Salicylic Acid Biosynthesis and Role in Modulating Terpenoid and Flavonoid Metabolism in Plant Responses to Abiotic Stress - Salicylic Acid: Plant Growth and Development - page 144

In plants, Wildermuth et al. ( 2001 ) characterized two putative ICS genes (ICS1
and ICS2) implicated in the SA biosynthesis from IC in A. thaliana. ICS1
expression correlated with SA accumulation and expression of the SA-induced
PR-1 gene. More recent analyses revealed that ICS1 transcripts are also accu-
mulated in response to several biotic and abiotic stresses, including UV light,
ozone, biotrophic pathogens, and exogenous SA treatment (Ogawa et al. 2005 ;
Nobuta et al. 2007 ; Harrower and Wildermuth 2011 ). Overexpression of bacterial
ICS and IPL in tobacco or A. thaliana led to higher levels of SA and SA glucoside
as compared to wild-type plants, provided the enzymes were targeted to the
chloroplasts (Verberne et al. 2000 ; Mauch et al. 2001 ). Biochemical and molecular
analyses of ICS1 further supported a role for this enzyme in SA biosynthesis. The
high affinity of ICS1 for chorismate allows ICS1 to compete successfully with
other pathogen-induced enzymes that use chorismate as their substrate, such as
anthranilate synthase (Ziebart and Toney 2010 ). ICS1 functions as a unifunctional
ICS (Strawn et al. 2007 ). Moreover, SA accumulation is not totally blocked by
mutations in ICS1, so ICS2 seems to have a redundant function (Garcion et al.
2008 ). Some studies in tobacco cells indicated that ICS2 is a functional, chloro-
plast-localized ICS (Garcion et al. 2008 ), and further analysis of the recombinant
ICS2 confirmed it to be a unifunctional ICS that is imported into the chloroplast
stroma (Strawn et al. 2007 ). Still, genetic analyses revealed that ICS2 accounted
for small level of SA biosynthesis (Garcion et al. 2008 ). In addition to Arabidopsis,
ICS homolog has been characterized in several other plant species (Van Tegelen
et al. 1999 ; Ogawa et al. 2005 ; Uppalapati et al. 2007 ; Catinot et al. 2008 ; Yuan
et al. 2009 ). Knowing their role in phylloquinone synthesis, it is very likely that
ICS homologues will be identified in all plant species. Hence, identification of ICS
gene in a given plant species is not enough to show that SA synthesis occurs via
IC. However, analysis of [1- 13 C]- D -glucose incorporation in Catharanthus roseus
cells, revealed that most of the SA synthesized after elicitation with a Pythium
aphanidermatum extract was generated via the IC pathway (Mustafa et al. 2009 ).
Still, it is remarkable that no plant gene encoding an IPL activity has been iden-
tified so far (Chen et al. 2009 ).
3 Roles of SA
3.1 Regulation of SA Accumulation
The SA metabolism may differ slightly between plant species. In some species, IC
metabolism is more complex than in A. thaliana, with a major flux of IC chan-
nelled to the formation of anthraquinones (Stalman et al. 2003 ). In other species,
including rice and poplar, SA synthesis is not markedly induced by biotic and
abiotic stress, while SA and their derivatives play a key role in disease and her-
bivore resistance (Smith and Mètraux 1991 ). Based on the current knowledge in A.