Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

ETHYLENE PERCEPTION AND GENE EXPRESSION 127
Sensor
GAF
Histidine kinase
Subfamily I
Sl-ETR1&ETR2
H
Receiver
Sl-ETR3/NR
H
Subfamily II
Sl-ETR4&ETR6
H
Receiver
Sl-ETR5
H
Receiver
Fig. 6.2 Schematic representation of the tomato ethylene receptor proteins and their functional domain structures
similar to Arabidopsis. The sensor domain contains three hydrophobic, transmembrane regions. Ethylene binding
occurs within this amino terminal hydrophobic region. Subfamily II has a fourth membrane spanning domain.
The GAF domain is conserved among a range of diverse group of proteins. Its function in ethylene signaling
is unknown. There are five subdomains that define the catalytic core of His kinase domain. While subfamily I
contains all of these subdomains, subfamily II lacks one or more of them.
to modulate the activity of a downstream factor (Chang et al., 1993; Hua et al., 1998; Sakai
et al., 1998). Recent studies with bacterial two-component systems support an important
role for receptor interactions in signal output. In plants, ETR1 and ERS1 receptors have
been shown to form homodimers, while ERS-type receptors have been postulated to use the
receiver domains of other receptors to form heterodimers with them (Schaller et al., 1995;
Takahashi et al., 2002; Wang et al., 2003).
Based on distinguishing structural features and overall sequence similarity, the members
of the ethylene receptor family can be divided into two subfamilies: subfamily I and
subfamily II (Stepanova and Alonso, 2005). Subfamily I ethylene receptors have three
transmembrane domains and a well-conserved histidine kinase domain. On the other hand,
subfamily II receptors contain a putative signal peptide in addition to the three conserved
transmembrane domains and a histidine kinase domain that lacks one or more elements
that are necessary for catalytic activity. (A schematic representation of the tomato receptors
structures similar to that in Arabidopsis is shown in Fig. 6.2.)
6.3 Ethylene perception in fruits and vegetables
Subsequent studies regarding ethylene perception have focused on the isolation and characterization
of the receptor gene family from various plant species. However, in fruit and
vegetable species, tomato has emerged as the most useful model to date, due to its commercial
importance, ease of genetic manipulation, rapid life cycle, year-round nonseasonal
greenhouse fruit production, well-characterized single gene-ripening mutants such as never
ripe (nr), nonripening (nor), ripening inhibitor (rin), and green ripe (gr) and the availability
of detailed genetic maps, EST collections, microarray chips, and full-length cDNA collections
(Alexander and Grierson, 2002; Barry et al., 2005; Barry and Giovannoni, 2006; Klee,
2006). Consequently, much of our understanding of ethylene perception in fruit species
comes from studies on tomato. The tomato has been renamed Solanum lycopersicum (formerly
Lycopersicon esculentum), and this had led to the renaming of its genes.