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Recent studies of the immune system of leguminous plants infected with nodular bacteria (rhizobia) are summarized. The possibility of blocking the invasion of rhizobia into plant organs not affected by the primary infection is discussed. The concept of local and systemic resistance of the leguminous plant to rhizobial infection is introduced. The Nod factors of rhizobia are considered, as well as the plant receptors that interact with these factors upon the formation of symbiosis of the plant and bacteria. The role of bacterial surface exopolysaccharides in the suppression of the protective system of the plants is discussed. The innate immunity of leguminous plant cells is assumed to affect the formation and functioning of the symbiosis of the plant and the bacteria.

Data from the literature and our own data on the participation and interrelation of bacterial signaling Nod-factors and components of the calcium, NADPH-oxidase, and NO-synthase signaling systems of a plant at the preinfection and infectious stages of the formation of a legume-rhizobium symbiosis are summarized in this review. The physiological role of Nod-factors, reactive oxygen species (ROS), calcium (Ca2+), NADPH-oxidase, nitric oxide (NO), and their cross influence on the processes determining the formation of symbiotic structures on the roots of the host plant is discussed.

The level of nitric oxide (NO) in roots of 2-day-old etiolated pea (Pisum sativum L.) seedlings was investigated by fluorescence microscopy using the fluorescent probe 4,5-diaminofluorescein diacetate. Segments representing transversal (cross) cuts of the roots having thickness of 100 to 150 µm (a segment of the root located 10 to 15 mm from the apex) were analyzed. A substantial concentration of NO in the roots was registered when the seedlings were grown in water (control). Addition of 4 mM sodium nitroprusside, 20 mM KNO₃, 2 mM NaNO₂, 2 mM L-arginine into the growth medium increased NO concentration with respect to the control by 1.7- to 2.3-fold. Inhibitors of animal NO-synthase - 1 mM Nω-nitro-L-arginine methyl ester hydrochloride and 1 mM aminoguanidine hydrochloride - reduced the intensity of fluorescence in the root segments in the presence of all the studied compounds. In medium with KNO₃, the inhibitor of nitrate reductase -150 µM sodium tungstate - lowered the fluorescence intensity by 60%. Scavengers of nitric oxide - 100 µM 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide and 4 µM hemoglobin - lowered NO concentration in all the studied variants. Potassium ferrocyanide (4 mM) as the inactive analog of sodium nitroprusside inhibited generation of NO. These results are discussed regarding possible pathways of NO synthesis in plants.