Endophytic Flavobacterium promotes root hair development and enhances drought tolerance via an ERF-CEP5 hormonal regulatory module.

Root hairs (RHs) are cellular outgrowths of plant root epidermal cells that are important for water uptake, nutrient acquisition and rhizosphere dynamics. Key genes controlling RH development have been identified, but the regulatory mechanisms of RH growth during drought stress remain largely elusive. Here we show that the bacterial root endophyte Flavobacterium sp. 98 (Flavo98) modulates root system architecture with a pronounced promotion of RH formation and cell elongation that is maintained under low-water conditions. We also show that Flavo98 improves plant performance under drought in diverse plant species, including Arabidopsis and wheat, and mitigates drought-associated seed yield loss. Using cellular, molecular and genetic analyses in Arabidopsis, we identified root stele-expressed ERF transcription factors, ERF115 and ERF114, as key mediators of Flavo98-induced RH development and plant drought responses. We show that both ERFs operate, in part, by inducing the expression of the small signalling peptide C-TERMINALLY ENCODED PEPTIDE 5 (CEP5). In addition, we present evidence for enhanced ethylene biosynthesis and signalling in Flavo98-induced ERF115 and ERF114 expression, revealing a new pathway that integrates ethylene signalling into RH regulation under drought. Together, our findings uncover a pathway that regulates RH growth as part of a broader, microorganism-dependent root system architecture plasticity under low-water conditions. This study highlights the potential of uncovering plant-microorganism mechanisms to strengthen crop resilience in a changing climate.