The ta-si RNA pathway is required for lateral organ development, including leaf patterning, flower differentiation and lateral root growth. Legumes can develop novel lateral root organs - nodules - resulting from symbiotic interactions with rhizobia. However, ta-si RNA regulation in nodule formation remains unknown. To explore ta-si RNA regulation in nodule formation, we investigated the roles of REL3, a key component of TAS3 ta-si RNA biogenesis, during nodulation in Lotus japonicus., We characterized the symbiotic phenotypes of the TAS3 ta-si RNA defective rel3 mutant, and analyzed the responses of the rel3 mutant to auxin and ethylene in order to gain insight into TAS3 ta-si RNA regulation of nodulation., The rel3 mutant produced fewer pink nitrogen-fixing nodules, with substantially decreased infection frequency and nodule initiation. Moreover, the rel3 mutant was more resistant than wild-type to 1-naphthaleneacetic acid ( NAA) and N-1-naphthylphthalamic acid ( NPA) in root growth, and exhibited insensitivity to auxins but greater sensitivity to auxin transport inhibitors during nodulation. Furthermore, the rel3 mutant has enhanced root-specific ethylene sensitivity and altered responses to ethylene during nodulation; the low-nodulating phenotype of the rel3 mutant can be restored by ethylene synthesis inhibitor L-α-(2-aminoethoxyvinyl)-glycine ( AVG) or action inhibitor Ag+., The REL3-mediated TAS3 ta-si RNA pathway regulates nodulation by integrating ethylene and auxin signaling. [ABSTRACT FROM AUTHOR]