Ethylene: traffic controller on hormonal crossroads to defense

Ethylene (ET) is an important hormone in plant responses to microbial pathogens and herbivorous insects, but also in the interaction of plants with beneficial microbes and insects. Early ET signaling events during these biotic interactions involve activities of MAP kinases and ERF transcription factors. Rather than being the principal regulator, ET often modulates defense signaling pathways, including those regulated by jasmonic acid and salicylic acid. Hormonal signal integrations with ET steer the defense signaling network to activate specific defenses that can have direct effects on attackers, or systemically prime distant plant parts for enhanced defense against future attack. ET also regulates volatile signals that attract carnivorous enemies of herbivores or warn neighboring plants. Conversely, ET signaling can also be exploited by attackers to hijack the defense signaling network in order to suppress effective defenses. In this review, we summarize recent findings on the significant role of ET in the plant’s battle against their enemies