Salicylic Acid Biosynthesis and Role in Modulating Terpenoid and Flavonoid Metabolism in Plant Responses to Abiotic Stress - Salicylic Acid: Plant Growth and Development - page 150

3.3 Role of SA in Modulating Terpenoid and Flavonoid
Metabolism
Unfavourable conditions such as drought or salt stress lead to an imbalance
between the energy harvested by plants and their ability to process it (Chaves et al.
2003 ). Plants display a number of mechanisms to avoid and dissipate this excess
excitation energy, some of which engage isoprenoid and flavonoid compounds
(Munné-Bosch and Alegre 2003 ; Delfine et al. 2005 ; Owen and Peñuelas 2005 ;
Agati and Tattini 2010 ). Many of these mechanisms represent conserved tools for
photoprotection. For instance, a-tocopherol and carotenoids display antioxidant
activity and contribute to keep cell functionality under stress (Havaux and
Kloppstech 2001 ; Havaux et al. 2005 ; Munné-Bosch 2005 ). Others, such as
monoterpenes and flavonoids (including anthocyanins and flavonols) represent
additional or alternative protective mechanisms (Feild et al. 2001 ; Peñuelas and
Munné-Bosch 2005 ; Hernández et al. 2009 ). Stress protection by terpenoids and
flavonoids has been ascribed to their antioxidant capacity, which in many cases,
especially terpenoids but also some flavonoids such as (-)-epigallocatechin gal-
late, is exerted in membranes (Munné-Bosch 2005 ; Hernández et al. 2006 ).
The biosynthesis of terpenoids has been extensively reviewed in the literature
(see Cheng et al. 2007 for a recent review). In summary, it consists in the
sequential condensation of isopentenyl diphosphate (IPP) molecules. In plants, the
biosynthesis of this 5-carbon intermediary is achieved by two different pathways,
the 2-methyl- D -erytritol-4-phosphate (MEP) and the mevalonate (MEV) pathways,
which operate in plastids and cytosol, respectively. All terpenoids are synthetized
by the sequential action of a set of prenyltransfereases that elongate the prenyl
chain, initially IPP, by the repetitive addition of this C 5 unit, generating mono-,
sesqui-, di-, tri-, tetraterpenes or even longer compounds. The genes encoding for
the 1-deoxy- D -xylulose-5-phosphate reductoisomerase, from the chloroplastic
MEP pathway, and 3-hydroxy-3-methylglutaryl-CoA reductase, from the cytosolic
MEV pathway, have been shown to be transcriptionally upregulated by SA in
Salvia miltiorhiza and Michelia chapensis, respectively (Yan et al. 2009 ; Cao et al.
2011 ). The expression of farnesyl diphosphate synthase (FDPS), geranylgeranyl
diphosphate synthase (GGDS) and squalene synthase (SQS), three prenyltransfe-
rases from the core terpenoid biosynthetic pathway, has been shown to be
upregulated by SA in different species as well (Cao et al. 2012 ; Shabani et al.
2010 ; Kai et al. 2010 ). In agreement, the levels of many isoprenoids such as a-
tocopherol or b-carotene have been shown to be upregulated during drought or salt
stress in parallel to increases in SA levels (see for instance Munné-Bosch and
Peñuelas 2003 ; Eraslan et al. 2007 ). In addition, holm oaks fumigated with SA
showed higher monoterpene levels in leaves and enhanced volatile monoterpene
emision (Peñuelas et al. 2007 ). Thus, as a general rule, SA upregulates the ter-
penoid biosynthetic pathway at the transcriptional level.
However, not all terpenoids behave equally upon drought or salt stress exposure
and SA levels, and some studies report on decreases of the levels of particular