Evolution of ethylene as an abiotic stress hormone in streptophytes

All land plants modulate their growth and physiology through intricate signaling cascades. The majority of these are at least modulated—and often triggered—by phytohormones. Over the past decade, it has become apparent that some phytohormones have an evolutionary origin that runs deeper than plant terrestrialization—many emerged in the streptophyte algal progenitors of land plants. Ethylene is such a case. Here we synthesize the current knowledge on the evolution of the phytohormone ethylene and speculate about its deeply conserved role in adjusting stress responses of streptophytes for more than half a billion years of evolution.