Histidine kinase MHZ1/OsHK1 interacts with ethylene receptors to regulate root growth in rice

We discovered that MHZ1 positively modulates root ethylene response through analysis of a rice ethylene-response mutant mhz1. MHZ1 encodes the rice histidine kinase OsHK1. MHZ1 autophosphorylates at the conserved histidine and can transfer the phosphoryl signal to response regulator OsRR21 via phosphotransfer proteins OsAHP1/2. This phosphorelay pathway is required for root ethylene response. Ethylene receptor OsERS2, via GAF domain, physically interacts with MHZ1 to inhibit its kinase activity. Our results suggest that MHZ1 mediates ethylene response in a way partially independent of OsEIN2, and is directly inhibited by ethylene receptors. Our study reveals a previously unidentified mechanism of ethylene signaling for root growth regulation.