Isolation, identification, and plant growth-promoting mechanisms of strain BN5, with a focus on exogenously Trp-independent IAA biosynthesis, and its impact on cucumber cultivation

• First systematic report of Niallia taxi BN5 with plant growth-promoting (PGP) activity, filling the Niallia PGP mechanism gap (previously studied for bioremediation/antibacterial functions). • Multi-omics (genome/transcriptome/qPCR) show BN5 has a complete trp operon for constitutive IAA biosynthesis (17.64 μg/mL); tryptophan regulates trp/motility genes. • BN5 enhances cucumber growth (height +24.29%, stem +11.23%), yield (+17.55%) and quality (vitamin C +17.2%, soluble sugar +37.41%) in pot/greenhouse trials. • BN5 reshapes rhizosphere microbiomes (enriches Bacillus / Pseudomonas ) and elevates soil enzymes, providing a microbial resource for sustainable agriculture. The genus Niallia was recently separated as a taxonomic group from Bacillus based on conserved signature insertions and deletions in the genome. Unlike its role in bioremediation, its function in plant growth promotion has not attracted widespread attention. This study identifies Niallia taxi BN5, focusing on the role of tryptophan in IAA synthesis and regulation, as well as the effects of cucumber. The study employed pot and greenhouse experiments to evaluate the impacts of BN5 on cucumber growth, yield, and quality. Genome analysis explored the tryptophan-regulated IAA biosynthesis pathway, with transcriptome and qPCR examining expression of IAA-related, tryptophan synthesis, and motility genes. It also evaluated impacts on rhizosphere microbial communities and soil enzyme activities. BN5 significantly enhanced cucumber growth (height: +24.29%, stem diameter: +11.23%), yield (+17.55%), and quality (vitamin C: +17.2% soluble sugar: +37.41%, all p