Ethylene Biosynthesis - Fruit Ripening: Physiology, Signalling and Genomics - page 195

MADS box proteins
RIN and TAGL1

SAM

RIN TAGL1

ACC synthase ( LeACS2 )

LeACS2

ACC

LeHB1, an HD-Zip protein

LeHB1 LeHB1

ACC oxidase ( LeACO1 )

LeACO1

ACC oxidase ( LeACO3 )

LeACO3

Ethylene

ERF2 ERF2

Ripening, senescence,
response to pathogens,
abscission, etc.

ERF2

Fig. 10.8. Transcription factors regulating LeACS2 , LeACO1 and LeACO3 and enhanced ethylene synthesis
in ripening tomato fruit. The RIN protein was identifi ed as the product of the rin gene in the non-ripening
rin mutant of tomato (Vrebalov et al. , 2002) and the normal protein was shown to bind the LeACO2
promoter (Ito et al. , 2008). The role of TOMATO AGAMOUS-LIKE 1 (TAGL1) was determined by Itkin et al.
(2009). Identifi cation and functional analysis of LeHB1, a homeobox protein, was performed by Lin et al.
(2008). The role of the ethylene response factor LeERF2 was proposed by Zhang et al. (2009).

operate downstream of the ethylene
signalling pathway (ethylene (C 2 H 4 ) o
ethylene receptor (ETR) proteins o
constitutive triple response (CTR) proteins
o EIN3-like (EIL) proteins) and have
diverse expression patterns and DNA-
binding capacity to the GCC box and other
sequences in the promoters of genes they
regulate (Tournier et al. , 2003; Pirrello et
al ., 2012). In addition to ripening, they
can be involved in responses to wound-
ing, biotic and salt stress, anaerobiosis
and signalling pathways involving
brassinosteroids, ethylene, jasmonic acid
and salicylic acid. One of these (LeERF2)
interacts with the GCC box in the
promoter of dehydration-responsive ele-
ment in the promoter of LeACO3 ,
resulting in transcriptional activation of
the gene. Inhibiting ERF2 expression by
gene silencing was shown to reduce
ethylene synthesis (Zhang et al. , 2009).

10.5 Post-translational Control of ACS

Some forms of ACS, such as those induced
by wounding or auxin, have a short half-
life (i.e. they are unstable and rapidly
degraded), and the demonstration that
LeACS2, but not LeACS4, is phos-
phorylated (Tatsuki and Mori, 2001) hinted
at a mechanism for regulating the stability
of some isoforms. Generally, three ACS
types are recognized, types 1, 2 and 3,
which differ in their structure and
regulation (Table 10.1). Type 1 ACSs are
phosphorylated by mitogen-activated pro-
tein kinase 6 (MAPK6) (Liu and Zhang,
2004), and probably also by calcium-
dependent protein kinase (Hernandez
Sebastia et al. , 2004); in the absence of
phosphorylation, they are rapidly degraded
by the 26S proteasome pathway for protein
degradation (Joo et al. , 2008). Type 2
isoforms have only one phosphorylation