Ethylene Perception and Signalling in Ripening Fruit - Fruit Ripening: Physiology, Signalling and Genomics - page 203

latter subgroup does not possess one or
more of the catalytic subdomains, in-
cluding the autophosphorylated domain. In
addition, the LeETR3 receptor protein
differs structurally from the remaining
receptor proteins in that it lacks the
receptor domain. Although these genes are
expressed ubiquitously in tomato, they
have been found to respond differently to
different external cues or at different stages
of fruit development. LeETR1 and LeETR2
genes are expressed ubiquitously in all
tissues, while expression levels of the
LeETR3-6 genes increase in reproductive
tissues such as fruits and fl owers (Klee and
Giovannoni, 2011). These receptor genes
have been utilized to manipulate ripening
in tomato; however, in many cases, no
effect of their altered expression was
observed on fruit ripening, indicating a
degree of functional redundancy among
them (Zhou et al. , 1996). For example,
knockdown antisense transgenic lines of
LeETR1 and LeETR3 did not show any
change in the ripening behaviour of their
fruits. Increased expression of LeETR4 in
the knockdown antisense LeETR3 lines
was found to compensate for reduced
expression of the LeETR3 gene (Tieman et
al. , 2000). Furthermore, functions of
LeETR4 and LeETR6 proteins have been
implicated in altering the time of fruit
ripening in tomato. These proteins
exhibited ethylene-induced degradation by
the 26S proteasome pathway. Transgenic
plants with attenuated expression of these
genes resulted in early fruit ripening,
suggesting that regulation at the protein
level is a major determinant of the ripening
initiation programme. The authors pro-
posed that, as the receptors are negative
regulators of ethylene signalling, depletion
of receptors levels would result in a
progressive increase in hormone sensitivity
causing an early ripening phenotype. Thus,
it was suggested that these receptors
themselves may act as regulators of ripen-
ing initiation (Kevany et al. , 2007).
Additionally, other tissue-specifi c regu-
lators of ethylene signalling have been
identifi ed in tomato. Green-Ripe ( Gr ) is a
dominant non-ripening mutant in tomato.

Fruits from Gr mutants do not fully ripen
and exhibit reduced ethylene responsive-
ness during ripening. Mutation in this gene
results in overaccumulation of otherwise
normal GR protein in fruit. This is
suggested to be a membrane protein of un-
known function that negatively regulates
ethylene responses only in fruits. Mutation
in its Arabidopsis homologue, REVERSION-
TO-ETHYLENE SENSITIVITY1 ( RTE1 ), was
found to restore the ethylene insensitivity
phenotype caused by etr1-2 gain-of-
function mutations. Together, evidence
from GR and its Arabidopsis homologue
(RTE1) suggests that these proteins interact
with ethylene receptors to alter/modify
their action in an as-yet-unknown way. In
the case of the Gr mutation, it has been
suggested that the GR protein can interfere
with the normal activity of related proteins
in tissue-dependent ethylene responses
unique to tomato fruit (Barry and Giovan-
noni, 2006).

11.2.2 Constitutive triple response (CTR)
proteins

In tomato, four CTR genes ( LeCTR1-4 ) are
members of a small multigene gene family
(Adams-Phillips et al. , 2004). These MAP
kinase kinase kinase (MAP3K) genes are
present downstream of the ethylene
receptors and act as negative regulators of
ethylene responses. They interact physic-
ally with the receptor proteins and form a
signalling complex (Cara and Giovannoni,
2008). These genes were identifi ed either
through heterologous hybridizations using
the Arabidopsis CTR1 gene as a probe or in
a differential display. The LeCTR1 , LeCTR3
and LeCTR4 genes are close homologues
of Arabidopsis CTR1 . These genes have
been used successfully in complementing
the Arabidopsis ctr1 mutation, although
with different effi cacies, suggesting that all
three CTR genes may be functional in
tomato and could have a degree of
functional redundancy. Similar to the
ethylene receptors, these genes also exhibit
tissue-specifi c transcript accumulation.
Expression of LeCTR1 is induced in an