Podbel’skogo, St. Petersburg, 196608 Russia, e-mail tsyganov@arriam.spb.ru Acknowledgements: Supported by Russian Science Foundation (project ¹ 14-24-00135) A b s t r a c t The process of symbiotic nodule formation resulting from interaction between legume plants and rhizobia is controlled by both partners. <...> From the plant side the important role belongs to a system of hormonal regulation, involving all classes of phytohormones identified in plants. <...> Moreover, nodules lacking nitrogen fixation might be formed during interaction with ineffective strain of rhizobia, and it is disadvantageous for the plant. <...> The first negative effect of ethylene appears at the level of calcium spiking, triggered by Nod-factors produced by rhizobia. <...> Further, ethylene negatively influences deformations of roots hairs, stimulated by Nod-factors, infection thread growth, as well as nodule primordium development. <...> In tropical legume Bremek. & Oberm. ethylene represses the activity of nodule meristem, leading to formation of determinate type of nodule (with temporal meristem activity), while at the absence of ethylene indeterminate nodules (with prolonged meristem activity) are formed. <...> At the same time, it was found that in soybean (L.) Merr., ethylene is not involved in regulation of nodulation. <...> It seems that ethylene involvement into regulation of nodule formation is not strictly dependent on the type of nodules, since in the other legume plants, forming determinate nodules, number of nodules is negatively affected by ethylene. <...> It is suggested that ethylene synthesis in inoculated roots is triggered by Nod-factors, which activate plant defense responses, leading to restriction of number of forming nodules. <...> Hypernodulating mutant of Gaertn. , carrying a mutation in the gene , which is the key component in ethylene signal transduction pathway, is characterized by decreased level of defense response activation, as it was shown by proteomic analysis. <...> One of such mechanisms is the synthesis of rhizobitoxin by some rhizobial strains, which has structural similarity with inhibitor of ethylene synthesis aminoethoxyvinilglycine (AVG). <...> The other mechanism is more widespread among rhizobia and it deals with synthesis of ACC deaminase, an enzyme, which cleaves the precursor of ethylene synthesis 1-aminocyclopropane-1-carboxylic acid (ACC). <...> Even more, it is necessary to remember that ethylene stimulates development of root hairs <...>