Date of Award

Spring 2004

Project Type

Dissertation

Program or Major

Biochemistry

Degree Name

Doctor of Philosophy

First Advisor

G Eric Schaller

Abstract

Ethylene is one of the most important plant hormones and regulates many processes during plant growth and development. In Arabidopsis, the ethylene receptor family consists of five members: ETR1 and ERS1 have a functional histidine kinase domain and form subfamily 1; members of subfamily 2, including ETR2, ERS2, and EIN4, possess a highly diverged histidine kinase domain predicted not to be functional. To analyze signal output by the ethylene receptor ETR1 from Arabidopsis, mutant-based approaches were taken. Initially, the role of the proposed signal output region of ETR1 in ethylene signaling was examined. For this purpose, the ability of mutant versions of ETR1 to rescue the constitutive response phenotype of the etr1--6;etr2--3;ein4--4 triple loss-of-function mutant line was examined. A truncated version of ETR1 that lacks both the histidine kinase domain and the receiver domain failed to rescue the triple mutant phenotype. A truncated ETR1 receptor that lacks only the receiver domain restored normal growth to the triple mutant in air, but the transgenic seedlings displayed hypersensitivity to ethylene. A mutation that eliminated histidine kinase activity had a modest effect upon the ability of the receptor to repress ethylene responses in air. These results demonstrate that the histidine kinase domain is required for the repression of ethylene responses. To further address whether histidine kinase activity is required for ethylene signaling, single loss-of-function allele of ERS1 was isolated and ers1;etr1 double null mutants were generated. The ers1;etr1 double mutants displayed a constitutive ethylene response phenotype when grown in the dark, and were dwarfed with small and epinastic leaves in the air and died without bolting when grown under light. The phenotype of ers1;etr1 is more profound than that observed in the etr2--3;ein4--4;ers2--3 triple loss-of-function mutant reported previously, indicating that subfamily 1 members (ETR1 and ERS1) play a predominant role in ethylene signaling. Addition of the kinase-inactivated ETR1(G2) into a background containing both ers1--3 and etr1--7 mutations results in ethylene insensitivity, demonstrating that the histidine kinase activity of ETR1 may play a role in the establishment of ethylene responses.

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