丛枝菌根真菌Rhizophagus irregularis菌丝分泌物可诱导解磷细菌Rahnella aquatilis向菌丝移动

Hyphal exudates of an arbuscular mycorrhizal fungus Rhizophagus irregularis induce phosphate-solubilizing bacterium Rahnella aquatilis to swim towards its hyphae

丛枝菌根真菌(AM真菌)和菌丝际解磷细菌是土壤中广泛存在的功能类微生物,两者通过合作能帮助植物获取更多磷,改善植物的磷素营养.AM真菌将来自植物的碳水化合物分泌到菌丝际,招募解磷细菌在菌丝际定殖并共同完成土壤难溶性磷的活化利用,但菌丝分泌物吸引解磷细菌向菌丝移动的机制仍不清楚.为了理解解磷细菌向菌丝移动过程中的趋化性作用,本文利用3D打印微流体芯片,研究菌丝分泌物对解磷细菌的趋化作用,揭示解磷细菌在土壤中向菌丝际移动并定殖的过程.试验结果表明,菌丝分泌物存在时解磷细菌的趋化性系数(C/C0)为0.243,而无菌丝分泌物对照组的趋化性系数仅为0.033;解磷细菌接触菌丝分泌物2 h后,其控制趋化性运动的组氨酸蛋白激酶基因CheA和连接蛋白基因CheW表达水平分别上调了4和2倍.上述结果表明,AM真菌对解磷细菌的招募涉及趋化性过程,菌丝分泌物中存在趋化因子并能够吸引解磷细菌的趋向性移动.

Arbuscular mycorrhizal fungi(AM fungi) not only form hyphal networks which expand their nutrient absorption range beyond roots,but also release carboxylates or protons to mobilize insoluble inorganic P,then improve plant phosphorus status.However,recent studies show that AM fungi have little ability to use organic P in soil,for example.This is because they don’t harbor the genes encoding phytases,which hydrolyze phytate,the predominant form of organic P in agricultural soil,which accounts for about 20%-80% of the total P in a wide range of soils.In order to utilize organic P,AM fungi have to cooperate with phosphate-solubilizing bacteria(PSB),which have a strong ability to produce phosphatase.To maintain the cooperation,AM fungi usually release carbohydrates to soil to recruit PSB to colonize in hyphosphere.The extraradical hyphae of AM fungi are not only the habitat of PSB,but also the "highway" that helps PSB to move to the organic P patch,by providing a water film to travel in and carbon source for energy in the form of hyphal exudates.However,before moving along the hyphae,how PSB migrate to the hyphae is not clear.Chemotaxis is the ability of microorganisms to sense chemical gradients and move either up the gradient towards the source(attraction) or down the gradient away from the source(repulsion).Here,we investigated whether chemotaxis plays a role in the process of bacteria migrating towards the AM fungal hyphae.We hypothesized that hyphal exudates act as an attractant to PSB,and during the contact process,the expression level of chemotaxis-related genes(e.g.,Che A,Che W) will be upregulated.Using a microfluidic chip made by 3 D printing technology,we observed that in 45 min PSB showed a significant positive response to the hyphal exudates and the chemotactic index(C/C0) is 0.243,was significantly greater than the control(culture bacteria in the medium without any carbon).Under strict in situ culture conditions,after 2 h of contact with the exudates,the expression of the gene-encoding histidine kinases(Che A) and connexin(Che W) increased 4 and 2 folds,respectively.These results provide evidence that hyphal exudates are an attractant and PSB can migrate up the exudates gradient.Our results suggest,for the first time,that the process of AM fungi R.irregularis recruiting phosphatesolubilizing bacteria R.aquatilis involves chemotaxis,and some chemokines in hyphal exudates can attract bacteria to swim chemotactically.