# Bacillus subtilis 8–32 enhances tomato growth and reshapes rhizosphere microbial communities: insights into plant-microbe interactions

## TL;DR

A strain of Bacillus subtilis boosts tomato growth and changes soil microbes, offering a sustainable agriculture solution.

## Contribution

Demonstrates how Bacillus subtilis 8–32 promotes tomato growth and reshapes rhizosphere microbial communities.

## Key findings

- Bacillus subtilis 8–32 colonized tomato roots and soil effectively over 21 days.
- Root drenching with the strain significantly improved plant growth and root vitality.
- The strain enriched beneficial microbes and reduced pathogenic fungi without altering diversity.

## Abstract

Plant growth-promoting rhizobacteria (PGPR) are crucial for sustainable agriculture, but their efficacy depends heavily on their colonization capacity within the rhizosphere and their interactions with native microbial communities. In previous studies, Bacillus subtilis strain 8-32 was screened for its potent antagonistic activity against pathogenic fungi and its high capacity for producing indole-3-acetic acid (IAA). This study aimed to investigate the colonization dynamics of strain 8-32 in tomato plants and evaluate its impact on the rhizosphere microbial community structure.

Green fluorescent protein (GFP) labeling was used to track the colonization of B. subtilis 8-32 in tomato plants. Tomato seedlings were randomly divided into three treatment groups: the control group (CK), the seed-soaking group (T1), and the root-drenching group (T2). During the 21-day experimental period, plant growth parameters, root activity, and malondialdehyde (MDA) content were monitored. Changes in bacterial and fungal community structures were analyzed via high-throughput sequencing of the 16S rRNA and ITS regions.

The results revealed that Bacillus subtilis 8-32 successfully colonized both the tomato root system and the surrounding soil. On days 14 and 21, the colonization levels in the root system reached 7.1130±0.0413 (log10 CFU/g), while in the soil, they were 6.4664±0.03620 (log10 CFU/g) and 7.111±0.0461 (log10 CFU/g), respectively. T1 group and T2 group exhibited significant growth improvements compared to CK. Specifically, on day 14, the root length, root weight, stem length, and stem weight of T2 group increased by 19.96%, 381.81%, 39.97%, and 145.33%, respectively, compared to CK. Root vitality in the T2 group was 39.77%, 177.24%, and 171.16% higher on day 7, 14, and 21, respectively, while malondialdehyde content decreased by 24.60%, 34.18%, and 71.34%, over the period. Microbial diversity analysis revealed that Bacillus subtilis 8-32 did not significantly alter the community α-diversity (P>0.05), but selectively reshaped the community composition: it enriched beneficial bacterial taxa such as Proteobacteria, Bacteroidota, and Actinobacteriota, enhanced the functional diversity of Ascomycota, and concurrently reduced the abundance of pathogenic fungi within Basidiomycota.

These findings confirm that Bacillus subtilis 8–32 exerts growth-promoting effects on tomatoes through efficient colonization, regulation of rhizosphere microecological structure, and synergistic enhancement of plant stress resistance. Application of this strain by root drenching exhibits promising potential in tomato production, offering a novel approach to reduce reliance on chemical fertilizers and pesticides and facilitate the development of sustainable agriculture.