Flavonoid biosynthesis and Arabidopsis genetics: more good music.

Abstract

In this issue of Journal of Experimental Botany (pages 1505–1517), Ishihara et al. report the identification of a gene responsible for the production of flavonol 3-O-gentiobioside 7-O-rhamnosides by elegantly tickling the ivories of Arabidopsis genetics and genetic resources combined with straightforward metabolite analysis: it is a model case of functional evaluation. Flavonoids are secondary metabolites derived from the phenylpropanoid biosynthetic pathway that occur in a huge number and variety in plants. They have been implicated in diverse processes, including pigmentation, redox and UV protection, plant–microbe interactions, development and regulation of auxin transport (Winkel-Shirley, 2006; Yonekura-Sakakibara et al., 2008; Kuhn et al., 2011; Peer et al., 2011; Grunewald et al., 2012; Buer et al., 2013; Emiliani et al., 2013; Saito et al., 2013; Yin et al., 2014; Ishihara et al., 2016). Genetics and flavonoid biosynthesis were already successfully engaged on the verge of Arabidopsis becoming the plant model organism. The transparent testa (tt) mutant loci, which affect the biosynthesis of flavonoids, defined easily scorable genetic markers due to the loss of seed coat pigmentation. Their molecular identification established many crucial steps in the biosynthesis of the flavonoid core structure (Koornneef et al., 1983; Shirley et al., 1995; Winkel-Shirley, 2006; Saito et al., 2013). This core is formed by a phenyl ring condensed with an oxygen-containing heterocycle in different oxidation states which is further substituted at different positions with another phenyl side group. These variable cores constitute different flavonoid classes occurring in plants and within a given plant species. However, only the decoration of these aglycones with various carbohydrate side chains and further chemical modification provides the full flavonoid range. Again, genetics in combination with biochemistry, metabolite analyses and, in particular, gene co-expression patterns led to the identification of several UDP-carbohydrate-dependent glycosyltransferases (UGTs) conjugating flavonoids with different carbohydrates at different positions (Jones et al., 2003; Yonekura-Sakakibara et al., 2008, 2012). Nevertheless, there are still unresolved cases, one of them being the production of the Arabidopsis accession-specific flavonol 3-O-gentiobioside 7-O-rhamnosides (F3GG7Rs), comprising the flavonols kaempferol, quercetin or isorhamnetin with the specific carbohydrate decoration 3GG7R, which are most probably derived through glucosylation from flavonol 3-O-glucoside 7-O-rhamnoside (F3G7R) precursors.