Internalization and induction of defense responses in tobacco by harpin_Pss conjugated gold nanoparticles as a foliar spray

Controlled and targeted delivery of elicitor molecules with improved bioavailability is required to treat plant diseases. Here we reported stable, reproducible, and nano-sized (20-22 nm) negatively (H-AuNPs) and positively (H-CAuNPs) charged harpin adsorbed nanoparticles to induce defense responses in tobacco. The foliar application of NPs facilitated their entry possibly through epidermal stomata accumulated in the apoplast and symplast eliciting the hypersensitive response (HR) with the up-regulation of pathogenesis-related gene transcripts (PR-1, PR-2, PR-3, chitinase, HSR203, and HR specific marker gene 'HIN1') in 6 h. Immunogold labeling of harpin(Pss) revealed its internalization (symplast & apoplast) and correlated with the induction of defense responses. Studies with fluorescently labeled NPs demonstrated the uptake and distribution within the leaf. Transmission electron microscopy observations revealed the internalization of H-CAuNPs and long-distance transport, eliciting systemic acquired resistance. Our observation holds promise in effectively minimizing the concentration of harpin for field application.