Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

128 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
The involvement of ethylene and its receptors in the ripening of climacteric fruits has
been reaffirmed in several fruits, even though it has also emerged that the role played
by ethylene is not exclusive, since ethylene-independent pathways are also involved in
the ripening process (Giovannoni, 2004). There appear to be significant differences in the
way ethylene signaling is regulated in Arabidopsis and tomato. But the building blocks
of ethylene signal transduction are very similar between the two species. A family of six
genes encoding tomato ethylene receptors (LeETR1-6/SlETR1-6) has been isolated and
characterized and the predicted structures are similar to Arabidopsis (Fig. 6.2) (Alexander
and Grierson, 2002; Klee and Tieman, 2002; Klee, 2002, 2004, 2006). The proteins encoded
by these genes are structurally diverse and, at the most, are less than 50% identical. The
mRNA expression patterns vary among the tomato ETR1 homologs. LeETR1 is expressed
constitutively in all tissues examined. LeETR2 is expressed at low levels in all tissues
with induction in seeds before germination and downregulation in elongating seedlings
and senescing leaf petioles. NR (LeETR3) mRNA is upregulated in ovaries and ripening
fruit. The LeETR4 mRNA is present at high levels in fruit but is low in vegetative tissues.
The LeETR5 expression pattern is similar to LeETR4, but absolute mRNA levels are lower.
The LeETR6 mRNA is abundantly expressed in flowers and fruits and less in vegetative
tissues.
Other than tomato fruit, the ethylene receptors have been isolated in several climacteric
and nonclimacteric fruits, and exhibits different expression patterns during ripening. The
melon fruit is second to tomato fruit when it comes to research work carried on ethylene
perception. Melon fruit is an ideal fruit for these studies due to the fact that its development
has three distinct stages: phase I, II, and III; the flesh, embryo, placenta, and seeds are
well ordered; the fruit development can be clearly divided into ethylene-insensitive and
ethylene-sensitive stage, and the developing fruit has a lower sensitivity to ethylene than
does the ripening fruit (Gillaspy et al., 1993; Takahashi et al., 2002). In muskmelon, Cm-
ERS1 mRNA increased slightly in the pericarp of fruit during ripening, followed by a
marked increase of Cm-ETR1 mRNA, which paralleled climacteric ethylene production.
The increase of Cm-ERS1 mRNA at a low concentration of ethylene before the increase of
Cm-ETR1 mRNA and ethylene production indicates that Cm-ERS1 may be sensitive to a
much lower concentration of ethylene, while Cm-ETR1 may be involved in the response at
a high concentration of ethylene (Sato-Nara et al., 1999). Studies carried out to examine
the temporal and spatial expression pattern of Cm-ERS1 protein, during fruit development,
revealed that a posttranscriptional regulation of Cm-ERS1 expression affects stage- and
tissue-specific accumulation of the protein (Takahashi et al., 2002). The melon receptor
CmERS1 was localized at the endoplasmic reticulum and its topology indicates that there
are three membrane-spanning domains, with its N-terminus facing the luminal space and the
large C-terminal portion being located on the cytosolic side of the ER membrane (Fig. 6.3)
(Ma et al., 2006a). The melon subfamily II ethylene receptor, Cm-ETR2 mRNA, exhibits
earlier accumulation compared to Cm-ETR1 during ripening, and its transcript accumulation
increased during melon ripening, and declined in parallel with a reduction in ethylene
production. Furthermore, the Cm-ETR2 mRNA was induced by ethylene treatment and
inhibited by 1-MCP (Owino et al., unpublished results).
The expression of two ethylene receptor genes in passion fruit (Passiflo a edulis),
PeETR1 and PeERS1, did not change significantly during ripening. However, the levels
of PeETR1 and PeERS1 mRNA were much higher in arils than in seeds (Mita et al., 1998).