Flavonoids are diverse polyaromatic secondary metabolites consisting of a 15-car-bon skeleton and are formed from a branch of the phenylpropanoid pathway. The first committed step of flavonoid biosynthesis is catalysed by chalcone synthase; this reaction involves the condensation of 4-coumaroyl-CoA with three molecules of malonyl-CoA to form a chalcone flavonoid precursor (Fig. 1). This precursor feeds into further biosynthetic reactions which yield either 5-deoxyflavonoids or 5-hydroxyflavonoids. Flavonoids can be further sub-classed into flavonoids and isoflavonoids according to whether the phenyl group is attached to C2 or C3 (as with the flavonol kaempferol or the isoflavone daidzein, respectively; Fig. 1).
Flavonoid production is ubiquitous in plants and these compounds are typically associated with plant defence responses, in addition to lignin and anthocyanin production (Winkel-Shirley 2001). The first proof for the role of root-exuded flavonoids during symbiosis was the induced expression of Sinorhizobium meliloti nodulation genes (nodABC) by luteolin, a flavone (Peters et al. 1986), and the induction of Rhizobium trifolii nod genes by flavones from clover (Redmond et al. 1986). Subsequent research has identified several other flavonoids involved in the nodulation signalling of many plant species (reviewed by Broughton et al. 2000). It has been noted that some flavonoid structures are only produced by particular plants; for example, isoflavonoid production is limited to the Papilionoideae (or Faboideae) subfamily of the Leguminosae (Dixon et al. 2002). This diversity of flavonoid production has been associated with determining, at least in part, the
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