Effects of endophite bacteria applications on some morphological and physiological characteristics and gene expressions in cucumbers under biotic and abiotic stress

Since the effectiveness of biological control agents (BCA) in biological control of plant diseases is generally evaluated on plants under a single stress, it cannot fully reflect the performance of plants and BCA that are exposed to more than one stress in nature. This study examined Pseudomonas syringae pv. lachrymans (Psl) and the effects of endophyte bacteria (EB) and stress factors on each other were investigated in cucumbers consecutively exposed to drought stress caused by polyethylene glycol (PEG). EB Acinetobacter calcoaceticus V33K2 isolate, which was found to be successful against both stresses separately, was applied to cucumbers and exposed to biotic (Psl) and abiotic (drought) stress sequentially in the hydroponic environment. In the groups where drought stress was applied first, HKYL disease severity increased by 131% and drought severity increased by 50%, and no change was observed compared to the positive control in the groups where Psl was applied before. The selected EB V33K2 isolate reduced HKYL disease (46%) and the effects of drought stress (11%). EB V33K2 increases disease severity in plants exposed to two stresses: drought and HKYL, respectively; It decreased by 22 and 36% before drought stress and by 11 and 20% before biotic stress. EB V33K2 applications provided significant increases in total chlorophyll, chlorophyll "a" and carotenoid content. MDA content is at the lowest level in EB application. Among the application groups, the highest proline accumulation was observed in the EB + PEG + Psl group. Among the application groups, the highest proline accumulation was observed in the EB + PEG + Psl group. It was observed that the gene expression was higher in the PR-1 gene under double stress and in the LOX-1 gene under all stress factors in the EB applied groups at the 96th hour compared to the non-treated groups. In the SnRK-2.5 gene, it was observed that in double stress applications, EB applied groups at 72 and 96 hours increased the upstream induction compared to the non-applied groups. EB V33K2 application made a significant contribution to the health and development of cucumber plants under dual stress conditions. However, more studies are needed to fully understand this issue.