Behind the Screen: How a Simple Seedling Response Helped Unravel Ethylene Signaling in Plants

In 1901, Dimitry Neljubov demonstrated that ethylene caused altered growth of etiolated pea (Pisum sativum) seedlings, a morphology that by 1913 came to be referred to as the “triple response.” This response was utilized by Guzman and Ecker in a genetic screen designed to identify mutants affected in various aspects of ethylene function in Arabidopsis (Arabidopsis thaliana), as described in their landmark article in The Plant Cell (Guzman and Ecker, 1990). At the time, relatively little was known regarding the molecular aspects of ethylene action, or really for any of the plant hormones. Previously, Anthony Bleecker, while a postdoc with Hans Kende, had exploited the triple response of etiolated seedlings to identify the etr1 mutation (Bleecker et al., 1988), which later turned out to encode the founding member of the ethylene receptor family in Arabidopsis, and Sakis Theologis’s lab had just identified a gene from zucchini (Cucurbita pepo) fruit encoding ACC synthase, the enzyme catalyzing the generally rate-limiting step in ethylene biosynthesis. The triple response provided a perfect tool to genetically dissect ethylene function. While biochemical analysis is facilitated by simple, specific assays for protein activity, geneticists dream of a simple screen directly targeting a pathway of interest in which mutants can be easily identified. The development of the triple response assay provided such a facile screen for mutants altered in ethylene action, which opened up the road to elucidation of the ethylene signaling pathway, as well as factors regulating ethylene biosynthesis and other aspects of ethylene function.