Fate map of Medicago truncatula root nodules

Abstract

Legume root nodules are induced by N-fixing rhizobium bacteria that are hosted in an intracellular manner. These nodules are formed by reprogramming differentiated root cells. The model legume Medicago truncatula forms indeterminate nodules with a meristem at their apex. This organ grows by the activity of the meristem that adds cells to the different nodule tissues. In Medicago sativa it has been shown that the nodule meristem is derived from the root middle cortex. During nodule initiation, inner cortical cells and pericycle cells are also mitotically activated. However, whether and how these cells contribute to the mature nodule has not been studied. Here, we produce a nodule fate map that precisely describes the origin of the different nodule tissues based on sequential longitudinal sections and on the use of marker genes that allow the distinction of cells originating from different root tissues. We show that nodule meristem originates from the third cortical layer, while several cell layers of the base of the nodule are directly formed from cells of the inner cortical layers, root endodermis and pericycle. The latter two differentiate into the uninfected tissues that are located at the base of the mature nodule, whereas the cells derived from the inner cortical cell layers form about eight cell layers of infected cells. This nodule fate map has then been used to re-analyse several mutant nodule phenotypes. This showed, among other things, that intracellular release of rhizobia in primordium cells and meristem daughter cells are regulated in a different manner.

Different Pathways Act Downstream of the CEP Peptide Receptor CRA2 to Regulate Lateral Root and Nodule Development

“…9) is that the peptide dampens MtEIN2/SKL-dependent ethylene signaling, therefore attenuating its negative effect on rhizobial infections. Consistent with such MtEIN2/SKL-related action, MtCEP1 peptide also increases the number of infection events, as well as fused nodule formation and nodule number, which are phenotypes that resemble those of skl (Penmetsa and Cook, 1997;Penmetsa et al, 2003Penmetsa et al, , 2008Xiao et al, 2014). Additionally, similarly to skl phenotypes, MtCEP1 peptides increased the root competency zone for nodulation, the frequency of nodules forming in the proximity of phloem poles, and the number of nodules that spanned two xylem poles and the phloem pole in between.…”

“…At early developmental stages LRs and nodules share developmental genes Larrainzar et al, 2015) and morphological features, such as reactivation of cortical, pericycle, and endodermal cell divisions Herrbach et al, 2014;Xiao et al, 2014;Mohd-Radzman et al, 2015), and it is therefore possible that overlapping genetic mechanisms may be involved. In addition, these two developmental pathways are initiated in cells in close proximity to the RAM Herrbach et al, 2014;Xiao et al, 2014). It is not known if MtCEP1 peptides affect LR and nodule development through the same or different pathways, or if common developmental components are involved.…”

“…S. meliloti induces two, parallel, nodule-specific processes to form indeterminate nodules adjacent to root proto-xylem poles: (1) the rhizobial infection pathway, which involves infection thread formation in root hairs and cortical cells; and (2) the nodule organogenesis pathway, which involves the activation of cell divisions in root cortical, endodermal, and pericycle cell layers to form a nodule primordium and, subsequently, a nodule meristem (Timmers et al, 1999;Desbrosses and Stougaard, 2011;Oldroyd et al, 2011Oldroyd et al, , 2013Xiao et al, 2014;Djordjevic et al, 2015). Rhizobial NFs rapidly trigger nuclear calcium oscillations in root hair cells (Lévy et al, 2004;Miwa et al, 2006), which transcriptionally activate key symbiotic (SYM) genes (e.g.…”

“…Rhizobia hyperinfect skl and form numerous underdeveloped nodules that are often fused (Penmetsa and Cook, 1997). Fused nodules have been defined as having two or more nodule meristems coalescing into one nodule structure confined by a single layer of endodermis (Xiao et al, 2014). In addition, skl nodules show spatially aberrant initiation sites in roots and can develop proximal to proto-xylem and -phloem poles (Penmetsa et al, 2003), whereas in wild-type roots, a localized ethylene production adjacent to proto-phloem poles is thought to prevent nodules developing in this area (Heidstra et al, 1997).…”

“…In addition, skl nodules show spatially aberrant initiation sites in roots and can develop proximal to proto-xylem and -phloem poles (Penmetsa et al, 2003), whereas in wild-type roots, a localized ethylene production adjacent to proto-phloem poles is thought to prevent nodules developing in this area (Heidstra et al, 1997). Consistently, ethylene synthesis inhibitors such as aminoethoxyvinyl-Gly (AVG) promote both proto-phloem pole-and fused-nodule formation (Heidstra et al, 1997), as well as an increase in infection thread and nodule numbers (Guinel and Larue, 1992;Xiao et al, 2014). Conversely, the ethylene precursor, 1-aminocyclopropane-1-carboxylic-acid (ACC), reduces nodule number and, at high concentration, inhibits calcium oscillations and abolishes nodulation (Penmetsa and Cook, 1997;Oldroyd et al, 2001).…”