A lipochito-oligosaccharide, Nod factor, induces transient calcium influx in soybean suspension-cultured cells

Lipochito-oligosaccharides (Nod factors) produced by Rhizobium or Bradyrhizobium are the key signal molecules for eliciting nodulation in their corresponding host legumes. To elucidate the signal transduction events mediated by Nod factors, we investigated the effects of Nod factors on the cytosolic [Ca²⁺] of protoplasts prepared from roots and suspension-cultured cells of soybean (Glycine max and G, soja) using a fluorescent Ca²⁺ indicator, Fura-PE3. NodBj-V (C18:1, MeFuc), which is a major component of Nod factors produced by Bradyrhizobium japonicum, induces transient elevation of cytosolic [Ca²⁺] in the cells of soybean within a few minutes. This effect is specific to soybean cells and was not observed in the tobacco BY-2 cells. Furthermore, NodBj-V without MeFuc did not induce any cytosolic [Ca²⁺] elevation in soybean cells. Exclusion of Ca²⁺ from the medium, as well as pre-treatment of the cells with an external Ca²⁺ chelator or with a plasma membrane voltage-dependent Ca²⁺ channel inhibitor, suppressed the Nod factor-dependent cytosolic [Ca²⁺] elevation. These results indicate that transient Ca²⁺ influx from extracellular fluid is one of the earliest responses of soybean cells to NodBj-V (C18:1, MeFuc) in a host-specific manner.