Massilia strains facilitate root growth and reshape root microbiota through diverse salicylic acid hydrolysis pathways

Rhizosphere microbiota play an important role in maintaining plant root growth. However, the physiological and molecular basis of microbial regulation of root traits remains poorly understood. Here, we report that Massilia efficiently colonizes roots and promotes root elongation. In particular, the M117 strain of Massilia significantly inhibits salicylic acid (SA)-related immune signaling, which is required for M117-mediated root elongation. M117 can directly degrade SA, thereby reducing SA levels in the roots. Integrated omics reveal the presence of multiple SA hydrolytic pathways in M117. Among them, the NagGHAaAb pathway is strongly induced by SA. This pathway regulates root growth and is nonrandomly distributed across Massilia species. Finally, we show that M117 colonization enriches specific bacterial taxa within roots. Our findings reveal a specific pathway employed by rhizosphere bacteria to colonize roots and promote their growth and highlight a useful microbial strategy and information for balancing host immunity and growth.