The relationship between ethylene binding and dominant insensitivity conferred by mutant forms of the ETR1 ethylene receptor

Ethylene responses in Arabidopsis are mediated by a small family of recepto rs, including the ETR1 gene product. Specific mutations in the N-terminal e thylene-binding domain of any family member lead to dominant ethylene insen sitivity. To investigate the mechanism of ethylene insensitivity, we examin ed the effects of mutations on the ethylene-binding activity of the ETR1 pr otein expressed in yeast. The etr1-1 and etr1-4 mutations completely elimin ated ethylene binding, while the etr1-3 mutation severely reduced binding. Additional site-directed mutations that disrupted ethylene binding in yeast also conferred dominant ethylene insensitivity when the mutated genes were transferred into wild-type Arabidopsis plants. By contrast, the etr1-2 mut ation did not disrupt ethylene binding in yeast. These results indicate tha t dominant ethylene insensitivity may be conferred by mutations that disrup t ethylene binding or that uncouple ethylene binding from signal output by the receptor. Increased dosage of wild-type alleles in triploid lines led t o the partial recovery of ethylene sensitivity, indicating that dominant et hylene insensitivity may involve either interactions between wildtype and m utant receptors or competition between mutant and wild-type receptors for d ownstream effecters.