Q&A: How do plants respond to ethylene and what is its importance?

Abstract

Ethylene gas is a major plant hormone that influences diverse processes in plant growth, development and stress responses throughout the plant life cycle. Responses to ethylene, such as fruit ripening, are significant to agriculture. The core molecular elements of the ethylene-signaling pathway have been uncovered, revealing a unique pathway that is negatively regulated. Practical applications of this knowledge can lead to substantial improvements in agriculture.

Fig. 1

The two-step ethylene biosynthesis pathway in plants. In the first committed step, which is generally the rate-limiting step, ACC is synthesized from SAM by the enzyme ACS. SAM is produced from methionine in the “Yang cycle” of methionine cycling (named after Shang Fa Yang and colleagues, who elucidated the ethylene biosynthesis pathway in the 1970s). In the second step, ACC is converted to ethylene by the enzyme ACO

Fig. 2

The triple response assay in Arabidopsis: a rapid method for screening for ethylene-response mutants. Dark-grown wild-type seedlings germinated on agar medium in the presence of exogenous ethylene display the triple response phenotype: a short and thick hypocotyl, an exaggerated apical hook (labeled in the figure) and a short root (not visible in this image). The single tall seedling (approximately 1 cm in length) is an ethylene-insensitive mutant with a long and thin hypocotyl (labeled in the figure) and no apical hook. From the cover of Science volume 241 (August 26, 1988); photograph by Kurt Stepnitz (Michigan State University). Reprinted with permission of AAAS

Fig. 3

Model of the core ethylene-signaling pathway as described in the text. Top: in the absence of the ethylene signal, the ethylene receptors (represented by the isoform ETR1) activate the CTR1 protein kinase, which represses EIN2 function. In the nucleus, the master transcription factors EIN3/EIL1 are degraded. Bottom: when ethylene is detected, the ethylene receptors no longer activate CTR1, resulting in the proteolytic release of the EIN2 C-END, which inhibits protein translation of the F-box proteins EBF1/2. EIN3/EIL1 are consequently stabilized and regulate an extensive transcriptional cascade involving the ERF1 transcription factor. Other elements that regulate the pathway can be found in [37]

Fig. 4

The five ethylene receptor isoforms in Arabidopsis. The ethylene-binding domain consists of three conserved transmembrane domains at the N-terminus (represented by the vertical blue bars). The receptors fall into two subfamilies. Subfamily II receptors have a fourth transmembrane domain (dark blue) at the N-terminus, which possibly serves as a signal sequence, and a degenerate histidine kinase domain (light blue) that displays serine/threonine kinase activity in vitro. Other plant species have similar ethylene receptor isoforms