Interactive effects of salt stress and Pseudomonas syringae pv. lachrymans infection in cucumber: Involvement of antioxidant enzymes, abscisic acid and salicylic acid

•NaCl-induced changes in cucumber performance promoted the pathogen growth.•Combined stress specifically induced the prooxidant and antioxidant mechanisms.•NaCl-induced ABA modulated the SA-dependent defence response.•Cucumber response under combined stress was predominantly influenced by salt stress.

Under natural conditions plants have to cope with a wide range of abiotic and biotic stressors that occur simultaneously or in sequence, and phytohormones and reactive oxygen species are key mediators of plant stress response. However, their role in defence against abiotic and biotic stresses acting in combination still needs elucidation. We studied the involvement of reactive oxygen species, antioxidant enzymes, abscisic acid and salicylic acid in the response of cucumber plants exposed to individual or sequential action of salt stress (50 mM or 100 mM NaCl) and Pseudomonas syringae pv. lachrymans. Salt stress was more harmful than infection, especially with respect to leaf growth and stomata opening. The effects were most severe when the stresses were combined. Cucumber response to the pathogen infection was negatively affected by prior NaCl treatment. NaCl-induced changes promoted the pathogen growth and intensified the disease symptoms. It also provoked specific interactions between stress signalling components at the levels of hormones and the prooxidant/antioxidant mechanisms, exemplified by stronger H2O2 accumulation and Fe superoxide dismutase activation, changed salicylic acid/abscisic acid balance and modified PR1 gene expression.