Analysis of Combinatorial Loss-of-Function Mutants in the Arabidopsis Ethylene Receptors Reveals That the ers1 etr1 Double Mutant Has Severe Developmental Defects That Are EIN2 Dependent

Ethylene responses in Arabidopsis are controlled by the ETR receptor family. The receptors function as negative regulators of downstream signal transduction components and fall into two distinct subfamilies based on sequence similarity. To clarify the levels of functional redundancy between receptor isoforms, combinatorial mutant lines were generated that included the newly isolated ers1-2 allele. Based on the etiolated seedling growth response, all mutant combinations tested exhibited some constitutive ethylene responsiveness but also remained responsive to exogenous ethylene, indicating that all five receptor isoforms can contribute to signaling and no one receptor subtype is essential. On the other hand, light-grown seedlings and adult ers1 etr1 double mutants exhibited severe phenotypes such as miniature rosette size, delayed flowering, and sterility, revealing a distinct role for subfamily I receptors in light-grown plants. Introduction of an ein2 loss-of-function mutation into the ers1 etr1 double mutant line resulted in plants that phenocopy ein2 single mutants, indicating that all phenotypes observed in the ers1 etr1 double mutant are EIN2 dependent.