A receptor antagonist counterbalances multiple systemin phytocytokines in tomato

Tight regulation of immune activation is crucial for plant health. How plants control the actions of their immunostimulatory phytocytokines is largely unknown. Here, we identify antiSYS as a natural inhibitor of the tomato cytokine systemin. AntiSYS is a systemin-like peptide encoded in a gene cluster with four additional paralogs, three of which comprise newly identified agonistic systemins. AntiSYS is a potent and specific antagonist of the systemin receptor. Tomato mutants lacking antiSYS show aberrant growth and reduced reproductive fitness. These symptoms of antiSYS deficiency are not observed in plants lacking functional systemin receptors, suggesting a role of antiSYS in counterbalancing agonistic systemins. Thus, reminiscent of antagonistic interleukins controlling immune homeostasis in animals, antiSYS serves a crucial role in the regulation of phytocytokine activity in tomato plants.