A strong loss-of-function mutation in RAN1 results in constitutive activation of the ethylene response pathway as well as a rosette-lethal phenotype

A recessive mutation was identified that constitutively activated the ethyl ene response pathway in Arabidopsis and resulted in a rosette-lethal phenot ype. Positional cloning of the gene corresponding to this mutation revealed that it was allelic to responsive to antagonist1 (ran1), a mutation that c auses seedlings to respond in a positive manner to what is normally a compe titive inhibitor of ethylene binding. In contrast to the previously identif ied ran1-1 and ran1-2 alleles that are morphologically indistinguishable fr om wild-type plants, this ran1-3 allele results in a rosette-lethal phenoty pe. The predicted protein encoded by the RAN1 gene is similar to the Wilson and Menkes disease proteins and yeast Ccc2 protein, which are integral mem brane cation-transporting P-type ATPases involved in copper trafficking. Ge netic epistasis analysis indicated that RAN1 acts upstream of mutations in the ethylene receptor gene family. However, the rosette-lethal phenotype of ran1-3 was not suppressed by ethylene-insensitive mutants, suggesting that this mutation also affects a non-ethylene-dependent pathway regulating cel l expansion. The phenotype of ran1-3 mutants is similar to loss-of-function ethylene receptor mutants, suggesting that RAN1 may be required to form fu nctional ethylene receptors. Furthermore, these results suggest that copper is required not only for ethylene binding but also for the signaling funct ion of the ethylene receptors.