光破坏起重要的防御作用。但让我们困惑的是AOX途径传递电子能力只占光合电子传递能力的1-3%，却起不可替代的光保护（光破坏防御）作用；此外AOX途径只在C3植物中起光保护作用而在C4植物不起作用。这说明植物中还有另一条尚未阐明的途径介导了AOX途径的光保护作用。我们研究显示这条未知途径与光呼吸存在某种联系。本研究将进一步探讨C3和C4植物AOX途径光保护作用差异的内在原因，通过构建C3植物AOX、光呼吸关键酶单/双价突变体、在酶活性、代谢产物和基因表达等不同层次上干涉AOX和光呼吸，从基因表达和酶活性调控等水平探讨光呼吸受抑或缺失对AOX途径光保护作用的影响，阐明光呼吸与AOX途径的互作及光呼吸产物对AOX的调控。揭示光呼吸在AOX途径光保护中的作用机制。为改善植物光保护能力提高光合效提供新思路。

本研究以拟南芥aox1a突变体和不同类型的C4植物和C3植物为为材料，探讨AOX途径在强光下, 对PSII光保护的贡献和机制。我们发现，C3植物的AOX途径在强光下有着重要的光保护作用。aox1a突变体在强光下, PSII光抑制明显重于野生型，抑制C3植物的AOX途径后，强光下，植物叶片的光抑制明显加重，但是强光下，抑制各种类型C4植物的AOX途径后，对植物叶片的光抑制程度均没有明显的影响。这说明AOX途径对C3植物的PSII有显著的光保护作用，但是对C4植物却不起任何光保护作用。. 进一步研究证明，AOX是通过维持光呼吸起光保护作用的，同时也可以受到光呼吸代谢产物的活化。光呼吸代谢中的甘氨酸会从过氧化物酶体转运至线粒体中，在线粒体内的GDC催化下转化为丝氨酸。甘氨酸转化为丝氨酸的过程中产生大量的NADH，这需要被AOX快速消耗。当AOX受抑或者缺乏时，NADH积累，导致NAD+不足，这会抑制GDC的活性，导致甘氨酸积累，进而反馈抑制上游的光呼吸代谢，最终可能导致对植物有毒的磷酸乙醇酸的积累，进一步通过抑制D1蛋白的合成加重PSII光抑制的发生。因为AOX的光保护作用是通过光呼吸起作用的，所以在光呼吸较强的情况下，AOX的光保护功能更明显，而当光呼吸较弱时，如低温下，AOX的光保护功能不明显；在没有光呼吸的C4植物中，AOX基本没有光保护作用。