Quality or Quantity? Increasing Legume Yield Using Traditional Inoculants and Rhizobial Nod Factors in the Context of Inter-Strain Competition

Rhizobia have been used for decades as biopreparations, successfully replacing synthetic nitrogen fertilizers in legume cultivation. They have a beneficial effect on the growth and yield of these plants when cultivated in soils that are deficient in both nitrogen and indigenous rhizobia. However, such preparations, containing strains that are characterized by high effectiveness in reducing atmospheric dinitrogen, are not universal. Their use is ineffective when plants are grown in soils that are already rich in strains with low effectiveness, because such inoculant strains are unable to effectively compete with native soil populations. This review discusses issues related to the rhizobia–legume symbiosis, with particular emphasis on inter-strain competition occurring in the soil and in the colonized plant tissues. The importance of Nod factors (NFs) in symbiosis and their broad impact on plant physiological and developmental processes are also discussed. Research results on the effects of NF-containing biopreparations on legume growth and yield are summarized. Moreover, this review explains how such preparations can support the growth and yield of legumes growing in soils containing numerous populations of low-effectiveness rhizobia. Finally, the potential for the application of this technology to non-legume plants is presented.