Colonization by the Mycorrhizal Helper Bacillus pumilus HR10 Is Enhanced During the Establishment of Ectomycorrhizal Symbiosis Between Hymenochaete sp. Rl and Pinus thunbergii.

Ya-Hui Wang,Wei-Liang Kong,Xiao-Qin Wu,Mei-Ling Zhu,Yun Dai

doi:10.3389/fmicb.2022.818912

Abstract

There are complex interactions between mycorrhizal helper bacteria (MHBs) and ectomycorrhizal (ECM) fungi, with MHBs promoting mycorrhizal synthesis and ECM fungi regulating plant rhizobacterial colonization, diversity, and function. In this study, to investigate whether the ECM fungus Hymenochaete sp. Rl affects the survival and colonization of the MHB strain Bacillus pumilus HR10 in the rhizosphere, the biomass of B. pumilus HR10 was measured in the rhizosphere and mycorrhizosphere. In addition, extracts of Hymenochaete sp. Rl and Pinus thunbergii were evaluated for their effect on B. pumilus HR10 colonization (growth, sporulation, biofilm formation, extracellular polysaccharide and extracellular protein contents, flagellar motility, and expression of colonization-related genes). The results showed that inoculation of Hymenochaete sp. Rl significantly increased the biomass of B. pumilus HR10 in the rhizosphere; however, while extracts of Hymenochaete sp. Rl and P. thunbergii did not affect the biomass or spore formation of HR10, they did affect its biofilm formation, extracellular polysaccharide and extracellular protein production, and flagellar motility. Furthermore, the addition of symbiont extracts affected the expression of chemotaxis-related genes in HR10. When the extracts were added separately, the expression of srf genes in HR10 increased; when the extracts were added simultaneously, the expression of the flagellin gene fliG in HR10 increased, but there was no significant effect on the expression of srf genes, consistent with the results on biofilm production. Thus, Hymenochaete sp. Rl and P. thunbergii roots had a positive effect on colonization by B. pumilus HR10 at the rhizosphere level through their secretions.

Highlights

Ectomycorrhizal (ECM) fungi establish symbiosis with the roots of most trees in boreal and temperate ecosystems and are major drivers of nutrient circulation between trees and the soil (Plassard and Dell, 2010; Martin et al, 2016; Moreau et al, 2019; Tedersoo et al, 2020)
Bacillus pumilus HR10 was originally isolated from the rhizosphere soil of mycorrhizal seedlings of Pinus thunbergii (Sheng et al, 2014); it is a mycorrhizal helper bacterium that promotes the formation of P. thunbergii—Hymenochaete sp
The Biomass of Bacillus pumilus HR10 Was Improved in a Pinus thunbergii-Hymenochaete sp

Summary

Introduction

Ectomycorrhizal (ECM) fungi establish symbiosis with the roots of most trees in boreal and temperate ecosystems and are major drivers of nutrient circulation between trees and the soil (Plassard and Dell, 2010; Martin et al, 2016; Moreau et al, 2019; Tedersoo et al, 2020). Mycorrhizal fungi interact with the soil bacterial and modify the rhizosphere microbial community This part of the study is mostly seen in the effect of inoculation of arbuscular mycorrhizal (AM) fungi on the growth of rhizosphere bacteria, and the effect of ECM fungi on rhizosphere bacteria is rarely reported. Nineteen of the 23 Pseudomonas strains promoted the growth of L. bicolor, three of them had positive effects on mycorrhizal formation and one strain inhibited mycorrhization; two strains significantly inhibited the growth of L. bicolor and inhibited mycorrhization. These bacterial strains that positively influenced the establishment and functioning of mycorrhizal symbioses were categorized as mycorrhizal helper bacteria (MHBs; Garbaye, 1994). Current theory holds that MHB play a role in promoting the mycelial growth of mycorrhizal fungi, reducing the concentration of toxic substances in the soil, promoting the development of host plant roots, and increasing mycorrhizal infection, thereby achieving the ultimate goal of improving the efficiency of mycorrhizal formation (Poole et al, 2001; Vivas et al, 2006; Deveau et al, 2007; Zhao et al, 2013; Armada et al, 2016; Shinde et al, 2019)

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