36 Secondary Metabolism in Arabidopsis

Abstract

Plants cannot remove themselves from environments where they may be subjected to various insults, such as attack by pathogens, irradiation by UV light, or damage by herbivores. It is thought that to provide protection against the adverse effects of their environment, plants have acquired the ability to accumulate secondary metabolites. Many of these same compounds are also important to humans because of their pharmaceutical, insecticidal, and organoleptic qualities. Arabidopsis has been reported to accumulate 36 secondary metabolites belonging to four distinct classes. Flavonoids and hydroxycinnamic acid esters are derived from the general phenylpropanoid pathway (Fig. 1). The other two classes of secondary metabolites known to be produced by Arabidopsis are the glucosinolates and the indole phytoalexins. More than 15,000 plant secondary metabolites have been identified, and this may represent only 5–10% of those that occur in nature (Wink 1988). How this diversity has arisen is unknown, as is the extent to which this diversity is represented within a single species, although some hypotheses have been offered (Williams et al. 1989; Jones and Firn 1991). For example, the pathways devoted to cyanogenic glycoside and glucosinolate biosynthesis appear to be closely related, yet they diverge to produce very different secondary metabolites. Cyanogenic glycosides have been found in more than 200 species of angiosperms (Conn 1980) but have never been found in species such as Arabidopsis that accumulate glucosinolates. What has led to this specialization and mutual exclusivity? Are the genes for cyanogenic glycoside biosynthesis present in Arabidopsis, but unexpressed? If so,...