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

The genus <i>Niallia</i> was recently separated as a taxonomic group from <i>Bacillus</i> 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 <i>Niallia taxi</i> 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 <i>p</i> <i><</i> <i>0.05</i>). It increased soil enzyme activities (urease by +52.12 %, dehydrogenase by +44.70 %, <i>p</i> <i><</i> <i>0.05</i>) and reshaped the microbial structure. Owing to its possession of a complete tryptophan synthase operon (<i>trp</i>E-D-G-C-F-B-A), strain BN5 is able to produce IAA at a concentration of 17.64 μg/mL via a tryptophan-dependent biosynthetic pathway, with no reliance on exogenous tryptophan. Exogenous tryptophan downregulated the <i>trp</i> cluster but upregulated motility genes (<i>mcp</i> and <i>fli</i>C, <i>p</i> <i><</i> <i>0.05</i>), indicating adaptive regulation of IAA precursors. This study clarifies the plant growth-promoting mechanisms of <i>Niallia</i> spp., focusing on tryptophan-mediated regulation of IAA biosynthesis that is independent of exogenous tryptophan. It lays a foundation for efficient microbial fertilizers, highlighting BN5's potential in sustainable agriculture.