# Bacillus velezensis BY6 Controls Armillaria Root Rot in Poplar by Reshaping Rhizosphere–Phyllosphere Microbiomes and Inducing Systemic Resistance

## TL;DR

Bacillus velezensis BY6 reduces poplar root rot by altering soil and leaf microbes and boosting plant defenses.

## Contribution

BY6's dual mechanism of microbiome reshaping and systemic resistance induction is newly demonstrated.

## Key findings

- BY6 reduced disease severity by 37.19% in poplar against Armillaria root rot.
- BY6 enriched beneficial bacteria like Pantoea ananatis and Acidobacteria in rhizosphere and phyllosphere.
- BY6 activated plant defenses via SA/JA pathways and suppressed auxin transporter gene AUX1.

## Abstract

Armillaria solidipes, the causal agent of Armillaria root rot, poses a severe and persistent threat to poplar forest plantations. This study evaluated the biocontrol efficacy of the endophytic bacterium Bacillus velezensis BY6 against this pathogen and elucidated its multimodal mechanisms of action. BY6 application significantly reduced disease severity by 37.19% at 30 days post-treatment. 16S rRNA (V3–V4) microbiome analysis revealed that BY6 reshaped both the rhizosphere and phyllosphere bacterial communities, consistently enriching beneficial taxa, including Pantoea ananatis and members of Acidobacteria, while suppressing opportunistic groups. Concurrently, BY6 activated systemic defenses in poplar, evidenced by enhanced activities of key enzymes PAL and POD, and the upregulated expression of SA/JA pathway marker genes (PR1, JAZ, and COI1), coupled with the downregulation of the auxin transporter gene AUX1. These data indicate that the biocontrol efficacy of B. velezensis BY6 was mediated by a dual mechanism: the modulation of both rhizospheric and phyllospheric bacterial communities, direct elicitation of systemic defense pathways in poplar, which synergistically enhanced resistance against A. solidipes.