Effects of Rhizobium leguminosarum bv. viceae Strains Different in Their Symbiotic Effectiveness on Changes in cAMP and Hydrogen Peroxide Concentrations in Cells of Pea Seedlings

Changes in hydrogen peroxide and cAMP concentrations in pea (Pisum sativum L.) seedlings inoculated with Rhizobium leguminosarum bv. viceae Frank (Rlv) were evaluated as related to a sorption rate of the bacteria. The tested bacterial strains differ in effectiveness of symbiotic nitrogen fixation. Both substances were analyzed in different growth zones of the root. Planktonic cultures of the effective RCAM 1022 strain and two ineffective highly competitive RCAM 1064 and RCAM 1065 strains were used. After 6 h postinoculation, the sorption rate of all the strains markedly differed from each other in the I–V root zones that are different in their susceptibility to Rlv infection. The sorption rate of the effective 1022 strain was the most and that of the ineffective 1065 one was the least. The effective strain increased the cAMP concentration up to 120–130% in the I–III zones, while this index remained almost as low as in the noninoculated control in the IV and V zones and epicotyl. The ineffective 1064 strain behaved similarly. Another ineffective strain, 1065, caused an inversed effect: the concentration of this signal molecule was close to the control in the I–IV zones and rose in the V zone and, especially, epicotyl. The different strains led to unequal changes in the hydrogen peroxide concentration in the root zones. Upon contact with the ineffective strains, the concentration was at the control (with RCAM 1064) or lower (with RCAM 1065) level in the I–II zones; both strains significantly decreased the H₂O₂ content in the III zone and slightly increased it in the IV–V zones. The effective 1022 strain significantly elevated the H₂O₂ concentration above the control in the I–II zones but decreased it in the III–V zones. In the epicotyl, the peroxide concentration increased mainly due to activity of the ineffective strains. Therefore, upon interaction of bacterial mutualists with their host plants, the activation of plant signaling depends on the degree of effectiveness of a particular strain and directs the interaction to the path of either mutualism or pathogenesis.