光强对植物C4光合途径形成的调节功能及作用机制研究

Typical C4 plants with highly active C4 photosynthesis key enzymes and "kranz" structure are of higher photosynthetic capacity under high temperature and high light intensity and drought condition. Light intensity can adjust the formation of C4 photosynthesis pathways and the differentiation of mesophyll cells and vascular bundle sheath cells and the chloroplasts in two types of cells. But the mechanism of the control function is not clear. Under high light intensity, PEPC activity is low and chloroplast PSⅡis inactive because of photoinhibition in mesophyll cells of C3 plants; while PEPC activity is high and chloroplast PSⅡ has high activity without photoinhibition in mesophyll cells of C4 plants; and vascular bundle sheath cells of C4 plants lack of PEPC activity and chloroplast PSⅡactivity. Thus we assume that the high light intensity may induce the differentiation of chloroplasts in photosynthetic cells and C4 pathways formation by adjusting the PEPC activity . The project will construct maize plants lacked of PEPC activity and rice plants with high PEPC expression in first, then place these genetically modified plants and their wild types under different light intensity, investigate the effect of light intensity on C4 key enzyme's gene expression and vascular bundle sheath cell differentiation and its related gene expression, elucidate the regulatory function and its mechanism of light intensity on C4 photosynthesis formation and vascular bundle sheath cell differentiation, provide new train of thought and theoretical basis for clarifying the mechanisms of C4 photosynthesis formation and vascular bundle sheath cell differentiation in C4 plants.

典型C4植物有高活性C4光合关键酶和花环结构，在高光强、高温和干旱条件下具更高的光合能力。光强可调节C4光合途径形成、叶肉细胞和维管束鞘细胞及两类细胞中叶绿体的分化。而这种调节功能的作用机制尚不清楚。强光下，C3植物叶肉细胞缺少PEPC活性，叶绿体PSⅡ易发生光抑制而失活；C4植物叶肉细胞具高PEPC活性，叶绿体PSⅡ不发生光抑制且活性高，而维管束鞘细胞缺少PEPC活性，同时叶绿体缺少PSⅡ活性。由此我们推测光强可能通过调节PEPC活性诱导光合细胞叶绿体分化和C4途径形成。本项目构建缺失PEPC的玉米植株和高表达PEPC的水稻植株，通过对这些转基因和野生型植株进行不同光强处理，研究光强变化对叶片C4循环关键酶基因表达和维管束鞘细胞分化及相关基因表达的影响，阐明光强对C4途径形成和维管束鞘细胞分化的调节功能和作用机制，为阐明C4植物的C4光合作用和维管束鞘细胞分化机制提供新的思路和理论依据。

与C3植物相比，C4植物在高光强、高温和干旱条件下具有更高光合能力和适应性。典型C4植物叶肉细胞具有高活性PEPC，叶绿体具有发达基粒和高活性PSⅡ；维管束鞘细胞缺少PEPC，叶绿体基粒不发达，无PSⅡ活性。在高光强下，C4植物很少发生光抑制；而C3植物，叶肉细胞缺少PEPC活性，叶绿体PSⅡ易发生光抑制而失活。由此我们推测PEPC参与维持高光强下叶绿体PSⅡ的活性。本项目以野生型水稻，转PEPC水稻，野生型玉米和PEPC专一抑制剂L-Asp处理的玉米为材料，通过不同光强处理，研究这些植株叶片和叶绿体光合特性对光强变化的响应，探讨光强对C4光合循环形成的调节功能和作用机制，为阐明C4植物的C4光合作用和维管束鞘细胞分化机制提供新的思路和理论依据。结果如下。.1.成功构建具有C4-PEPC的水稻植株。.2.成功构建玉米C4-PEPC基因RNAi植物表达载体。.3.野生型水稻叶片、转PEPC水稻叶片、玉米叶片和玉米叶肉细胞的PEPC活性均随光强增强而升高，其PSⅡ活性指标（Fv/Fm、电子传递速率和希尔反应放氧速率）均随光强增强先升后降。且在各光强下各指标均为玉米高于转PEPC水稻高于野生型水稻。在L-Asp处理玉米后，所有光强下叶肉细胞的PEPC几乎完全丧失活性，PSⅡ希尔反应放氧活力和电子传递速率均降低。对各光强下野生型水稻、转PEPC水稻、玉米叶片和加入L-Asp前后的玉米叶肉细胞的PSⅡ活性与PEPC活性的相关分析表明二者显著正相关。光强通过调节PEPC活性来诱导PSⅡ活性， PEPC参与维持高光强下PSⅡ的活性。.4.随光强增加，玉米叶片C4关键酶（PEPC、PPDK、NADP-MDH、NADP-ME）活性和基因表达均逐渐增加，加入L-Asp后PEPC活性被抑制，而PPDK、NADP-MDH、NADP-ME等活性也大幅度下降。.5.随光强增加，玉米叶片叶肉细胞叶绿体基粒更发达，基粒片层排列更紧密。加入L-Asp后基粒数目减少，基粒片层排列松散。.综上，光强调节PEPC活性，PEPC活性进一步诱导其它C4光合关键酶和PSⅡ活性以及叶绿体的发育，从而通过C4途径完成高光合运转。