Ethylene Perception and Signalling in Ripening Fruit - Fruit Ripening: Physiology, Signalling and Genomics - page 204

ethylene-dependent manner but not that of
LeCTR3 and LeCTR4 , suggesting that these
three genes might be subjected to both tran-
scriptional and post-translational regulation
during ripening (Lin et al. , 1998; Leclercq et
al. , 2002).

LeEIL4 exhibits a ripening-associated in-
crease during ripening. Reduced expres-
sion levels of LeEIL1-3 resulted in ethylene
insensitivity, indicating that these proteins
are positive regulators of ethylene
responses (Cara and Giovannoni, 2008).
Yokotani et al. (2009) reported that, in
transgenic plants with attenuated tran-
script accumulation of LeEIL genes,
expression of the LeACS2 and LeACS4
genes exhibited a limited increase during
fruit ripening that was not abolished even
after 1-methylcyclopropene treatment. This
study concluded that ethylene production
is developmentally regulated in an
ethylene-independent manner during fruit
ripening in tomato. EIL proteins have also
been found to control the expression of
genes associated with ethylene bio-
synthesis, such as 1-aminocyclopropane-1-
carboxylic acid oxidase ( ACO ) genes,
directly by interacting with their promoters
in kiwifruit and melon fruits. It has been
suggested that EIL proteins could also be
involved in the regulation of ethylene
biosynthesis (Yin et al. , 2010). Recently,
analysis of phosphorylation-dependent
regulation of ethylene responses at the
level of EIN3/EIL has led to the
identifi cation of a phosphorylation site in
the tomato EIL1 protein. This phos-
phorylation region, named EPR1 (EIN3/EIL
phosphorylation region 1), was found to be
essential for its transcriptional activity.
Mutation in this region prevented phos-
phorylation of EIL1, which in turn led to
the complete loss of its transactivation
potential and fi nally abolished the ethylene
constitutive responses. Furthermore, func-
tional EPR1 was found to be necessary for
mediating the dimerization step of EIL1
proteins, indicating that this region might
represent a molecular mechanism for the
regulation of EIN3/EIL activity. In con-
clusion, the authors proposed a model
where phosphorylation within EPR1
caused dimerization of EIL1 protein, and
the homodimer complex then bound to the
target genes allowing transcriptional
activation of ethylene-regulated genes (Li et
al. , 2011). Moreover, two EBF (EIN3-
binding F-box) proteins have been found to

11.2.3 Ethylene Insensitive 2 (EIN2)

In the ethylene signalling cascade, EIN2, a
member of the Nramp family protein of
metal transporters, is present downstream
of the CTR genes (Roman et al. , 1995).
Expression of the EIN2 gene is ethylene
independent and does not exhibit sub-
stantial changes at different stages of fruit
development and ripening. Transgenic
plants with attenuated EIN2 expression
show a delayed ripening phenotype. In
addition, inhibited transcript accumulation
of ethylene- and ripening-related genes
suggests that this gene is a positive
regulator of ethylene-mediated responses
during tomato fruit development and
ripening. This protein has been suggested
as a converging point for the signalling
pathways of various phytohormones, in-
cluding auxin, abscisic acid, jasmonic acid
and ethylene, indicating that these hor-
monal pathways can cross-talk to each
other during various developmental
aspects in plants (Cara and Giovannoni,
2008).

11.2.4 EIN3-like (EIL) proteins

A family of trans -acting proteins, namely
EIL proteins, is present downstream of
EIN2. These proteins act as transcription
regulators and control the expression of
another class of transcription factors, the
ethylene-response factor (ERFs). EILs recog-
nize specifi c motifs known as ethylene-
response elements (EREs) present in the
promoters of various ripening-associated
genes. In tomato, four EIL genes constitute
a small multigene family (Tieman et al. ,
2001; Yokotani et al. , 2009). The transcript
accumulation of LeEIL1 , LeEIL2 and LeEIL3
genes is ethylene independent, while