Endophytic Bacterium Acinetobacter calcoaceticus V33K2 Modulates Cucumber Tolerance to Combined Drought and Pathogen Stress

Abstract

Plants in natural environments frequently encounter simultaneous or sequential abiotic and biotic stresses, which substantially alter physiological responses and may affect the efficacy of biological control agents. This study evaluated the effects of combined drought stress, induced by polyethylene glycol (PEG), and Angular Leaf Spot Disease (ALSD), caused by Pseudomonas syringae pv. lachrymans (Psl), on cucumber plants, together with the performance of the endophytic bacterium (EB) Acinetobacter calcoaceticusV33K2. When drought stress preceded pathogen infection, disease severity increased by131% and drought symptoms by50% compared to the control. In contrast, EBV33K2 application under combined stress conditions reduced ALSD severity by 36% and drought symptoms by 22%. EB treatment also significantly enhanced chlorophyll content, biomass, and proline levels while simultaneously decreasing the Psl population. At the molecular level, PR1 expression, a marker of the salicylic acid(SA), was induced under individual stresses but was suppressed at 72 h under combined stress conditions. This suppression coincided with a concomitant increase in the expression of SnRK2.5, a key component of the abscisic acid(ABA) signaling pathway. The LOX1 gene, involved in jasmonic acid(JA) biosynthesis, was significantly upregulated at 96 h-particularly in EB-treated plants-despite being downregulated at earlier time points. Moreover, the expression of ACS3, a gene associated with ethylene biosynthesis, however, its transcript levels were reduced at 72 h following EB application. Collectively, these findings demonstrate that the chronological sequence of stress exposure critically shapes plant physiological and molecular responses, and that EB V33K2 isolate can mitigate the detrimental effects of combined stresses and enhance plant resilience.