A research group led by Professor Chuanchao Dai, from the School of Life Sciences of NNU, has made important progress in the research of endophytic fungi promoting legume nodulation. Recently, the research results were published online under the title of "Mycelial network-mediated rhizobial dispersal enhances legume nodulation" in the ISME Journal, a famous journal of ecology with an impact factor of 9.49.
His research group has been devoted to the mechanism of promoting peanut nodulation and nitrogen fixation. Previous studies have found that endophytic fungal colonization with P. liquidambaris significantly increases the productivity, nodulation, and N2 fixation of peanuts through the secretion of specific root exudates.
At the same time, it was found that the plant hormone auxin and jasmonic acid’s signals in the roots of peanuts were activated by p.liquidambar through transcription to activate a nodule signaling pathway and change the distribution of photosynthesis products, which provides more carbon sources for peanut nodules, and energy for the development of nodule primordium, nodule formation and nitrogen fixation. Relevant research results were published in mBio with the journal impact factor of 6.75, Plant,Cell & Environment with an impact factor of 5.62 and other magazines in 2019.
The access of rhizobia to a legume host is a prerequisite for nodulation. Rhizobia are poorly motile in soil, while filamentous fungi are known to grow extensively across soil pores. Since root exudates-driven bacterial chemotaxis cannot explain rhizobial long-distance dispersal, mycelia could constitute ideal dispersal networks to help rhizobial enrichment in the legume rhizosphere from bulk soil.
Thus, the research group hypothesized that mycelia networks act as vectors that enable contact between rhizobia and legume and influence subsequent nodulation. By developing a soil microcosm system, the research group found that a facultatively biotrophic fungus, Phomopsis liquidambaris, helps rhizobial migration from bulk soil to the peanut (Arachis hypogaea) rhizosphere and, hence, triggers peanut–rhizobium nodulation but it is not seen in the absence of mycelia. Assays of dispersal modes suggest that cell proliferation and motility mediate rhizobial dispersal along mycelia, and fungal exudates might contribute to this process.
Furthermore, transcriptomic analysis indicates that genes associated with the cell division, chemosensory system, flagellum biosynthesis, and motility were regulated by Ph. liquidambaris, thus accounting for the detected rhizobial dispersal along hyphae. Our results indicate that rhizobia use mycelia as dispersal networks that migrate to the legume rhizosphere and trigger nodulation. This work highlights the importance of mycelial network-based bacterial dispersal in legume–rhizobium symbiosis.
Wei Zhang, a doctoral student from the school of Life Sciences, is the first author of this paper. The corresponding author of the paper is professor Chuanchao Dai of the school of Life Sciences. The co-authors of the paper are professor Xiaogang Li (co-first author) from Nanjing Forestry University, Kai Sun, Mengjun Tang and Fangji Xu, doctoral students from the school of Life Sciences of NNU, and Zhang Ming, a PhD candidate from the Danish University of Science and Technology. The research work is supported by the general program of NSFC and the program of provincial advantageous disciplines.