**1. Introduction**

Plants are attached immobile organisms and thus have to adapt to their environment in order to survive and reproduce successfully. Usually, plants experience multiple simultaneous

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impacts from different sources, so they developed complex signaling pathways to effectively detect these impacts and adequately respond to them [1]. Various microorganisms, which are constantly present in the environment, form one of the major factors affecting the life cycle of plants [2, 3]. Although many plant-associated microbes are pathogens that impair plant growth and reproduction, there are also a lot of beneficial (mutualistic) microorganisms able to provide plants with nutrition and additional defense mechanisms. Cooperation with such microor‐ ganisms constitutes the universal and highly effective strategy of plants' ecological adaptation, so they tend to form long-lasting associations, which sometimes grow into highly integrated symbioses where one or both partners can develop novel features useful for their survival. Establishing of such symbiotic relationships involves the complicated developmental pro‐ grams implemented under the joint control by plant and microbial partners and based on the cross-regulation of their genes.

Leguminous plants (or Legumes, family Fabaceae) are known to form symbioses with extremely broad range of beneficial soil microorganisms (BSM), representing examples of almost all plant-microbe mutualistic systems. One of the most ecologically important and wellstudied legume-beneficial symbioses is root nodule (RN) symbiosis (symbiotic association with nitrogen-fixing bacteria). Compared with other interactions of legumes with BSM, RN symbioses demonstrate high level of genetic and metabolic integrity, which implies, *inter alia*, highly specific mutual recognition of partners. As legume plant plays a central role in establishing of RN symbiosis, performing functions of initiation, coordination, and regulation of all developmental processes, it possesses complex receptor system capable of accurate identification of microsymbiotic partner. In this chapter, we describe the mechanisms of plantmicrobe recognition during initial steps of RN symbiosis using the interaction of model legumes – pea (*Pisum sativum* L.), barrel medic (*Medicago truncatula* Gaertn.), and *Lotus japonicus* (Regel.) K. Larsen – with rhizobia as an example. We pay particular attention to symbiotic system of *P. sativum* since pea, besides its importance as a model object of genetics, is also a valuable crop plant. Hence, in conclusion, we discuss the potential to use obtained knowledge for optimizing the broad spectrum of plant adaptive functions and to improve the sustainability of legume crop production.
