Green synthesis of silver nanoparticles and their potential to enhance defense in cabbage crop against Xanthomonas campestris pv. campestris

The green synthesis of metallic nanoparticles (MNPs) is a promising approach for plant disease management. In this study, eco-friendly nanoparticles were produced to fight black-rot disease of cruciferous crops caused by Xanthomonas campestris pv. campestris (Xcc). Silver nanoparticles (AgNPs) were synthesized by using aqueous extracts of cabbage (Brassica oleracea var. capitata L.), thale cress (Arabidopsis thaliana [L.] Heynh), neem (Azadirachta indica L.), and noni (Morinda citrifolia L.) leaves and fruit parts (peel and pulp/seeds), as reducing and stabilizing agents. The AgNPs syntheses were performed at six different concentrations of extracts in aqueous solutions of silver nitrate (AgNO3), at 1 mmol L−1. In total, 42 samples of AgNPs were produced, out of which, 14 were selected according to their hydrodynamic diameter (HD), polydispersity index (PDI), and Zeta potential (ZP) and further tested in vitro to evaluate their antibacterial activity against Xcc. The AgNPs with the highest antibacterial activity displayed the lowest HD (192.3 ± 1.1 nm) at 64 µmol L−1, particularly those synthesized using aqueous extract of noni fruit peel (AENFP) at 60 mg mL−1. Furthermore, susceptible plants of cabbage were treated with AENFP- AgNPs, and the positive modulation of defense-related marker genes were captured by RT-qPCR. Plants treated with AENFP- AgNPs at 64 µmol L−1 when challenge with Xcc, showed a more tolerant phenotype, with leaves displaying reduced black rot symptoms up to 7 days post-infection. These results indicate that the application of AgNPs seems to trigger an effective defense response, probably due to the elicitor properties present in AgNPs synthesized from AENFP. The present study proposes an interesting alternative approach to control plant diseases.