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be regulated by auxin-response factors
(ARFs) directly during ripening in tomato
(Kumar
et al.
, 2011). These observations
have been further supported by the
analyses of expression profi les of genes
involved in early auxin responses, such as
ARFs and Gretchen Hagen3 (GH3). These
studies have resulted in the identifi cation
of four ARFs (
SlARF3
,
SlARF5
,
SlARF6
and
SlARF13
) and two GH3 genes (
SlGH3
-1 and
SlGH3
-2) with ripening-associated expres-
sion (Kumar
et al.
, 2011, 2012a). In
addition, the role of abscisic acid in the
regulation of fruit ripening alone and/or in
relation to ethylene is also being studied
(Sun
et al.
, 2012). Together, these studies
give support to the observation that other
hormones, besides ethylene, are also
involved in the regulation of fruit ripening.
cell-wall dynamics such as polygalacturon-
ase (
PG
), Expansin1 (
LeEXP1
) and pectin
methyl esterase (
PME
), genes controlling
carotenoid biosynthesis and volatile
production such as phytoene synthase
(
PSY
), lipoxygenases (
TomloxA
,
TomloxB
and
TomloxC
) and some other ethylene-
responsive genes with unidentifi ed func-
tions such as
E4
and
E8
(Cara and
Giovannoni, 2008). Some of the other less-
well-characterized ethylene-responsive
genes include
ER24
,
ER49
,
ER50
and
ER68
and the gene encoding an enzyme of the
Rab GTPase family,
LeRab11a
(Zegzouti
et
al.
, 1999; Lu
et al.
, 2001). Transcriptome
analyses of non-ripening mutants with
altered ethylene responses have revealed
multiple ethylene-associated events during
tomato ripening (Alba
et al.
, 2005; Osorio
et al.
, 2011; Kumar
et al.
, 2012b). Recently,
two microRNAs, miR828 and miR1917,
whose targets are EIN2 and the serine/
threonine protein kinase CTR1, were
identifi ed in tomato fruit, suggesting their
role in the regulation of ethylene signalling
during ripening (Zuo
et al.
, 2012).
Characterization of promoter regions of
several ethylene-induced and ripening-
related genes have provided some insights
into their transcriptional regulation. These
studies resulted in the identifi cation of
several motifs that contribute to their
ethylene responsiveness. For example, two
repeat regions and multiple EREs (A(A/T)
TTCAAA) along with a stress-related motif
(TCATCTTCTT) were found in the pro-
moters of the
ACO1
,
2A11
and
E4
genes.
Promoter analysis of two ethylene-
responsive genes (
E4
and
E8
) with con-
trasting expression patterns in response to
ethylene resulted in the identifi cation of
motifs responsible for the tissue-specifi c
and developmentally regulated responses
of
E8
.
Cis
-element analysis of the pro-
moters of ripening-associated genes,
including these two genes, has also led to
the identifi cation of a similar region
necessary for their ethylene responsiveness
in several such genes (Cara and Giovan-
noni, 2008; Kumar
et al.
, 2012b).
11.4 Ethylene-regulated Transcriptional
Aspects of Fruit Ripening
In addition to our improved knowledge of
the components of ethylene signalling,
numerous ethylene-regulated genes that are
responsible for the outcome of the fi nal
ripened fruit phenotype have been
identifi ed across the plant species,
including tomato (Cara and Giovannoni,
2008). Along with the coding sequence, the
upstream regulatory sequences of several
such genes have also been characterized
for their ethylene responsiveness during
ripening. Initially, ethylene-regulated
aspects of ripening were studied using both
ripening mutants and antisense transgenic
plants altered in ethylene production/
responses or by upregulating expression of
genes related to these aspects. It has been
clearly shown that ethylene controls
ripening by regulating the expression of
genes at the mRNA or protein level (Cara
and Giovannoni, 2008). Some of the best-
characterized genes identifi ed in this
manner include genes involved in ethylene
biosynthesis, such as 1-aminocyclopropane-
1-carboxylic acid synthases (
ACS
s) and
ACO
s, genes encoding proteins related to
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