Ethylene Biosynthesis - Fruit Ripening: Physiology, Signalling and Genomics - page 190

Turning Orange Red Soft red

8

6

Ethylene production
by ripening tomato
fruit

4

ACO
mRNA

2

ACS mRNA

0 2 4

Time (days)

6 8

Fig. 10.3. Ethylene evolution and ACS and ACO mRNA levels during tomato fruit ( Lycopersicon
esculentum ) ripening. During ripening, ethylene synthesis is autocatalytic: its production is initiated in a few
cells and this recruits other cells to switch on ethylene synthesis, so that a sustained burst of ethylene
production moves throughout the fruit. The turning stage of ripening is when the fl esh is beginning to
change from green to orange. The increase in ethylene biosynthesis (dotted-dashed line) is closely
paralleled by increases in mRNA for LeACS2 and LeACS4 (dotted line), and LeACO1 , LeACO3 and LeACO4
(solid line), as discussed in the text. High ethylene production levels continue for days, without feedback
inhibition, and production then declines slowly as the fruit become fully ripe. Compare this with the
kinetics of ethylene production in wounded leaves in Fig 10.4. The fi gure shows a diagrammatic
representation of the results of Holdsworth et al. (1987), Barry et al. , (1996, 2000) and Nakatsuka et al.
(1998).

4

3

Ethylene production
by wounded tomato leaves

2

1

TOM13 ( ACO1 mRNA)

0 3 6

Time (h after wounding)

9 12

Fig. 10.4. Ethylene evolution and levels of TOM13 ( ACO1 ) mRNA in wounded tomato leaves. Wounding
initiates a rapid burst of ethylene production within 20 min of wounding (cutting) (dotted-dashed line). This
is preceded by a rise in TOM13 ( ACO1 ) mRNA (solid line). Ethylene and ACO1 mRNA start declining
within a few minutes and continue to decline for a few hours to reach the basal levels. Redrawn from
Holdsworth et al. (1987).

inhibiting its expression in transgenic
tomato plants, using an antisense TOM13
gene-silencing construct, led to inhibition
of ethylene biosynthesis (Hamilton et al. ,
1990) (Fig. 10.5). By expressing it in yeast,

TOM13 was shown to encode a protein
that converted ACC to ethylene with the
stereospecifi city predicted by Yang
(Hamilton et al. , 1991), and the identity of
TOM13 was confi rmed by expressing a