The CLAVATA3/embryo-surrounding region (CLE) peptides control the fine balance between proliferation and differentiation in plant development. We studied the role of CLE peptides during indeterminate nodule development and identified 25 MtCLE peptide genes in the Medicago truncatula genome, of which two genes, MtCLE12 and MtCLE13, had nodulation-related expression patterns that were linked to proliferation and differentiation. MtCLE13 expression was up-regulated early in nodule development. A high-to-low expression gradient radiated from the inner toward the outer cortical cell layers in a region defining the incipient nodule. At later stages, MtCLE12 and MtCLE13 were expressed in differentiating nodules and in the apical part of mature, elongated nodules. Functional analysis revealed a putative role for MtCLE12 and MtCLE13 in autoregulation of nodulation, a mechanism that controls the number of nodules and involves systemic signals mediated by a leucine-rich repeat receptor-like kinase, SUNN, which is active in the shoot. When MtCLE12 and MtCLE13 were ectopically expressed in transgenic roots, nodulation was abolished at the level of the nodulation factor signal transduction, and this inhibition involved long-distance signaling. In addition, composite plants with roots ectopically expressing MtCLE12 or MtCLE13 had elongated petioles. This systemic effect was not observed in transgenic roots ectopically expressing MtCLE12 and MtCLE13 in a sunn-1 mutant background, although nodulation was still strongly reduced. These results suggest multiple roles for CLE signaling in nodulation.

Growth and development are coordinated by an array of intercellular communications. Known plant signaling molecules include phytohormones and hormone peptides. Although both classes can be implicated in the same developmental processes, little is known about the interplay between phytohormone action and peptide signaling within the cellular microenvironment. We show that genes coding for small secretory peptides, designated GOLVEN (GLV), modulate the distribution of the phytohormone auxin. The deregulation of the GLV function impairs the formation of auxin gradients and alters the reorientation of shoots and roots after a gravity stimulus. Specifically, the GLV signal modulates the trafficking dynamics of the auxin efflux carrier PIN-FORMED2 involved in root tropic responses and meristem organization. Our work links the local action of secretory peptides with phytohormone transport.

The Clavata3 (CLV3)/endosperm surrounding region (CLE) signaling peptides are encoded in large plant gene families. CLV3 and the other A-type CLE peptides promote cell differentiation in root and shoot apical meristems, whereas the B-type peptides (CLE41-CLE44) do not. Instead, CLE41 inhibits the differentiation of Zinnia elegans tracheary elements. To test whether CLE genes might code for antagonistic or synergistic functions, peptides from both types were combined through overexpression within or application onto Arabidopsis thaliana seedlings. The CLE41 peptide (CLE41p) promoted proliferation of vascular cells, although delaying differentiation into phloem and xylem cell lineages. Application of CLE41p or overexpression of CLE41 did not suppress the terminal differentiation of the root and shoot apices triggered by A-type CLE peptides. However, in combination, A-type peptides enhanced all of the phenotypes associated with CLE41 gain-of-function, leading to massive proliferation of vascular cells. This proliferation relied on auxin signaling because it was enhanced by exogenous application of a synthetic auxin, decreased by an auxin polar transport inhibitor, and abolished by a mutation in the Monopteros auxin response factor. These findings highlight that vascular patterning is a process controlled in time and space by different CLE peptides in conjunction with hormonal signaling.cambium ͉ hypocotyl ͉ RAM ͉ SAM ͉ TDIF I n higher plants, postembryonic organogenesis is mediated by meristems. These specialized structures provide a reservoir of undifferentiated stem cells as well as a limited population of proliferating cells, often referred to as transit-amplifying (TA) cells that are fated for differentiation (1). To date, molecular research has focused on the Arabidopsis primary meristems of root and shoot apices, but more recent studies have sought to dissect molecular programs underpinning the control of secondary meristems, including the vascular cambium (2, 3) that is a circumferential stem cell niche including the fusiform initials from which secondary xylem and phloem originate (2). During the course of differentiation, fusiform initial daughters take on a TA state to increase the population of xylem and phloem mother cells. Positioning and size of stem cell and TA cell populations, at least in root and shoot meristems, are controlled in a noncell-autonomous manner (1,4,5).This noncell-autonomous control of stem cell size and positioning has best been described in Arabidopsis primary meristems where a stable pool of undifferentiated stem cells are maintained by a feedback loop mechanism involving the Clavata (CLV) signaling pathway and the Wuschel (WUS) homeodomain transcription factor (6, 7). Similar regulatory mechanisms may be at work within root meristems, where the transcription factor WUS-related-homeobox 5 (WOX5) controls stem cell maintenance (8) and CLV3 and related genes can influence root patterning (9-11), although to date the regulation of WOX5 by a CLV-like pathway has not been demonstrated...