Enterobacter sp. SA187-induced coordinated regulation of high-affinity nitrate transporters and ethylene signaling enhances nitrogen content and plant growth under low nitrate

Sustainable crop production demands solutions to reduce the over use of synthetic nitrogen fertilizers, and plant-growth-promoting bacteria (PGPB) offer a promising alternative by enhancing nutrient acquisition under limiting conditions. This study shows that the endophytic bacterium Enterobacter sp. SA187 significantly improved the growth of Arabidopsis thaliana seedlings under low nitrate conditions. SA187 inoculation led to marked increases in biomass, primary root length, and lateral root density, with growth-promoting effects becoming more important with the severity of nitrate limitation. Analyses suggested enhanced nitrate uptake and improved nitrogen partitioning as indicated by reduced shoot carbon:nitrogen ratios. Transcriptomic profiling identified SA187-driven reprogramming of host gene expression, including the induction of high-affinity nitrate transporters (HATs) and hormone-responsive pathways. Functional analyses confirmed that the growth-promoting effect of SA187 required both ethylene signaling and specific HATs. The ein2, nrt2.5, and nrt2.6 mutants failed to show SA187-induced growth promotion, and expression analysis revealed that NRT2.5 was regulated downstream of ethylene signaling, while NRT2.6 was partially independent. These findings position SA187 as a conditionally beneficial endophyte that can improve plant growth through coordinated, context-dependent regulation of nitrate uptake and allocation. Such results highlight the potential of SA187 as a bioinoculant for improving plant performance under nitrate-limited conditions and thus reduce fertilizer dependency.