Ethylene Biosynthesis - Fruit Ripening: Physiology, Signalling and Genomics - page 194

10

LeACS4

C 2 H 4 ETR CTR1 EIL

LeACS2
Autocatalytically
regulated

1

Developmental
factor System 2

10 -1

Developmentally
regulated

LeACS4

LeACS1A

LeACS2

LeACS6

System 1

10 -2

Fig. 10.7. Expression of different LeACS and LeACO genes related to system 1 and system 2 ethylene
production in developing and ripening tomato. The contribution of each isoform to total ethylene evolution
is indicated for each stage of development and ripening. Note that developmental factors are shown
promoting upregulation of LeACS4 and LeACS2 . The ethylene response pathway (ethylene (C2H4) o
ethylene receptor (ETR) proteins o constitutive triple response (CTR) proteins o EIN3-like (EIL) proteins) is
shown inhibiting LeACS6 and promoting transcription of LeACS4 and LeACS2 . From Yokotani et al. (2009);
see also Barry et al. (2000).

(Fig. 10.8), using a combination of genetic,
molecular and biochemical procedures.
The rin tomato mutant fails to express some
ethylene biosynthetic genes (Fig. 10.6) and
does not ripen. The TF gene responsible,
LeMADS-RIN , was identifi ed (Vrebalov et
al. , 2002) and loss of activity was shown to
be the result of the rin mutation. Fujisawa
et al. (2011) subsequently showed that the
normal RIN protein binds to the LeACS2
gene promoter and causes transcription of
the gene. A second MADS-box TF,
TOMATO AGAMOUS-LIKE1 (TAGL1), has
also been shown to bind the LeACS2
promoter (Itkin et al ., 2009) (Fig. 10.8). It is
known that MADS-box proteins can form
homo- and heterodimers and higher-order
complexes, but it is not clear whether RIN
and TAGL1 operate separately or in com-
bination. A different type of TF, a
homeodomain-leucine zipper (HD-Zip) pro-
tein named LeHB-1, was discovered by Lin
et al . (2008) during a search for TFs that

bind the LeACO1 promoter. LeHB-1 binds
to a putative binding site in the tomato
LeACO1 gene and appears to activate
transcription at the onset of ripening, as
inhibiting LeHB-1 expression in tomato
fruit leads to inhibition of ripening (Lin et
al ., 2008). Interestingly, there are also
putative LeHB-1-binding sites in the gene
promoters of LeACO2 and LeMADS_RIN ,
and the promoter for the tomato cell-wall-
modifying enzyme polygalacturonase (Lin
et al ., 2008). RIN itself activates LeACS2 , so
RIN and LeHB-1 appear to be crucial for
switching on system 2 ethylene synthesis
and ripening. Interestingly, TAGL1 expres-
sion was enhanced in the presence of
the ethylene perception/action inhibitor
1-methylcyclopropene and appears not to
be regulated by ethylene or RIN (Itkin et al .,
2009).
Members of a large superfamily of
plant-specifi c TFs, called APETALA2/
ethylene responsive factor (AP2/ERF),