Activities of Adenylate Cyclase and Changes in cAMP Concentration in Root Cells of Pea Seedlings Infected with Mutualists and Phytopathogens

Work was carried out on pea (Pisum sativum L.) seedling roots to assess the attachment of the nitrogen-fixing symbiotic bacteria Rhizobium leguminosarum bv. vicea (Rlv) and the bacterial phytopathogens—specific Pseudomonas syringae pv. pisi (Psp) and nonspecific Clavibacter michiganensis ssp. sepedonicus (Cms). Different root zones were examined: (I) the meristem, 2 mm from the root tip; (II) the root hair-free zone, 27 mm; (III) the zone of root hair anlages, 712 mm; (IV) the young root hair zone, 1217 mm; and (V) the zone of root hair that completed the growth, 1722 mm. It was found earlier that the zones differed in their susceptibility to Rlv. In the present work, reactions of particular components of the adenylate cyclase signaling system (ACSS) were estimated, i.e., concentration of cAMP and activities of transmembrane adenylate cyclase (tAC) and soluble adenylate cyclase (sAC) in these zones after different times post inoculation (5, 15, 120, and 360 min). It was revealed that the degree of activation of particular components of ACSS did not depend on the sorption rate of differently specialized bacteria. Upon contact with Rlv, the character of changes in tAC and sAC activities was almost the same in different root zones and resembled the dynamics of the cAMP content. Inoculation with Psp changed the cAMP level similarly to that with Rlv, but the dynamics of tAC and sAC was opposite to each other in most cases. Inoculation with Cms, in spite of the absence of its attachment, elevated the cAMP content and activated tAC and sAC. It is suggested that the above-mentioned changes in ACSS is associated with exometabolites of Rlv, Psp, and Cms, which activate the PAMP-induced immunity of the pea seedling cells. The uniform dynamics of cAMP in different root zones upon the exposure to Rlv and Psp seems to reflect the specific reaction and, presumably, fulfills different functions—regulatory with Rlv and defensive with Psp. Upon short-term contact with Cms, the cAMP dynamics in the same root zones displayed a nonspecific character that might be related to the rate of adsorption of exopolysaccharides by the root hair. The systemic response of ACSS was observed in the hypocotyls of the seedlings exposed to any of the three organisms.