Root cortex development is fine-tuned by the interplay of MIGs, SCL3 and DELLAs during arbuscular mycorrhizal symbiosis.

Root development is a crucial process that determines the ability of plants to acquire nutrients, adapt to the substrate and withstand changing environmental conditions. Root plasticity is controlled by a plethora of transcriptional regulators that allow, in contrast to tissue development in animals, post-embryonic changes that give rise to new tissue and specialized cells. One of these changes is the accommodation in the cortex of hyperbranched hyphae of symbiotic arbuscular mycorrhizal (AM) fungi, called arbuscules. Arbuscule-containing cells undergo massive reprogramming to coordinate developmental changes with transport processes. Here we describe a novel negative regulator of arbuscule development, MIG3. MIG3 induces and interacts with SCL3, both of which modulate the activity of the central regulator DELLA, restraining cortical cell growth. As in a tug-of-war, MIG3-SCL3 antagonizes the function of the complex MIG1-DELLA, which promotes the cell expansion required for arbuscule development, adjusting cell size during the dynamic processes of the arbuscule life cycle. Our results in the legume plant Medicago truncatula advance the knowledge of root development in dicot plants, showing the existence of additional regulatory elements not present in Arabidopsis that fine-tune the activity of conserved central modules.

Regulation of distinct branches of the non-canonical Wnt-signaling network in Xenopus dorsal marginal zone explants.

BACKGROUND: A tight regulation of the Wnt-signaling network, activated by 19 Wnt molecules and numerous receptors and co-receptors, is required for the establishment of a complex organism. Different branches of this Wnt-signaling network, including the canonical Wnt/ß-catenin and the non-canonical Wnt/PCP, Wnt/Ror2 and Wnt/Ca(2+) pathways, are assigned to distinct developmental processes and are triggered by certain ligand/receptor complexes. The Wnt-signaling molecules are closely related and it is still on debate whether the information for activating a specific branch is encoded by specific sequence motifs within a particular Wnt protein. The model organism Xenopus offers tools to distinguish between Wnt-signaling molecules activating distinct branches of the network. RESULTS: We created chimeric Wnt8a/Wnt11 molecules and could demonstrate that the C-terminal part (containing the BS2) of Wnt8a is responsible for secondary axis formation. Chimeric Wnt11/Wnt5a molecules revealed that the N-terminus with the elements PS3-1 and PS3-2 defines Wnt11 specificity, while elements PS3-1, PS3-2 and PS3-3 are required for Wnt5a specificity. Furthermore, we used Xenopus dorsal marginal zone explants to identify non-canonical Wnt target genes regulated by the Wnt5a branch and the Wnt11 branch. We found that pbk was specifically regulated by Wnt5a and rab11fip5 by Wnt11. Overexpression of these target genes phenocopied the overexpression of their regulators, confirming the distinct roles of Wnt11 and Wnt5a triggered signaling pathways. Furthermore, knock-down of pbk was able to restore convergent extension movements in Wnt5a morphants. CONCLUSIONS: The N-terminal part of non-canonical Wnt proteins decides whether the Wnt5a or the Wnt11 branch of the Wnt-signaling network gets activated. The different non-canonical Wnt branches not only regulate cellular behavior, but, surprisingly, also regulate the expression of different target genes. One of these target genes, pbk, seems to be the relevant target gene executing Wnt5a-mediated regulation of convergent extension movements.