Plant immunity inducers: new peptide motifs from Prosystemin to face plant stressors

Chemical pesticides represent a pivotal tool in agriculture to control pest agents and secure both quality and yield in plant production. Nevertheless, their extensive use in crop protection led to a widespread toxicity that involved also non-target organisms as well as the surrounding environment, endangering ecosystems sustainability. Therefore, the increasing demand for food products low or free in pesticide residues, spurred the scientific community to develop more effective, sustainable, and eco-friendly solutions for pest control. Promising biotechnological strategies include molecules that act as plant immunity inducers, known as elicitors. Among them, tomato Systemin (Sys) has often been the focus of attention as a key component of plant resistance phenotype towards a wide range of stressors. This 18-amino acids peptide is released upon wounding from the C-terminal end of its 200 amino acids precursor called Prosystemin (ProSys), activating an array of local and systemic defenses. However, ProSys is not only a mere scaffold for the Sys peptide, but itself contains other biologically active regions. In addition, the defence mechanism underpinned by the protein is likely linked to the intrinsic disorder of its sequence that may have its own biological function beyond the release of Sys. Indeed, our recent findings suggests that ProSys hides other functional peptide motifs able to activate multiple stress-related pathways. To contribute to this knowledge, we identified and produced different synthetic peptides derived from ProSys precursor that when exogenously supplied to tomato plant, induce defense-related genes protecting plants against necrotrophic fungal pathogens such as Botrytis cinerea and chewing insects like Spodoptera littoralis larvae. Larvae fed on treated leaves are unable to reach the required weight to moult, dying either during moulting or shortly afterwards. It is intriguing to notice that dying larvae exhibit a strange “albino” phenotype, remaining contracted and trapped in the old cuticle and lacking sclerotization of the new cuticle. Since peptides act only on plant defenses rather than directly on larvae, the observed unusual phenotype is possibly linked to the induction of genes that promote the production of substances with insect hormone-like activity that impact on larvae development. Overall, our data open new perspectives on ProSys functionality, since it contains multiple biologically active sequences that may be novel exploitable tools in crop protection programs.