Fruit Ripening: Primary Metabolism - Fruit Ripening: Physiology, Signalling and Genomics - page 16

the synthesis and degradation of this
hormone is highly regulated. Moreover,
signal transduction is also a critical aspect
and is regulated at multiple levels. The role
of ethylene in ripening of climacteric fruits
has been known for more than 50 years.
Since its discovery, considerable effort
has been focused on studies of ethylene
biosynthesis (involving the enzymes
S -adenosylmethionine (SAM) synthetase,
1-aminocyclopropane carboxylic acid (ACC)
synthase (ACS) and ACC oxidase (ACO)),
ethylene perception (by ethylene receptors),
signal transduction (by ethylene response
factors) and ethylene-regulated genes such
as cell-wall-disassembling genes (endo-
polygalacturonase, pectin methyl esterase
and pectate lyase).
Tomato fruit has been used as a model
for climacteric fruits because ethylene is an
agriculturally important hormone. In
ethylene biosynthesis, only the ACS and
ACO genes are involved. SAM is converted
to ACC by ACS, and this step is considered
the limiting step. ACC is subsequently
converted to ethylene by ACO. There are a
few characterized ACS and ACO genes in
tomato, but only ACO1 , the most highly
induced ACO during ripening, prevents
ethylene synthesis and ripening (Hamilton
et al ., 1990). On the other hand, only two
ACS genes, ACS2 and ACS4 , are
signifi cantly increased during ripening
(Rottmann et al ., 1991; Barry et al ., 1996),
and both genes, as with ACO1 , prevent
ethylene synthesis and ripening (Oeller et
al ., 1991). Recently, genomics approaches
have provided insight into the primary
ripening control upstream of ethylene. The
tomato pleiotropic ripening mutations
ripening inhibitor ( rin ), non-ripenin g ( nor )
and Colourless non-ripening ( Cnr ) have
added much insight in this regard. The rin,
nor and Cnr mutations are affected in all
aspects of the tomato fruit-ripening process
that are unable to respond to ripening-
associated ethylene genes (Vrebalov et al. ,
2002; Manning et al. , 2006). Furthermore,
in fruits from these mutants, the ripening-
associated ethylene genes are induced by
exogenous ethylene, indicating that all
three genes operate upstream of ethylene

biosynthesis and are involved in a process
controlled exclusively by ethylene. The
three mutant loci encode putative tran-
scription factors. The rin mutant encodes
a partially deleted MADS-box protein of
the SEPALATTA clade (Hileman et al. ,
2006), whereas Cnr is an epigenetic change
that alters the promoter methylation of
SQUAMOSA promoter binding (SPB)
proteins. It has been suggested that the nor
loci encodes a transcription factor (J.
Vrebalov and J. Giovannoni, unpublished
results), although not a member of MADS-
box family (Giovannoni, 2004). The
observed ethylene-independent aspect of
ripening suggests that the RIN, NOR and
CNR proteins are candidates for conserved
molecular mechanisms of fruit in both the
climacteric and non-climacteric categories.
Another study (Osorio et al. , 2011) in
which transcriptome, proteome and tar-
geted metabolite analysis were combined
during development and ripening of nor
and rin mutants has helped to refi ne the
ethylene-regulated transcriptome and has
added to our knowledge of the role of
ethylene in both protein and metabolite
regulation in tomato ripening. These data
support the contention that nor and rin act
together in a cascade to control ripening
(Giovannoni et al. , 1995; Thompson et al. ,
1999) and also suggest that nor has a more
global effect on ethylene/ripening-related
gene expression than rin , which indicates
that nor probably operates upstream of rin .
Biochemical evidence suggests that
ethylene production may be infl uenced or
regulated by interactions between its
biosynthesis and other metabolic pathways.
One such example is provided by the fact
that SAM is the substrate for both the
polyamine pathway and nucleic acid
methylation; competition for substrate has
been demonstrated by the fi nding that
overexpression of a SAM hydrolase is
associated with inhibited ethylene pro-
duction during ripening (Good et al. , 1994).
On the other hand, the methionine cycle
directly links ethylene biosynthesis to the
central pathways of primary metabolism.
Non-climacteric fruits such as straw-
berry and grape do not increase respiration,