article · Frontiers in Plant Science
In greenhouse evaluations, a nitrogen-fixing bacterium isolated from faba bean root nodules, Rhizobium leguminosarum bv. viciae strain 33504-Alex1, was tested as a soil inoculum and foliar spray against Bean yellow mosaic virus. Both application methods improved plant growth and total chlorophyll levels while substantially lowering disease incidence and severity. The treatments reduced non-enzymatic oxidative stress markers, specifically hydrogen peroxide and malondialdehyde, while boosting total phenolic content, free radical scavenging, and protective enzymes such as superoxide dismutase and polyphenol oxidase. Furthermore, the bacterial strain stimulated the expression of pathogenesis-related genes PR-1, PR-3, and PR-5 and restored beneficial polyphenols suppressed by the virus. Chemical profiling of the bacterial culture filtrate identified compounds that appear to act as elicitors of systemic acquired resistance, indicating the strain can provide both nutritional and antiviral protective benefits.
Viral infections such as Bean yellow mosaic virus severely compromise legume productivity, and direct chemical cures for plant viruses do not exist. Identifying a native nitrogen-fixing bacterium that stimulates plant growth while priming systemic defences against viral disease offers an integrated, sustainable approach to crop protection. This reduces dependence on synthetic agrochemicals and promotes ecological health in farming systems.
The findings could enable the development of dual-action biological inputs that serve as both biofertilisers and antiviral biocontrol agents for legume producers. This is early-stage applied research conducted entirely under greenhouse conditions. Commercial adoption will require multi-location open-field validation, shelf-life and formulation testing, and clearance through biological agricultural input regulatory frameworks before reaching market viability.
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<i>Rhizobium</i> 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), <i>Rhizobium leguminosarum</i> bv. <i>viciae</i> 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 <i>Bean yellow mosaic virus</i> (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 (H<sub>2</sub>O<sub>2</sub>) 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 (<i>PR-1</i>, <i>PR-3</i>, and <i>PR-5</i>) 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 <i>R. leguminosarum</i> bv. <i>viciae</i> 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.
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DOI: 10.3389/fpls.2022.933498
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