Green-Synthesized Silver Nanoparticles Boost Tomato Defenses at the Right Dose

Tomato growers battling bacterial wilt and speck may soon have a new tool that works in two directions at once. A research team at Shahid Beheshti University in Tehran has shown that silver nanoparticles coated with chitosan, a sugar derived from crustacean shells, can both kill harmful bacteria directly and nudge tomato plants into mounting their own stronger defenses. The catch, the study reveals, is that the same particles that help at low concentrations can turn toxic at higher ones. Published in BMC Plant Biology, the work maps that narrow therapeutic window with a precision that could shape how nano-pesticides are designed for sustainable agriculture.

The nanoparticles themselves were built using a green sonochemical method, meaning ultrasonic energy drove the chemical reduction of silver ions while an extract of Calotropis procera leaves served as the natural reducing agent. This approach avoids the harsh synthetic chemicals typically used in nanomaterial fabrication. Chitosan, a biodegradable polymer with its own antimicrobial reputation, was then used to cap the particles, stabilizing them and improving their bioavailability while reducing safety concerns. Structural characterization confirmed the team had produced phase-pure crystalline metallic silver with a face-centered cubic lattice, and that the chitosan coating had remained intact throughout synthesis.

On the pathogen-fighting front, the results were striking. The chitosan-capped silver nanoparticles, abbreviated Ch-Ag NPs, showed strong antibacterial activity against two of the tomato crop’s most damaging bacterial enemies: Ralstonia solanacearum, the agent of bacterial wilt, and Pseudomonas syringae pv. tomato, which causes bacterial speck. In laboratory assays, the particles produced inhibition zones measuring between 22.5 and 25.5 millimeters, a performance significantly better than the crude plant extract alone. The particles likely work by releasing silver ions that disrupt bacterial membranes and proteins, an action amplified by the chitosan coating that keeps the nanoparticles dispersed and in close contact with bacterial cells.

But the more intriguing findings came from greenhouse experiments, where tomato plants were inoculated with P. syringae pv. tomato and then treated with varying concentrations of the nanoparticles. The researchers observed a classic hormetic response, the biological phenomenon in which a low dose of a potentially harmful agent produces a beneficial effect while a high dose causes damage. At concentrations of 5 to 10 milligrams per liter, the nanoparticles acted almost like a vaccine for the plants’ metabolism, enhancing chlorophyll content and stimulating the biosynthesis of phenolic compounds and flavonoids, the chemical workhorses of plant defense.

At the molecular level, the low-dose treatments switched on a coordinated genetic program. Expression of antioxidant enzyme genes including superoxide dismutase, catalase, and ascorbate peroxidase rose alongside defense-related genes such as phenylalanine ammonia-lyase, chalcone synthase, and the pathogenesis-related protein marker PR1. The trigger for this activation appears to be a carefully controlled pulse of hydrogen peroxide, a reactive oxygen species that plants normally use as a signaling molecule. In effect, the nanoparticles create a mild, manageable oxidative stimulus that primes the plant’s immune machinery without overwhelming it, a state researchers call metabolic priming.

The story changes sharply above the hormetic threshold. At concentrations of 15 milligrams per liter and higher, the same particles disrupted the plants’ redox balance, generating oxidative stress that manifested as chlorosis, the yellowing of leaves, and broad metabolic inhibition. The study attributes this toxicity to excessive release of silver ions and direct damage to thylakoids, the membrane structures inside chloroplasts where photosynthesis takes place. Nutrient imbalance data reinforced the picture: at high doses, the nanoparticles interfere with the plant’s mineral uptake and internal chemistry, turning a potential ally into a stressor.

This dual, dose-dependent mechanism is what makes the study notable in the crowded field of agricultural nanotechnology. Many papers evaluate nanomaterials solely for their pathogen-killing power or solely for their effects on plant physiology. This research integrates green synthesis, antibacterial testing, and physiological and molecular analysis into a single framework, and by doing so defines an optimal dose window in which pathogen suppression and metabolic conditioning occur together. Applying nanoparticles within that window could mean treating a crop while simultaneously training it to defend itself, reducing the need for repeated interventions.

The environmental implications are significant. Conventional copper-based bactericides, long the mainstay against tomato bacterial diseases, accumulate in soils and face increasing regulatory restrictions. Silver nanoparticles carry their own ecological questions, but chitosan capping addresses some of them by enhancing stability and biocompatibility, which can lower the effective dose required and limit free silver release into the environment. The use of a plant extract in synthesis further reduces the chemical footprint of production. If the hormetic dose window proves robust across field conditions and different tomato cultivars, Ch-Ag NPs could position themselves as environmentally friendly nano-pesticides for sustainable production.

Important caveats remain before that vision materializes. The greenhouse findings, while encouraging, were obtained under controlled conditions with a single pathogen challenge, and hormetic responses are notoriously sensitive to species, growth stage, climate, and formulation details. How the nanoparticles behave in complex soil environments, whether they accumulate in edible fruit, and what effects they have on beneficial microbes and pollinators are all questions that will need dedicated study. Regulatory frameworks for nanomaterials in agriculture are still evolving, and any commercial product would need to clear toxicological and environmental hurdles that go well beyond plant health.

Still, the study offers a compelling demonstration that the difference between a remedy and a poison in nanotechnology can be a matter of concentration, and that this boundary can be measured and exploited. By pairing rigorous structural characterization with gene expression profiling and physiological measurements, the Tehran team has provided a template for how agricultural nanomaterials should be evaluated, not just as weapons against pathogens but as modulators of plant biology. For the tomato, one of the world’s most economically important vegetable crops, the message from this research is precise: the right dose of chitosan-capped silver nanoparticles does not merely protect the plant from bacteria, it teaches the plant to protect itself.

Subject of Research: Chitosan-capped silver nanoparticles inducing hormetic defense responses and antibacterial activity in tomato plants

Article Title: Chitosan‑capped silver nanoparticles trigger hormetic defense responses and gene expression changes in Solanum lycopersicum L.

Article References: Aghamir, F., Alvand, Z. M., Farzaneh, M., & Alvand, T. M. (2026). Chitosan‑capped silver nanoparticles trigger hormetic defense responses and gene expression changes in Solanum lycopersicum L.. BMC Plant Biology. https://doi.org/10.1186/s12870-026-09982-w

Image Credits: AI Generated

DOI: 10.1186/s12870-026-09982-w

Keywords: silver nanoparticles, chitosan, tomato, hormesis, green synthesis, antibacterial activity, plant defense genes, oxidative stress, Calotropis procera, nano-pesticides, BMC Plant Biology, capped