Ethylene Perception and Gene Expression - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 126

Fig. 6.1 The triple response of dark-grown melon seedlings to ethylene. Wild-type seedlings grown in the
absence (left) or presence (right) of ethylene.
response) (Bleecker et al., 1988; Chang et al., 1993) and etr2 (Sakai et al., 1998). Cloning and
characterization of genes disrupted in these mutants have defined a mostly linear pathway
for ethylene signal transduction leading from initial hormone perception to transcriptional
regulation.
In Arabidopsis , ethylene perception initiates with the binding of ethylene to a family of
five receptors, namely, ETR1, ERS1, ETR2, EIN4, and ERS2. ETR1 was the first member
of the receptor family identified, and has been characterized in the most detail. Ethylene
binding is mediated by a copper cofactor (Rodriguez et al., 1999) that is provided to the
receptors by the copper transporter RAN1. All of the encoded receptor proteins show sim-
ilarity to bacterial two-component His kinases, which allows bacteria to adapt to changing
environmental conditions (Chang et al., 1993; Hua et al., 1995, 1998; Sakai et al., 1998).
As with the two-component regulators in bacteria, the ethylene receptors can be divided
into multiple functional domains including a sensor domain that consists of a transmembrane
region responsible for ethylene binding (Schaller and Bleecker, 1995; Hall et al., 2000); a
GAF domain of unknown function (GAF domains are ubiquitous motifs present in cyclic
g uanosine monophosphate (cGMP)-regulated cyclic nucleotide phosphodiesterases, certain
a denylyl cyclases, the bacterial transcription F actor FhlA, and hundreds of other signaling
and sensory proteins from all three kingdoms of life) (Aravind and Ponting, 1997); a His
kinase domain, and, in the case of ETR and EIN4 type receptors, a receiver domain predicted