Ethylene signaling: new levels of complexity and regulation.

The gaseous plant hormone ethylene plays important roles in plant growth and development. Recent discoveries have expanded our linear view of ethylene signaling by revealing an elaborate signaling network with multiple regulatory circuits. At the membrane, the ethylene receptors form heteromeric and higher order complexes providing enhanced sensitivity and fine-tuning of signaling. Ethylene sensitivity is further enhanced by the rapid degradation of ethylene receptors upon ethylene binding and by dependence on a novel protein REVERSION-TO-ETHYLENE SENSITIVITY1 (RTE1)/GREEN-RIPE (GR). In the nucleus, EIN3-BINDING F-BOX1 and 2 (EBF1/2) coordinately control 26S proteasome degradation of the critical transcription factors EIN3 and EIL1. EBF1/2 expression is repressed by ETHYLENE-INSENSITIVE5 (EIN5), which encodes the exoribonuclease XRN4. Additionally, EIN3 possesses two mitogen-activated protein kinase (MAPK) phosphorylation sites that have opposing effects on EIN3 stability.