Ethylene Biosynthesis - Fruit Ripening: Physiology, Signalling and Genomics - page 196

Table 10.1. Classifi cation and regulation of ACS protein isoforms.

Examples in Arabi-
dopsis ACS type Properties Examples in tomato

Type 1 Extended C-terminal domain, multiple

phosphorylation sites for mitogen-activating
protein kinase (MAPK) and (probably) calcium-
dependent protein kinases (CDPKs);
unphosphorylated proteins are rapidly degraded
by the ubiquitin 26S proteasome system

AtACS1, AtACS2,
AtACS6

LeACS1A,
LeACS1B,
LeACS2, LeACS6

Type 2 Shorter C terminus and phosphorylated by CDPK

only, at a single site; type 2 isoforms are
recognized and interact with proteins such as
ATO1, EOL1 and EOL2, which leads to their
degradation by the ubiquitin 26S proteasome

AtACS9, AtACS8,
AtACS5, AtACS4

LeACS8, LeACS7,
LeACS3

Type 3 No known phosphorylation sites AtACS11, AtACS7 LeACS5, LeACS4

site, and type 3 have no known phos-
phorylation sites (Table 10.1). Mutations
affecting the C termini of type 2 isoforms
ACS5 and ACS6 from the eto2 and eto3
mutants of Arabidopsis caused dominant
overproduction of ethylene (Chae et al. ,
2003), indicating regulation involved the C
termini of the proteins. The recessive
mutation eto1 , which also causes ethylene
overproduction, revealed a class of
regulatory proteins (ETO1, EOL1 and
EOL2) that interact specifi cally with type 2
isoforms in their C termini and regulate
their degradation (Wang et al. , 2004;
Yoshida et al. , 2005; Christians et al. ,
2009). Cytokinins and brassinosteroids can
increase ethylene synthesis (Woeste et al. ,
1999; Yi et al. , 1999; Arteca and Arteca,
2008) by increasing ACS5 protein stability
(Chae et al. , 2003), and each may affect
type 2 ACS protein stability independently
(Hansen et al. , 2009). Controlling ACS
protein stability provides a means of
tightly controlling ethylene synthesis.
There is little evidence that ACO activity is
regulated post-transcriptionally, but it has
not been studied in the same detail as ACS.

many, if not all, climacteric fruits, and it is
important to understand the biological and
technological control of ethylene synthesis,
as this offers the opportunity to infl uence
fruit storage, ripening and fruit quality.
Intriguingly, ethylene controls its own
synthesis in some situations, including
ripening. Control of ethylene production in
response to stress factors and the molecular
regulation of these responses should be a
fertile area for future research. ACS and
ACO are both actively regulated at different
stages of development at either the mRNA
or protein level, or both. Regulation of
mRNA concentrations is achieved partly
by transcriptional control of families of
ACS and ACO genes with differential
expression patterns. This transcriptional
regulation involves MADS-box, ERF and
HD-zip TFs and it is predicted more TFs
that participate in different situations will
be discovered as our understanding of
the regulation of ethylene biosynthesis
improves. There is sparse information
about the control of mRNA turnover, but
this may also represent a potential control
point. Interplay between different hor-
mones makes an important contribution to
developmental and environmental regu-
lation of ethylene production, and an
important mechanism is post-translational
control of the stability of type 2 ACS
isoforms.

10.6 Conclusion

Ethylene biosynthesis, perception and
response are essential for full ripening of