Responses of a model legume Lotus japonicus to lipochitin oligosaccharide nodulation factors purified from Mesorhizobium loti JRL501

Lotus japonicus has been proposed as a model legume for molecular genetic s tudies of symbiotic plant-microbe interactions leading to the fixation of a tmospheric nitrogen. Lipochitin oligosaccharides (LCOs), or Nod factors, we re isolated from the culture of Mesorhizobium loti strain JRL501 (MAFF30309 9), an efficient microsymbiont of L. japonicus B-129 cv. Gifu. High-perform ance liquid chromatography and mass spectrometric analyses allowed us to id entify at least five different structures of LCOs that were produced by JRL 501. The major component was NodMl-V(C18:1, Me, Cb, AcFuc), an N-acetyl-glu cosamine pentamer in which the nonreducing residue is N-acylated with a C-1 8:1 acyl moiety, N-methylated, and carries a carbamoyl group and the reduci ng N-acetyl-glucosamine residue is substituted with 4-O-acetyl-fucose. Addi tional novel LCO structures bearing fucose instead of acetyl-fucose at the reducing end were identified. Mixtures of these LCOs could elicit abundant root hair deformation on L. japonicus roots at a concentration of 10(-7) to 10(-9) M. Spot inoculation of a few nanograms of LCOs on L. japonicus root s induced the formation of nodule primordia in which the early nodulin gene s, ENOD40 and ENOD2, were expressed in a tissue-specific manner. We also ob served the formation of a cytoplasmic bridge (preinfection thread) in the s wollen outermost cortical cells. This is the first description of cytoplasm ic bridge formation by purified LCOs alone in a legume-forming determinate nodules.