Plant Hormones Play Many Roles (IMAGE) Washington University in St. Louis Caption In plants, hormone signaling substitutes for an immune system. Above, the plant is exhibiting a “hypersensitive response,” characterized by deliberate cell death to deny a pathogen food and water. The hypersensitive response is triggered by the plant hormone salicylic acid. Credit Wikmedia Commons Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.