Endophytic Fungus Drives Nodulation and N ₂ Fixation Attributable to Specific Root Exudates

Abstract

Endophytic fungi play important roles in the modification of ecosystem productivity; however, the underlying mechanisms are only partly understood. A 2-year field plot experiment verified that the endophytic fungus Phomopsis liquidambaris increased peanut (Arachis hypogaea L.) yields and significantly increased nodulation and N2 fixation regardless of whether N fertilizers were added. Root exudates collected from P. liquidambaris-colonized plants significantly improved nodulation and N2 fixation. Rhizosphere stimulation experiments further showed that colonized root exudates had significantly decreased soil nitrate (NO3−) concentrations, with decreased abundance and diversity of ammonia oxidizing archaea (AOA). In contrast, the abundance and diversity of diazotrophs significantly increased, and most diazotrophs identified were peanut nodulation-related strains (Bradyrhizobium sp.). P. liquidambaris symbiosis increased the expression of phenolic and flavonoid synthesis-related genes, and the derived phenolics and flavonoids could effectively increase the chemotaxis, biofilm formation, and nodC gene expression (nodulation-related biological processes) of the Bradyrhizobium strain. Metabolic pattern analysis showed that phenolics and flavonoids are more likely to accumulate to higher levels in the rhizosphere soil of peanuts colonized with P. liquidambaris. Finally, a synthetic root exudate experiment further confirmed the underlying mechanisms for the P. liquidambaris-induced improvement in nodulation and N2 fixation, i.e., that the specific root exudates derived from P. liquidambaris colonization decrease nitrate concentration and increase the population and biological activities of peanut nodulation-related Bradyrhizobium species, which beneficially enhance peanut-Bradyrhizobium interactions. Therefore, this study is the first to provide new insight into a positive relationship between an exotic endophytic fungus, crop nodulation, and N2 fixation increase. IMPORTANCE Endophytic fungi play an important role in balancing the ecosystem and boosting host growth; however, the underpinning mechanisms remain poorly understood. Here, we found that endophytic fungal colonization with P. liquidambaris significantly increased the productivity, nodulation, and N2 fixation of peanuts through the secretion of specific root exudates. We provide a reasonable mechanism explaining how P. liquidambaris promotes peanut nodulation and N2 fixation, whereby the specific root exudates produced by P. liquidambaris colonization decrease rhizosphere soil nitrate (NO3−) and increase the population and biological activities of peanut-nodulating-related Bradyrhizobium strains, which is beneficial to enhancing the peanut-Bradyrhizobium symbiotic interaction. Our study provides reliable empirical evidence to show the mechanism of how an exotic endophytic fungus drives an increase in nodulation and N2 fixation, which will be helpful in erecting a resource-efficient and sustainable agricultural system.

Impact of Soybean Nodulation Phenotypes and Nitrogen Fertilizer Levels on the Rhizosphere Bacterial Community

“…The similar bacterial community compositions in rhizospheres of the treatments D1 and H3 (Figures 1, 2) might imply that similar root exudates were produced by soybean plants in these two treatments, since the rhizosphere microbiome was strongly regulated by root exudates (Haichar et al, 2008;Subbarao et al, 2009;Huang et al, 2014;White et al, 2015;Szoboszlay et al, 2016) and compounds sloughed off root tips (Bulgarelli et al, 2013), which could be related to N fertilization, colonization of endophytic microbe, and N 2 fixation of legume plant (Xie et al, 2019). To confirm this estimation, a comparative study on the root exudates is needed.…”

Plant Growth and Soil Microbial Impacts of Enhancing Licorice With Inoculating Dark Septate Endophytes Under Drought Stress

“…Although some studies have reported that DSE can act as plant growth promoters (Zhu et al, 2015; He et al, 2019; Xie et al, 2019), their effects on plants are still limited under drought stress conditions (Li et al, 2018). The results of existing studies on the effects of inoculation of DSE on plant growth under water deficiency conditions are variable.…”

Mycelial network-mediated rhizobial dispersal enhances legume nodulation

“…Experiments were performed with Ph. liquidambaris strain B3 and peanut-nodulating rhizobium, Bradyrhizobium yuanmingense (hereafter Bradyrhizobium) [8,22]. To mimic soil conditions, soil extraction agar (SEA, 1.5% agar) was used to activate Ph.…”

“…Although no direct interactions between two root symbionts and concomitant effects on nodulation are known, Ph. liquidambaris inoculation increased bradyrhizobial diversity in peanut rhizosphere [22], suggesting the possible role of Ph. liquidambaris in rhizobial rhizosphere enrichment.…”

“…Several mechanisms have been elucidated regarding Ph. liquidambaris-mediated nodulation enhancement, including plant hormone modifications [11], leaf CO2 fixation increasement [9] and soil microflora improvement [22]. Although no direct interactions between two root symbionts and concomitant effects on nodulation are known, Ph.…”