Green silver nanoparticles display protection against Clavibacter michiganensis subsp. michiganensis in tomato plants (Solanum lycopersicum L.)

The quarantined bacterium Clavibacter michiganensis subsp. michiganensis (Cmm) causes significant losses on tomato plants worldwide, therefore efforts to reduce its damage are imperative. The alternatives offered by nanotechnology in the near future to be applied for the control of Cmm should be studied. Hence, this study aimed to determine the antibacterial activity of silver nanoparticles (AgNPs) biosynthesized on Moringa (Mo) leaves against Cmm in tomato plants (Solanum lycopersicum L.) and the possible protective mechanisms associated. UV-Vis, TEM and FTIR, elucidated some structural and physicochemical characteristics of biosynthesized AgNPs. The preventive and curative effect of Mo extract and Mo-AgNPs at 50 ppm against Cmm in tomato plants was evaluated. Cmm was inoculated in the stems of tomato plants with 1×109 UFC / mL. Biometric parameters, disease severity, PAL, CAT, SOD activities, H2O2 levels, as well as chalcone synthase and pr1a gene expression were measured in greenhouse during 29 days. The UV-Vis spectrum revealed the maximum absorption at 330 nm. Mo-AgNPs (32 nm; quasi-spherical shape) exhibited antibacterial potential against Cmm decreasing the disease severity by 86.29% at 29 days after the treatments were applied. Besides, the Mo extract decreased the disease severity in 72 %, both compared to the control, being these two treatments the most promising for controlling Cmm. The Mo extract in preventive and curative treatments had the highest numbers of leaves, 33.31%. The enzymatic activity exhibited no discernible pattern when comparing healthy and diseased plants, measured 29 days after the confrontations. Gene expression suggested a role of SAR pathway in the protective treatment by Mo Ag-NPs. In conclusion, the application of AgNPs derived from Mo leaf extracts effectively reduces the disease severity caused by Cmm on tomato plants under greenhouse conditions.