Computational and Experimental Evidence That Auxin Accumulation in Nodule and Lateral Root Primordia Occurs by Different Mechanisms

The formation of root lateral organ formation typically requires the de novo generation of a primordium, initiated at the site of local auxin accumulation. Legume roots are a particularly interesting example in this respect, as they can give rise to both lateral roots and root nodules. The initiation of the latter primordium is much less understood. Using computer simulations we start with an unbiased comparison of different mechanisms that a priori seemed capable of producing local auxin accumulation. These mechanisms all produce different characteristic signatures, of which a reduction of auxin efflux best matches the morphology of nodule primordia. This leads to the prediction that root nodule positions would not be affected by root curvature – contrary to lateral root primordia that are initiated by increased influx – which we experimentally confirmed. We further investigate how changes in the in silico root segment affect the induction and shape of local auxin maxima and discuss the functional implications of these findings with respect to nodule type and bounding the developmental zone where nodulation takes place.