植物内生菌促进宿主氮吸收与代谢研究进展

内生菌与植物共生能够提高宿主的氮吸收与氮代谢水平,这可能是由于内生菌在植物体内引发的多种效应的综合结果.植物内生菌能够通过促进植物根系发育和固氮作用为宿主植物提供更多的无机氮素；能够通过分泌多种胞外酶系如漆酶、蛋白水解酶等使宿主植物更好地利用有机氮素；能够提高宿主氮代谢关键酶如硝酸还原酶(NR)、谷氨酰胺合成酶(GS)等酶的活性；能够提高宿主植物激素水平和维生素含量从而促进宿主氮代谢；能够通过影响宿主植物氮代谢促进宿主植物分蘖、提高宿主植物叶绿素含量和光合速率等等.综述了国内外关于植物内生菌促进宿主氮代谢的相关报道,归纳了植物内生菌影响宿主氮素吸收与代谢的可能机制,并展望了关于植物内生菌促进宿主氮代谢机制方面的研究方向.

Research progress on endophyte-promoted plant nitrogen assimilation and metabolism

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.