A vitamin B₃-driven root bacterial metabolite primes systemic immunity in Arabidopsis

The microbiota is being increasingly recognized for its ability to regulate host physiology through the production of small bioactive molecules. However, how host-derived nutrients are metabolically transformed by root-associated microbes to influence plant immunity remains poorly understood. Here, we show that vitamin B₃ (VB₃; niacin) secreted by plant roots shapes the assembly of a functionally specialized root microbiota, which, in turn, metabolizes VB₃ into an immune-active signal that enhances plant disease resistance. VB₃ secretion selectively increases the abundance of root-associated bacteria harboring a conserved nic biosynthetic gene cluster (BGC), which enables the conversion of VB₃ into 6-hydroxynicotinate (6-OHNA), a previously uncharacterized microbial metabolite involved in plant-microbe interactions. Microbially produced 6-OHNA is transported from roots to shoots, where it primes systemic immune responses in a salicylic acid-dependent manner. Disruption of the microbial nic BGC abolishes immune priming, whereas increased VB₃ exudation from plant roots enhances disease resistance. Together, these findings reveal a metabolically mediated dialog between plant hosts and their microbiota that links host nutrient secretion to microbial functional specialization and the activation of systemic plant immunity.