Sugars altered fungal community structure and caused high network complexity in a Fusarium wilt pathogen infested soil

Root exudate plays a pivotal role in plant pathogen growth, microbial community assembly, and thus the plant health. Although sugar is a dominant component of root exudate, scare is known about its influence on the growth of fungal pathogen, such as Fusarium wilt pathogen, the associated the fungal communities, and especially the interaction among fungi, as well as the relationship between network keystone taxa and the Fusarium wilt pathogen. In this study, microcosm incubation experiment was constructed by adding four simple sugars (including two monosaccharides (glucose and fructose,) and two disaccharide (sucrose and maltose)) to Fusarium wilt pathogen (i.e., F. oxysporum, FO) infested soils. The natural soil, collected from a Fusarium wilt pathogen infested filed, was used as low-FO soil and the natural soils after receiving further Fusarium wilt pathogen inoculation was used as High-FO soil. Real-time PCR and Illumina sequencing were used to investigate the fungal community changes, and molecular ecological network analysis was used to understand the fungal interactions. After sugar addition, the Fusarium wilt pathogen showed a higher biomass throughout entire 42 days of incubation. Sugar-added soils harbored significantly (P<0.05) different fungal community structure from the sugar-unadded soil while the community structure from four sugar treatments were similar. Moreover, sugar addition caused decrease in fungal richness but increase in evenness in Low-FO soil while it led to decrease in both fungal richness and evenness in High-FO soil. At the genus level, Humicola was the only taxa that not only had a comparable relative abundance with Fusarium but also was significantly increased in both relative abundance and absolute abundance after sugar addition in Low-FO soil throughout 42 days of incubation. More importantly, Humicola could have more rapid biomass increase than Fusarium after addition of certain sugars, including glucose, fructose, and maltose, at some time points. In High-FO soil, the relative abundance (a reflection of the dominance/survival ability) of Humicola was increasing but the relative abundance of decreasing over time under all treatments, such that at 42d, the relative abundance of Humicola under all sugar-added treatments even exceeded that under sugar-unadded. Sugar-added network were more complex and connected than those in sugar-unadded, indicating a greater potential for fungal interaction and niche-sharing. The positive interaction between the keystone taxa and the most abundant OTUs within Fusarium in sugar-added soil in most time points indicated a suppressive effect of the keystone taxa on Fusarium wilt pathogen while the negative or no interaction between them indicated a potential promotion or no constraint effect. This study enhanced our understanding of the fate of Fusarium wilt pathogen, the changes in fungal communities and the fungal networks, and thus the mechanisms of the vigorous proliferation of this pathogen after sugar addition from the viewpoint of fungal interaction.