Systemin

Systemin is a plant peptide hormone first described in tomato (Solanum lycopersicum) as a wound-induced signaling molecule that mobilizes systemic defense responses. It is a small peptide, roughly 18 amino acids in length, produced from a larger precursor protein called prosystemin. Following tissue damage, proteolytic processing releases active systemin, which then moves through the plant to distant tissues to prime defenses.

Systemin is perceived by a cell-surface receptor, triggering a signaling cascade that strongly activates the jasmonic acid (JA) pathway. JA and its bioactive conjugate JA-Ile accumulate, leading to the transcription of defense-related genes, including proteinase inhibitors and enzymes involved in secondary metabolism. This results in enhanced resistance to chewing insects and some pathogens. The systemic signal can spread via the vascular system, enabling a whole-plant response from a localized damage site.

Interactions with other hormonal pathways are a feature of systemin signaling; it intersects with ethylene signaling and interacts with cross-talk involving salicylic acid pathways. The exact receptor and some downstream components have been the subject of ongoing research, with various receptor models proposed and debated in the literature.

Distribution and species range show that while systemin was originally identified in tomato, related systemin-like peptides have been found in other plant species, particularly within the Solanaceae. The systemin pathway is viewed as a paradigm for peptide-based systemic signaling in plants.

Systemin is notable as one of the earliest plant peptide hormones identified, providing insight into long-distance signaling mechanisms that coordinate whole-plant defense.