LAUSR

Abstract Relationships between changes in the levels of intracellular cAMP and H 2 O 2 were studied in various growth zones of pea ( Pisum sativum L.) seedling roots upon infection with Rhizobium leguminosarum bv. viceae (Rlv) , Pseudomonas syringae pv. pisi (Psp), or Clavibacter michiganensis ssp. sepedonicus ( Cms ). Previous studies have shown that these root zones are differentially sensitive to Rlv. In the present work, the highest level of H 2 O 2 in growth zones of pea roots was observed after a short-term (5 min) inoculation with Rlv . In roots treated with Psp and Cms , the H 2 O 2 contents were lower and showed little variation between Psp and Rlv treatments as well as between various growth zones. In all three treatments, the H 2 O 2 content in seedling epicotyls decreased by approximately 20% with respect to the control level. Under these conditions, the level of intracellular cAMP increased substantially in almost all growth zones of pea seedling roots. The largest increase was observed upon the inoculation with Cms . The intracellular cAMP level was artificially increased by applying various concentrations of exogenous n -butyryl-cAMP known to readily permeate into the cells. At concentrations of 5 and 10 nM, n -butyryl-cAMP induced equal increases of H 2 O 2 content in all analyzed zones, whereas the treatment with 50 nM n -butyryl-cAMP diminished H 2 O 2 content in root zones and epicotyls. A 30-min incubation of samples in the presence of 10 or 50 nM n -butyryl-cAMP followed by a 5‑min infection with Rlv, Psp, and Cms significantly modified the H 2 O 2 level in root growth zones. In order to decrease the intracellular content of cAMP, an inhibitor of transmembrane adenylate cyclase, suramin was applied. Under the action of suramin, the cAMP concentration in the root growth zones decreased by approximately 40%; in addition, the intracellular H 2 O 2 content increased in the same root zones and in epicotyls. The treatment of seedlings with suramin followed by a 5-min incubation of plants in the presence of Rlv and Psp microorganisms led to insignificant lowering in the content of a signaling molecule H 2 O 2 , whereas the “suramin + Cms ” treatment induced a larger decrease in H 2 O 2 content, thus shifting it to the control level. It is concluded that, in plant cells exposed to biotic stressors, a certain concentration status of secondary messengers is established by virtue of their mutual influence on the components of other plant signaling systems.