Auxin, Strigolactones, and Nitric Oxide Coordinate Lateral Root Development in Rice During Early Interaction With Rhizophagus irregularis.

Arbuscular mycorrhizal fungi (AMF) form symbiotic associations with plant roots, profoundly shaping root system architecture (RSA) and influencing nutrient acquisition in crops. This modulation begins during the early pre-symbiotic stage, when plants and fungi interact without physical contact. Root formation is orchestrated by signaling molecules, including hormones such as auxin (IAA) and strigolactones (SLs), as well as reactive nitrogen species such as nitric oxide (NO). However, the mechanisms by which AMF spores modulate these pathways to influence root development in rice remain largely unexplored. Here, we investigated the effects of Rhizophagus irregularis spores on root formation in Oryza sativa L., focusing on IAA, SLs, and NO modulation. Exposure to both live and autoclaved spores enhanced lateral root formation in adventitious roots, whereas only live spores promoted elongation and secondary branching of large lateral roots (LLRs), a rice-specific feature. These effects were correlated with increased IAA levels, transcriptomic changes, and decreased SL accumulation, revealing an integrated signaling network controlling LLR development. Histochemical analyses revealed NO accumulation in the root elongation zone and apex, accompanied by the upregulation of the high-affinity nitrate transporter OsNRT2.1 in LLRs. Together, our findings reveal a root-type-specific involvement of IAA, SLs, and NO in shaping RSA during the pre-symbiotic stage of AMF interactions. This study provides new insights into early signaling events that mediate host discrimination and regulate root architecture during the pre-symbiotic phase of AMF establishment.