Fig. 6

Outline of partial nodule number control mechanism. Our observations of the transient rootward auxin depression during primordium initiation suggest the following mechanism that contributes to the spatial separation of nodules. a An early stage primodrium serves as an auxin sink (Figs. 5, Additional file 9: Figure S6), which temporarily reduces the auxin availability downstream (rootward) of it. This temporarily inhibits the formation of new primordia. The forming primordium will not be deprived of its auxin source by the initiation of a new upstream (shootward) primodrium, because there, the root is no longer susceptible: the susceptible zone (hatched area) moves rootward as the root continues its growth. b As the primordium matures, it stops functioning as a net auxin sink (i.e., becomes “independent”) and auxin supply is restored to the susceptible zone (at its progressed location). Local auxin reflux ensures auxin-autonomy at the root tip (c.f. [66])