The Roles of Ethylene Receptors in Plants and Cyanobacteria

Last Friday, Brad Binder of the University of Tennessee spoke at the weekly biology department seminar at Dartmouth College. Binder focused on his research regarding the functional differences between ethylene receptors in the Arabidopsis plant. Ethylene is known to inhibit plant growth through interactions with five unique receptors in the endoplasmic reticulum of Arabidopsis. Binder attempted to isolate each of the receptors to identify the ways in which they associated with ethylene molecules.

The presence of ethylene in Arabidopsis is associated with growth inhibition that results in shorter, fatter seedlings with an exaggerated apical hook. Ethylene has also been observed to lead to “knotting” of plant stems, as well as premature ripening and germination. For the effects of ethylene to take hold in Arabidopsis, the molecule must bind to one of five different ethylene receptors. In the absence of ethylene, receptors are inhibited by the CTR-1 protein, which blocks “downstream ethylene signaling” and allows the plant to grow and develop normally.

Binder created several mutant varieties of the Arabidopsis plant with unique combinations of the five receptors to determine if ethylene signaling behaved similarly in different receptor-ethylene interactions. Through repeated tests, Binder was able to identify that the ETR-1 receptor was both “necessary and sufficient” for the observed knotting behavior caused by ethylene. This means that Arabidopsis mutants missing the ETR-1 gene did not exhibit twisting of the stem in the presence of ethylene. Plants with intact ETR-1 proteins consistently knotted, even when all four of the other receptors were missing.

Binder also studied the antagonistic effects of silver on ethylene signaling in Arabidopsis. While silver has been shown to serve as a cofactor for ethylene-receptor binding, it also effectively blocks downstream ethylene perception. Binder showed that silver functions to keep the ETR-1 receptor “on,” which in turn maintains the function of CTR-1 even when ethylene is present.

Binder’s research showed ETR-1 to be the most important receptor for ethylene-related growth inhibition; however each of the other receptors appears to act antagonistically to limit the negative effects of ethylene in Arabidopsis. Those mutants missing ETR-2 and EIN-4 receptors were shown to recover more slowly after the removal of ethylene from the environment than wild-type Arabidopsis.

Although the effects of ethylene on Arabidopsis and other plant species have been well documented, Binder’s efforts represent the first successful identification of specific receptor roles in ethylene-receptor binding. Moving forward, Binder hopes to use a similar protein-isolation approach to better define the signaling pathway in cyanobacteria.