Ethylene Perception and Signalling in Ripening Fruit - Fruit Ripening: Physiology, Signalling and Genomics - page 202

facilitated a better understanding of the
molecular and biochemical mechanisms
behind ethylene-dependent aspects of
ripening (Lanahan et al. , 1994; Wilkinson
et al. , 1995; Thompson et al. , 1999). As
tomato is the most studied plant for fruit
ripening, the major emphasis in this
chapter is on the knowledge accumulated
using this model system, although sig-
nifi cant insights obtained from other plants
are also discussed. Although the botanical
name of tomato has been changed recently
from Lycopersicum esculentum to Solanum
lycopersicum , the genes characterized
before the name change have been named
using the old prefi x 'Le' in the main text.

linked dimers and exhibit similarity to
bacterial two-component regulators. These
proteins are endoplasmic reticulum-
associated integral membrane proteins. The
receptor proteins possess two to three
domains, including the N-terminal sensor,
a kinase domain in the middle and the
C-terminal receiver domain. The sensor
domain is involved in ethylene perception,
dimerization and binding to the copper
cofactor, while the kinase domain is
involved in autophosphorylation. Transfer
of this phosphate group to an aspartate
residue on the receiver domain of either
the same or another ethylene receptor
protein activates the receiver domain and
results in the initiation of ethylene
signalling. Detailed characterization of fi ve
ethylene-receptor genes in Arabidopsis has
revealed their negative regulatory nature
during ethylene signalling and provided
the basic framework needed for their
characterization during fruit ripening in
tomato. When ethylene is absent, these
receptors actively suppress ethylene
responses, while during its availability,
receptors bind to ethylene causing removal
of this suppression and leading to the
ethylene responses.
Seven ethylene-receptor proteins
(LeETR1-7) have been identifi ed in tomato
and the fi rst fi ve of these have been shown
to bind ethylene. LeETR7 is the least
studied among the tomato receptor pro-
teins (Klee and Giovannoni, 2011). The
ethylene receptor NR (Never-Ripe) was the
fi rst receptor to be characterized during
fruit ripening (Lanahan et al. , 1994). Later,
the ETR1 gene of Arabidopsis was used as
a heterologous probe to screen and identify
additional receptor proteins in tomato
(Wilikinson et al. , 1995; Zhou et al. , 1996).
Based on their structural similarity, tomato
ethylene-receptor proteins have been
divided into two subgroups. LeETR1,
LeETR2 and LeETR3 proteins possess three
N-terminal membrane-spanning domains
and a conserved kinase domain, while
LeETR4, LeETR5 and LeETR6 proteins are
characterized by the presence of an add-
itional transmembrane domain. Further-
more, the kinase domain of members of the

11.2 Ethylene Signal Transduction

Ethylene plays a pivotal role during
various developmental events and environ-
mental responses in plants. Its role in
ripening has received the most attention
(Yang and Hoffman, 1984; Mattoo and
Suttle, 1991; Abeles et al. , 1992).
Discussions on the ethylene biosynthesis
pathway and its regulation are described in
detail by Grierson (Chapter 10, this
volume). Much knowledge about ethylene
signalling components in plants has been
obtained using various ethylene-response
mutants of Arabidopsis (Guo and Ecker,
2003). A high level of similarity was found
in various components of ethylene sig-
nalling in Arabidopsis and tomato. This
knowledge has served as a starting point
for the identifi cation of its components in
other plants as well. During ethylene
signalling, any change at the phenotype
level can be regulated at three steps of this
pathway: fi rst, at the level of ethylene
perception; secondly, during transduction
of this signal to downstream regulators;
and thirdly, at the level of expression of
ethylene-responsive genes (see Plate 6).

11.2.1 Ethylene receptors

The receptor proteins responsible for
perception of ethylene exist as disulfi de-