Regulation of Nodule Organogenesis by Auxin Transport and Flavonoids

Auxin gradients are known to specify meristematic activity throughout the plant. We investigated the role of auxin in nodule initiation and differentiation, as well as in the control of nodule numbers by local and long distance regulation in the indeterminate legume, Medicago truncatula. Since the discovery that auxin transport inhibitors can induce spontaneous formation of nodule-like structures in legumes (Hirsch et al., 1989), it has been suggested that auxin transport regulation might be part of the mechanism for nodule organogenesis. Nodule initiation is preceded by local inhibition of auxin transport in indeterminate legumes (Boot et al., 1999; Mathesius et al. 1998), but this might not be required for nodule initiation in determinate legumes (Pacios-Bras et al., 2003; Subramanian et al., 2006). This inhibition is accompanied by changes in PIN gene expression (Huo et al., 2006) and is followed by auxin accumulation at the site where nodules are initiated (Mathesius et al., 1998; Pacios-Bras et al., 2003). An ethyleneinsensitive hypernodulating mutant, sickle, shows higher auxin accumulation and PIN-gene expression at the site of nodule initiation than the wild type in M. truncatula (Prayitno et al., 2006). The role of the accumulation of auxin is likely to be in the regulation of cell division (Kondorosi et al., 2005). Proteome analysis supported a role for auxin at an early stage of nodulation as most of the proteins regulated by rhizobia are also regulated by auxin (G.E. van Noorden et al., unpublished data, 2006).