DIR12 Confers Rice Salt Tolerance by Mediating Lignin Deposition in Root Diffusion Barrier

Soil salinity severely threatens rice production worldwide, and root lignification-mediated apoplastic barrier formation is a key adaptive mechanism for rice salt tolerance. Here, we characterised DIR12, a plasma mnm embrane-localised dirigent-like protein in rice, which is transcriptionally induced by salt stress and highly expressed in root and stem tissues. DIR12 loss-of-function mutants exhibited significantly reduced salt tolerance, accompanied by disrupted Na⁺/K⁺ homoeostasis, compromised root lignification, and delayed and discontinuous lignin deposition in endodermal Casparian strips, resulting in a defective apoplastic diffusion barrier. Specifically, DIR12 mutation attenuates salt stress-stimulated lignin deposition in root. Consistently, transcriptomic analysis revealed that DIR12 is essential for the salt-induced upregulation of lignin biosynthesis-related genes in the phenylpropanoid pathway. Furthermore, DIR12 physically interacts with ascorbate peroxidase APX1 on the plasma membrane, and apx1 mutants showed salt sensitivity and reduced root lignin content. Moreover, dir12 mutants had decreased stem lignin levels and mechanical strength but no impairment in grain yield, and also exhibited altered mineral element accumulation in roots and shoots due to the defective root barrier. Collectively, our findings reveal that DIR12 positively regulates salt tolerance by promoting root lignification and apoplastic barrier formation, while also modulating stem mechanical strength and mineral homoeostasis in rice.