A vitamin B₃-driven root bacterial metabolite primes systemic immunity in <i>Arabidopsis</i>.

Abstract

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 <i>nic</i> 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 <i>nic</i> 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.