State Key Development Program for Basic Research of ChinaState Key Development Program for Basic Research of China [2006CB101904]; Commonwealth Specialized Research Fund of China Agriculture [200803008]; National Natural Science Foundation of ChinaNational Natural Science Foundation of China (NSFC) [30871335, 30828022]; Ph.D. Programs Foundation of Ministry of Education of ChinaMinistry of Education, China [20060537004]; Hunan Provincial Education Department, ChinaHunan Provincial Education Department [06A029]