抗坏血酸缺失介导水稻叶片衰老过程中光合作用“光碳失衡”的机理

Carbon assimilation decreases faster than the absorption of light energy in photosynthesis during senescence of rice leaves, resulting in photosynthetic "photon-carbon imbalance". The failure to use the surplus absorbed light energy in carbon assimilation leads to the production of reactive oxygen species (ROS), degradation of photosynthetic enzymes and photosynthetic pigments, and a rapid loss of photosynthetic function, these being consequences of photon-carbon imbalance. As GLDH is a key enzyme in ascorbic acid biosynthesis, this project is designed to study the relationships between ascorbic acid content and photon-carbon imbalance using GLDH RNA interference and overexpression mutants of rice and Arabidopsis as plant materials. The changes of light energy absorption, distribution of photosynthetic electron flow, excitation pressure, degradation and activity of Rubisco, photosynthetic pigments, metabolism of ROS and expression of the GLDH gene will be analyzed during leaf senescense in different mutants and wild- types. Several measures have been designed to improve photon-carbon imbalance such as reduction of absorption light energy by shading, enhancement of the ability for scavenging ROS by feeding exogenous ascorbic acid to the interference materials and wild-types, or stimulation of synthesis of endogenous ascorbic acid. In short, we aim to explore ways to prevent early leaf senescence so as to increase seed set and yield in rice. The broader purpose of this project is to elucidate photon-carbon imbalance theory in rice leaves, which might provide some theoretical guidance to rice breeding for high yield.

水稻叶片的衰老过程中，光合作用的碳固定能力的下降快于光能吸收能力的下降，导致光能吸收与碳固定的不平衡，吸收的光能不能及时被利用而使活性氧大量迸发，光合酶及光合色素降解，光合功能快速丧失，即"光碳失衡"。本项目利用分子生物学手段获得的抗坏血酸合成关键酶基因GLDH干涉与超表达水稻或拟南芥纯合体为材料，通过分析不同突变体及野生型叶片衰老过程中叶片光能吸收与光合电子的分配、激发压的变化、Rubisco的降解与活性变化、光合色素变化、活性氧代谢及GLDH基因表达等规律，探讨叶片不同抗坏血酸含量与光碳失衡的关系。同时通过在衰老过程中遮光的方式减少叶片光能的获得，或对干涉材料及野生型施加外源的抗坏血酸或刺激内源抗坏血酸合成等来增强清除活性氧的能力等方式，探讨改善叶片衰老过程中光碳失衡的状况。探究防止水稻早衰并增加其结实率和产量的措施，系统阐明水稻叶片衰老的光碳失衡理论，为水稻的高产育种提供理论指导。

衰老与农作物的产量息息相关并受多种因素调控。本项目提出了水稻衰老的“光碳失衡”假说，水稻叶片的衰老过程中，光合作用的碳固定能力的下降快于光能吸收能力的下降，导致光能吸收与碳固定的不平衡，吸收的光能不能及时被利用而使活性氧大量迸发，光合酶及光合色素降解，光合功能快速丧失，即"光碳失衡"。内源抗氧化剂-抗坏血酸通过影响衰老相关基因的表达影响叶片的光合能力，从而进一步影响水稻的产量。外源喷施抗坏血酸可以减轻水稻“光碳失衡”的状况，延缓水稻的衰老并增加产量，反之，喷施茉莉酸甲酯则加速衰老并导致产量降低。夏天中午强光时田间部分遮荫也能减轻水稻叶片“光碳失衡”状况，提高叶片光合能力从而增加产量。研究结果证实了水稻衰老的“光碳失衡”假说的合理性，增加抗氧化剂的浓度可以增强清除活性氧（ROS）的能力或者降低叶片的过剩光能的获得等措施，都可以改善叶片的“光碳失衡”，从而达到延缓叶片衰老的目的。另一方面，ROS在水稻籽粒的白垩形成方面起关键作用。抗坏血酸的缺乏导致水稻叶片的抗氧化能力下降，H2O2积累，光合功能及淀粉合成相关酶活性下降，影响了水稻籽粒的灌浆，最终导致籽粒的白垩增加，品质下降。抗坏血酸缺乏的拟南芥突变体vtc2-1比野生型对高光敏感，高光下vtc2-1积累活性氧导致PSII受伤害，结果显示抗坏血酸在高光胁迫下维持PSII的正常功能方面起关键作用。通过本项目的研究还揭示了另外一种重要的抗氧化物质花色素苷的新的光保护机制。与大多数叶片花色素苷分布在叶片叶肉细胞的液泡中不同，黧蒴幼叶的花色素苷分布于叶片表面的表皮毛中，许多富含花色素苷的表皮毛构成一层可去除的“红色外套”。由于花色素苷分布于叶片表面，随着叶片发育自然脱落和降解，这种花色素苷保护方式可能代表一种经济的、可根据需要调节花色素苷含量的保护方式。