An essentially pure substance has a structure of a Nod factor or one of its analogues. The Nod factor is characterized by the fact that its biosynthesis is controlled by at least one nodulation gene (nodA,B,C) common to the Rhizobiaceae, in particular to the genera Rhezobium, Bradyrhizobium, Sinorhizobium and Azorhizobium. This substance consists of a lipo-oligosaccharide which is not a derivative of the exopolysaccharides and which has the general formula (I). In formula (I), the Nod factor of which it has the strucutre is a plant-specific symbiotic signal and is capable of enhancing the capacity of the bacteria to infect the host plant with which it is associated and/or of accelerating the formation of nodules on the host plant with which it is associated and/or of inducing the transcription of symbiotic genes of the Leguminoseae. Applications to the treatment of plants and as an active therapeutic agent in humans and animals. G stands for a hexosamine variously substituted, for example by an acetyl group on the nitrogen, a sulphate group, an acetyl group and/or an ether group on oxygen R1, R2, R3, R5, R6, R7, which may be identical or different, stand for H, CH3CO-, CXHYCO- where X is a whole number between 0 and 17 and Y is a whole number between 1 and 35, or any other acyl group, for example a carbamyl groupe, R4 stands for a saturated or mono-, di, or tri-unsaturated aliphatic chain containing at least 12 carbon atoms and n is a whole number between 1 and 4.