Ethylene
A gas! The only gaseous plant hormone (C2H4)
Discovered in early 1900’s: Trees in Berlin lost leaves in summer; they were close to gas lights that leaked city gas (ethylene)
Production induced by auxins (especially in fruits)
Accelerates senescence (degradation of chlorophyll) and leave abscission
Accelerates fruit ripening (complex carbohydrates broken down to glucose, chlorophyll degradation, cell wall softening, induction of production of volatile compounds for flavor/smell of ripe fruits)
Economic application: fruits (banana, tomatoes) are harvested green; gassed with ethylene during transport or before store sale
Transgenic tomatoes: ethylene gene modified; cannot produce ethylene, remain green; be gassed before sale