Regulation of Nodule Development by Short- and Long-Distance Auxin Transport Control

Auxin is a key regulator of plant morphogenesis and areas of local auxin maxima have been hypothesized to control organ initiation. During the formation of symbiotic root nodules, rhizobia initiate a Nod-factor signaling cascade in the root that leads to changes in auxin responses in the root cortex. This auxin accumulation is likely to be necessary for the initiation of cell division to form a nodule primordium. There is growing evidence that rhizobia control the polar transport of auxin in the host and that this could lead to altered auxin accumulation and responses necessary for nodule organogenesis, although this auxin transport control differs between legumes forming determinate and indeterminate nodule types. Auxin transport is modulated by flavonoids during nodule formation. In addition, there is evidence that cytokinin signaling is necessary for auxin transport control. In addition to these local changes in auxin transport control, rhizobia trigger the long-distance transport of a signal to the shoot, where the control of nodule numbers by autoregulation control occurs. Autoregulation leads to changes in the shoot-to-root auxin transport and subsequent reduction in nodule numbers on the whole root system. The availability of nitrate in the growth medium exerts an additional level of control over nodule numbers, and nitrate signaling partially acts via the control of long-distance auxin transport.