Enhancing systemic resistance in faba bean ( L.) to soil application and foliar spray of nitrogen-fixing bv. strain 33504-Alex1.

spp. manifests strong nitrogen fixation ability in legumes. However, their significance as biocontrol agents and antivirals has rarely been investigated. Under greenhouse conditions, the molecularly identified nitrogen-fixing plant growth-promoting rhizobacteria (PGPR), bv. strain 33504-Alex1, isolated from the root nodules of faba bean plants, was tested as a soil inoculum or a foliar application to trigger faba bean plants' resistance against (BYMV) infection. Compared to the non-treated faba bean plants, the applications of 33504-Alex1 in either soil or foliar application significantly promoted growth and improved total chlorophyll content, resulting in a considerable reduction in disease incidence and severity and the inhibition index of BYMV in the treated faba bean plants. Furthermore, the protective activities of 33504-Alex1 were associated with significant reductions in non-enzymatic oxidative stress markers [hydrogen peroxide (HO) and malondialdehyde (MDA)] and remarkably increased DPPH free radical scavenging activity and total phenolic content compared to the BYMV treatment at 20 days post-inoculation. Additionally, an increase in reactive oxygen species scavenging enzymes [superoxide dismutase (SOD) and polyphenol oxidase (PPO)] and induced transcriptional levels of pathogenesis-related (PR) proteins (, , and ) were observed. Of the 19 polyphenolic compounds detected in faba bean leaves by high-performance liquid chromatography (HPLC) analysis, gallic and vanillic acids were completely shut down in BYMV treatment. Interestingly, the 33504-Alex1 treatments were associated with the induction and accumulation of the most detected polyphenolic compounds. Gas chromatography-mass spectrometry (GC-MS) analysis showed hexadecanoic acid 2,3-dihydroxypropyl ester, tetraneurin-A-Diol, oleic acid, and isochiapin B are the major compounds in the ethyl acetate extract of 33504-Alex1 culture filtrate (CF), suggesting it acts as an elicitor for the induction of systemic acquired resistance (SAR) in faba bean plants. Consequently, the capacity of bv. strain 33504-Alex1 to enhance plant growth and induce systemic resistance to BYMV infection will support the incorporation of 33504-Alex1 as a fertilizer and biocontrol agent and offer a new strategy for crop protection, sustainability, and environmental safety in agriculture production.