Root developmental programs shape the Medicago truncatula nodule meristem

Nodules on the roots of legume plants host nitrogen-fixing Rhizobium bacteria. Several lines of evidence indicate that nodules are evolutionarily related to roots. We determined whether developmental control of the Medicago truncatula nodule meristem bears resemblance to that in root meristems through analyses of root meristem-expressed PLETHORA genes. In nodules, MtPLETHORA 1 and 2 are preferentially expressed in cells positioned at the periphery of the meristem abutting nodule vascular bundles. Their expression overlaps with an auxin response maximum and MtWOX5, which is a marker for the root quiescent center. Strikingly, the cells in the central part of the nodule meristem have a high level of cytokinin and display MtPLETHORA 3 and 4 gene expression. Nodule-specific knockdown of MtPLETHORA genes results in a reduced number of nodules and/or in nodules in which meristem activity has ceased. Our nodule gene expression map indicates that the nodule meristem is composed of two distinct domains in which different MtPLETHORA gene subsets are expressed. Our mutant studies show that MtPLETHORA genes function redundantly in nodule meristem maintenance. This indicates that Rhizobium has recruited root developmental programs for nodule formation.