Legumes are distinguished by their ability to develop root nodules in symbiosis with nitrogen-fixing rhizobia. Flavonoids are essential signals during the symbiosis, first as Nod gene inducers acting on the rhizobia, secondly as regulators of auxin transport and auxin localisation (Wasson et al., 2006). Due to their autofluorescence and the specificity of their fluorescence emission spectra, fluorescence microscopy can be used to easily localise and party identify flavonoids in situ (Mathesius et al., 1998) We have combined the localisation of flavonoids with mass spectrometry to identify and quantify flavonoid structures, and have linked this to their functions in controlling auxin localisation in legume roots (Ng et al., 2015). While flavonoids accumulate specifically in dividing cells of a number of roots organs, including nodules, lateral roots and root galls, flavonoids are only essential for nodule development (Wasson et al., 2009). This suggests that nodulation in legumes has recruited flavonoids for this novel organogenesis process. The localisation of specific flavonoids is correlated with the accumulation of auxin, with flavonoids acting as auxin transport inhibitors in those cells, as well as in regulating enzymes involved in generation and quenching of reactive oxygen species (Mathesius 2001; Ng et al., 2015).
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