国家自然科学基金资助项目(30770073); 江苏高教优势学科建设工程资助项目; 南京市科委工程中心创新能力提升项目资助(201105058)
College of Life Science,Nanjing Normal University,Jiangsu Key Laboratory for microbes and functional genomics,Jiangsu Engineering and Technology Research Center for Industrialization of Microbial Resources,College of Life Science,Nanjing Normal University,Jiangsu Key Laboratory for microbes and functional genomics,Jiangsu Engineering and Technology Research Center for Industrialization of Microbial Resources,Institute of Food Crops,Jiangsu Academy of Agricultural Sciences Nanjing,,College of Life Science,Nanjing Normal University,Jiangsu Key Laboratory for microbes and functional genomics,Jiangsu Engineering and Technology Research Center for Industrialization of Microbial Resources
Endophytes are microbes that reside within the living tissues of host plants without substantively harming them. They are ubiquitous in most plant species, latently residing in or actively colonizing the tissues. Infection by endophytes can improve the level of host plant nitrogen assimilation and metabolism. The mechanisms mediated by endophytes are complex; when symbiotic with plants, the ability to fix and solubilize mineral nutrients otherwise unavailable to plants allows plant-associated bacteria to act as biofertilizers. Endophytes are able to improve the level of host plant nitrogen assimilation and metabolism by promoting the growth and development of roots and by the fixation of nitrogen (diazotrophy). Nitrogen is often a limiting factor for plant growth because atmospheric nitrogen exists as dinitrogen (N2), a form that is inaccessible to all organisms except a few specially adapted prokaryotes. For all other organisms (including plants), nitrogen needs to be in the form of either ammonia or nitrate before it can be used. Diazotrophic bacteria possess nitrogenase, an O2-sensitive enzyme that catalyzes the reduction of atmospheric nitrogen to ammonia. Higher plants cannot carry out this process in the absence of these bacteria. Endophytes can also produce multiple ectoenzymes, such as laccase and proteolytic enzymes. These ectoenzymes facilitate the hydrolyzing and mineralizing of some polypeptides and soluble proteins for plant use. The function of ectoenzymes suggests that endophytes can help the host plants to use organic nitrogen as well as atmospheric. In addition, endophytes also clearly enhance the activities of key enzymes in the nitrogen assimilation and metabolism pathways of their hosts. In plants, glutamine synthetase (GS) and nitrate reductase (NR) are the enzymes that are mainly responsible for the assimilation of ammonium, and enhanced activity in these two key enzymes greatly improves plant metabolism. Furthermore, endophytes are also able to promote the production of phytohormones such as ethylene, auxins, indole-3-acetic acid (IAA), cytokinins, abscisic acid or combinations thereof, as well as the production of vitamins such as zeatin, vitamin B1, and vitamin B6. Both phytohormones and vitamins are important in nitrogen metabolism. Interestingly, endophyte infection results in higher NH4+ concentrations in the plant tissue, but NO3- concentrations are decreased. Tiller production, chlorophyll content and photosynthetic rate are also affected by endophytes; infected plants produce more tillers and have greater chlorophyll content than uninfected ones, and photosynthetic rates are also increased. In this thesis, we will give a brief summary of the aspects that deserve special attention in researching the role of endophytes in enhancing plant nitrogen assimilation and metabolism. We will also attempt to outline the mechanisms of endophyte-affected plant nitrogen assimilation and metabolism to highlight future research prospects.