The tomato ethylene receptors NR and LeETR4 are negative regulators of ethylene response and exhibit functional compensation within a multigene family

The plant hormone ethylene is involved in many developmental processes, inc luding fruit ripening, abscission. senescence, and leaf epinasty. Tomato co ntains a family of ethylene receptors, designated LeETR1, LeETR2, NR, LeETR 4, and LeETR5, with homology to the Arabidopsis ETR1 ethylene receptor. Tra nsgenic plants with reduced LeETR4 gene expression display multiple symptom s of extreme ethylene sensitivity, including severe epinasty, enhanced flow er senescence, and accelerated fruit ripening. Therefore, LeETR4 is a negat ive regulator of ethylene responses. Reduced expression of this single gene affects multiple developmental processes in tomato, whereas in Arabidopsis multiple ethylene receptors must be inactivated to increase ethylene respo nse. Transgenic lines with reduced IVR mRNA levels exhibit normal ethylene sensitivity but elevated levels of LeETR4 mRNA, indicating a functional com pensation of LeETR4 for reduced NR expression. Overexpression of IVR in lin es with lowered LeETR4 gene expression eliminates the ethylene-sensitive ph enotype, indicating that despite marked differences in structure these ethy lene receptors are functionally redundant.