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Auxin Perception Is Required for Arbuscule Development in Arbuscular Mycorrhizal Symbiosis

Most land plant species live in symbiosis with arbuscular mycorrhizal fungi. These fungi differentiate essential functional structures called arbuscules in root cortical cells from which mineral nutrients are released to the plant. We investigated the role of microRNA393 (miR393), an miRNA that targets several auxin receptors, in arbuscular mycorrhizal root colonization. Expression of the precursors of the miR393 was down-regulated during mycorrhization in three different plant species: Solanum lycopersicum, Medicago truncatula, and Oryza sativa. Treatment of S. lycopersicum, M. truncatula, and O. sativa roots with concentrations of synthetic auxin analogs that did not affect root development stimulated mycorrhization, particularly arbuscule formation. DR5-GUS, a reporter for auxin response, was preferentially expressed in root cells containing arbuscules. Finally, overexpression of miR393 in root tissues resulted in down-regulation of auxin receptor genes (transport inhibitor response1 and auxin-related F box) and underdeveloped arbuscules in all three plant species. These results support the conclusion that miR393 is a negative regulator of arbuscule formation by hampering auxin perception in arbuscule-containing cells

Lotus japonicus NOOT-BOP-COCH-LIKE1 is essential for nodule, nectary, leaf and flower development

[EN] The NOOT-BOP-COCH-LIKE (NBCL) genes are orthologs of Arabidopsis thaliana BLADE-ON-PETIOLE1/2. The NBCLs are developmental regulators essential for plant shaping, mainly through the regulation of organ boundaries, the promotion of lateral organ differentiation and the acquisition of organ identity. In addition to their roles in leaf, stipule and flower development, NBCLs are required for maintaining the identity of indeterminate nitrogen-fixing nodules with persistent meristems in legumes. In legumes forming determinate nodules, without persistent meristem, the roles of NBCL genes are not known. We thus investigated the role of Lotus japonicus NOOT-BOP-COCH-LIKE1 (LjNBCL1) in determinate nodule identity and studied its functions in aerial organ development using LORE1 insertional mutants and RNA interference-mediated silencing approaches. In Lotus, LjNBCL1 is involved in leaf patterning and participates in the regulation of axillary outgrowth. Wild-type Lotus leaves are composed of five leaflets and possess a pair of nectaries at the leaf axil. Legumes such as pea and Medicago have a pair of stipules, rather than nectaries, at the base of their leaves. In Ljnbcl1, nectary development is abolished, demonstrating that nectaries and stipules share a common evolutionary origin. In addition, ectopic roots arising from nodule vascular meristems and reorganization of the nodule vascular bundle vessels were observed on Ljnbcl1 nodules. This demonstrates that NBCL functions are conserved in both indeterminate and determinate nodules through the maintenance of nodule vascular bundle identity. In contrast to its role in floral patterning described in other plants, LjNBCL1 appears essential for the development of both secondary inflorescence meristem and floral meristem.This work was supported by the CNRS and by the grants ANR-14-CE19-0003 (NOOT) from the Agence National de la Recherche (ANR) to PR. This work has benefited from the facilities and expertise of the Servicio de Microscopia Electronica Universitat Politecnica de Valencia (Spain, http://www.upv.es/entidades/SME/) and of the IMAGIF Cell Biology Unit of the Gif campus (France, www.imagif.cnrs.fr) which is supported by the Conseil General de l'Essonne. The authors thank Dr Mathias Brault from the Institute of Plant Sciences Paris-Saclay (France) for providing the pFRN: RNAi plasmid, A. rhizogenes ARqua1 strain and control GUS:RNAi construction, and Dr Simona Radutoiu from the University of Aarhus (Denmark), for providing the Na-Borate/TRIZOL RNA extraction protocol. We are grateful to Dr Cristina Ferrandiz from the Instituto de Biologia Molecular y Celular de Plantas (Spain) for help in interpreting the identity of the meristems in the SEM pictures and Professor Frederique Guinel from the University of Wilfrid Laurier (Canada) for help in interpreting the identity of L. japonicus nodule vascular tissues. 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<em>NODULE ROOT</em> and <em>COCHLEATA</em> Maintain Nodule Development and Are Legume Orthologs of Arabidopsis <em>BLADE-ON-PETIOLE</em> Genes

During their symbiotic interaction with rhizobia, legume plants develop symbiosis-specific organs on their roots, called nodules, that house nitrogen-fixing bacteria. The molecular mechanisms governing the identity and maintenance of these organs are unknown. Using Medicago truncatula nodule root (noot) mutants and pea (Pisum sativum) cochleata (coch) mutants, which are characterized by the abnormal development of roots from the nodule, we identified the NOOT and COCH genes as being necessary for the robust maintenance of nodule identity throughout the nodule developmental program. NOOT and COCH are Arabidopsis thaliana BLADE-ON-PETIOLE orthologs, and we have shown that their functions in leaf and flower development are conserved in M. truncatula and pea. The identification of these two genes defines a clade in the BTB/POZ-ankyrin domain proteins that shares conserved functions in eudicot organ development and suggests that NOOT and COCH were recruited to repress root identity in the legume symbiotic organ

Exploiting a fast neutron mutant genetic resource in Pisum sativum (pea) for functional genomics

A fast neutron mutagenised population was generated in Pisum sativum L. (pea) to enable the identification and isolation of genes underlying traits and processes. Studies of a number of phenotypic traits have clearly demonstrated the utility of the resource by associating gene deletions with phenotype followed by functional tests exploiting additional mutant sources, from both induced and natural variant germplasm. For forward genetic screens next generation sequencing methodologies provide an opportunity for identifying genes associated with deletions rapidly and systematically. The application of rapid reverse genetic screens of the fast neutron mutant pea population supports conclusions on the frequency of deletions based on phenotype alone. These studies further suggest that large deletions affecting one or more loci can be non-deleterious to the pea genome, yielding mutants that could not be obtained by other means. Deletion mutants affecting genes associated with seed metabolism and storage are providing unique opportunities to identify the products of complex and related gene families, and to study the downstream consequences of such deletion