Ethylene Biosynthesis - Fruit Ripening: Physiology, Signalling and Genomics - page 192

because, although reducing ethylene bio-
synthesis does reduce the rate of ripening,
overripening and deterioration, it also
inhibits fl avour development, which makes
this particular technological approach un-
attractive to consumers (Ayub et al. , 1996;
Flores et al. , 2002).

expressed in different plant organs at
specifi c times, particularly during fruit
development and ripening (Figs 10.6 and
10.7). The expression of members of the
ACS multigene family in ripening tomato
fruit and senescing fl owers was demon-
strated by Rottmann et al. (1991). ACO1
mRNA (when it was still referred to as
TOM13) was shown very early on to
accumulate within minutes of cutting or
wounding plant material and during
ripening (Smith et al. , 1986; Holdsworth et
al. , 1987; Blume and Grierson, 1997), leaf
senescence (Davies and Grierson, 1989;
Picton et al. , 1993; John et al. , 1995; Blume
and Grierson, 1997) and following fungal
infection (Blume and Grierson, 1997).
Barry et al. (1996) demonstrated that
mRNA from ACO1 , ACO2 and ACO3
accumulated in a variety of developmental
situations, including fruit ripening. ACO
genes also show differential expression in
fl ower petals in response to fertilization
(Llop-Tous et al. , 2000).
A gene called E8 , which encodes an
Fe(II) oxygenase with some similarity to
ACO genes, was reported to modulate
LeACO1 and LeACS2 expression and
ethylene production in tomato (Kneissl and
Deikman, 1996), but the mechanism
remains to be elucidated. The normal
accumulation of ACO1 and other mRNAs
during tomato ripening is prevented by
heat shock at 35°C (Picton and Grierson,
1988), which can occur under fi eld
conditions. Ozone, on the other hand,
induces rapid accumulation of ACS2 and
ACO mRNA, and the induced ethylene
biosynthesis is linked to oxidative stress
and induction of cell death (Tuomainen et
al. , 1997; Moeder et al. , 2002). Barry et al.
(2000) proposed that, in tomato, system 1
ethylene involves the expression of
LeACS1A and LeACS6 , and that during the
transition from system 1 to system 2, the
rin (or LeMADS-RIN ) gene, which is
mutated in the ripening inhibitor ( rin )
mutant of tomato, enhances expression of
LeACS1A and induction of LeACS4 and
that the maintenance of system 2 ethylene
production is due to the ethylene-
dependent induction of LeACS2 (Barry

10.3 Differential Expression of ACO and
ACS Genes: System 1 and System 2
Ethylene

All plant parts synthesize small amounts of
ethylene all the time, including unripe
fruits. This basal level is approximately
0.05 nl g -1 h -1 in tomato, but can vary with
the variety, temperature and other
environmental conditions. In climacteric
fruits, there is a ripening-related burst of
ethylene biosynthesis, accompanied by the
respiratory climacteric. This may reach
10 nl g -1 h -1 in tomato but can be much
higher in other fruits, such as banana.
Before ripening, ethylene production
increases rapidly if the fruit or leaves are
cut, or subjected to some other injury, but
this is generally reduced again after a few
hours (Fig. 10.4), because at this stage of
development ethylene biosynthesis is
subject to feedback inhibition. At the onset
of ripening, however, a burst of auto-
catalytic ethylene production begins (i.e.
ethylene stimulates its own synthesis;
there is no feedback inhibition) and this
stimulates ripening. McMurchie et al.
(1972) proposed the existence of two
systems (system 1 and system 2) involved
in ethylene biosynthesis. System 1
functions during normal vegetative growth,
is autoinhibited by ethylene and is
responsible for producing the basal levels
of ethylene that are synthesized by all
plant tissues. System 2 operates during
ripening of climacteric fruit, during
senescence and in some other situations.
The molecular and biochemical data
accumulated subsequently support this
proposal, and we now know that both
systems utilize different isoforms of ACS
and ACO, which are regulated differently.
ACO and ACS genes are differentially