土壤微生物与荒漠先锋植物羽毛针禾根鞘形成的关系

Stipagrostis pennata is a typical pioneer psammophyte species found in deserts of the Junggar Basin in Northern Xinjiang. This psammophyte is resistant to high temperatures, drought, and sandstorms. The presence of rhizosheath makes its ecological function stronger. Rhizosheaths are structures composed of mucilage secreted from plants and adherent soil particles that form a hard cylinder around the root. Its main part is a chamber which is segmented a few little rooms．S. pennata may store water in the chamber to resist drought．The rhizosheath of S. pennata is an important adaptation to arid environments, and an integral part of soil microbe ecology. However, lots of scholars argued the factors influenced the formation of the rhizosheath, especially for the function of microbes. .Our previous study showed that rhizosheath microhabitats were favorable environment for the growth and development of microorganisms. This project supports the hypothesis that the microbe will be the necessary condition for the formation of rhizosheath of S. pennata. Both the mycelium and secretion will be the key factor to affect the stability of rhizosheath. The microbial diversity will be higher in the rhizosheaths and the surrounding soil. The growth of dominant species will be significant influenced by root exudation. We designed a series of controlled lab experiments to verify the hypothesis. First, in order to determine whether microbe is the necessary condition for the formation of rhizosheath, we will carry out the rhizosheath culturing experiment both on the sterile sand and nature sand in the laboratory. Second, the eukaryote （fungi) and prokaryote of bacteria contribution to rhizosheath formation will be determined with selective inhibition techniques applying to sands. Scanning electron microscopy will help us to observe the structure of rhizosheath in details. Third, general methods of culture identifying fatty acid, BIOLOG and PCR-DGGE will be utilized to contrast the microbial diversity between rhizosheaths and the surrounding soil. Focus on the dominant species of the microbe in the rhizosheaths, we will design selective culture medium to obtain the functional strain. Crude polysaccharlde was extracted separately from strain mucilaginosus and identified by ultraviolet spectrum. At last, after cultivating the plant in Hoagland liquid, the root exudation will be extracted with C2 C12, and identified by the way of gas chromatography-mass spectrometry . To discuss the relationship between the exudation from the plant and microbe, we will further investigate not only the influence of root exudation on the growth of dominant species of the microbe in the rhizosheaths, but also their soil binding properties, their mix-effect to the character of rhizosheaths. .This study will expand our knowledge of the relationship between microbe and the formation of rhizosheath of S. pennata. We will well understand the anatomical structure and ecological adaptability

羽毛针禾是古尔班通古特沙漠重要的先锋物种，对流动沙丘优势群落演替有显著推动作用。其根区特殊的根鞘是其有别于木本沙生植物的重要抗旱结构。根鞘中独特的腔室能明显提高羽毛针禾对有限降水的吸收、利用效率；根鞘具足够的强度，有别于其它植物根系对沙粒的简单粘附，能有效保护根系免受损伤。然而，根鞘的形成机理，尤其是微生物在其中的重要作用尚不明确。.本项目拟以微生物为切入点展开室内控制条件下的根鞘培养实验，对自然沙土进行不同处理，对微生物进行不同程度的干扰，结合根鞘结构的电镜观测及抗压强度分析，验证土壤微生物对根鞘形成的必要性。针对根鞘中微生物分泌物成分及作用了解匮乏的现状，拟基于微生物多样性的分析结果，有目的地设计选择性培养基，分离根鞘中独特的优势类群，对比分析其分泌物与根系分泌物的组分差异，其生长状况与根系分泌物的关系等，进一步揭示微生物与根鞘形成的关系，从结构适应性的角度理解荒漠先锋禾草的抗旱机制。

羽毛针禾是古尔班通古特沙漠重要的先锋物种，对流动沙丘优势群落演替有显著推动作用。其根区特殊的根鞘是重要抗旱结构，具有多重生态功能。然而，根鞘的形成机理，尤其是微生物在其中的重要作用尚不明确。本项目基于显微结构观测、微生物多样性解析、室内控制实验等探讨了微生物与根鞘形成的关系，结果表明：（1）显微观测结果证实，细菌分泌的胞外多糖和真菌菌丝体在根鞘形成中分别发挥着黏结和捆绑沙粒的作用。（2）根鞘的存在有效扩大了根系与土壤的接触面积，使根区环境相对稳定。根鞘在生长旺季成为潜在的水库和氮库，促进了根鞘微生物的生长和羽毛针禾植株体发育。（3）灭菌处理减缓了羽毛针禾种子萌发速率，降低了种子萌发率、幼苗成活率，导致幼株地上部分萎蔫枯黄最后整株死亡，地下部分出现黑根、根毛失水萎蔫、沙粒粘附性变差。进一步证明土壤微生物在羽毛针禾种子萌发、幼苗存活、根鞘初期形成中均发挥重要作用。（4）根鞘中可培养微生物优势类群为芽胞杆菌属(Bacillus)、链霉菌属(Streptomyces)、青霉属(Penicillium)，具有固沙、固氮、解磷、产抗生素等生态潜能，有助于改善根区营养吸收，促进植株生长。羽毛针禾种子中的内生菌不足以促进生长，需从土壤召集外源微生物；受根系分泌物影响强烈，根鞘内生菌与根系内生菌组成接近。根鞘中的土壤细菌群落代谢活性、群落多样性指数、对糖类、氨基酸类等各类碳源的利用率均显著高于外围土壤。高通量测序结果表明根鞘细菌中变形菌门（Proteobacteria）占绝对优势，蕴藏着丰富的根瘤菌目（Rhizobiales）固氮菌资源。根鞘中丛枝菌根真菌以球囊菌门（Glomeromycota）占绝对优势，可分泌具有 “超级胶水”功能的球囊霉素，不仅促进根鞘的稳固，而且在土壤碳转化方面发挥重要作用。总之，作为对高温、缺水恶劣环境的一种应答结构，根鞘的形成促进了植物根土界面的信息交流及水分和养分的交换，有利于植物、土壤、微生物建立互惠互利的关系，对植物忍耐干旱逆境胁迫尤为重要。