Olivier Catrice

Turning meiosis into mitosis.

The mutation of as few as three genes in a sexual plant transforms meiosis into mitosis and results in diploid gametes that are genetically identical to the mother plant. This phenotype resembles apomeiosis, which is a major step in apomixis.

Mutations in AtPS1 (Arabidopsis thaliana Parallel Spindle 1) Lead to the Production of Diploid Pollen Grains

Polyploidy has had a considerable impact on the evolution of many eukaryotes, especially angiosperms. Indeed, most—if not all—angiosperms have experienced at least one round of polyploidy during the course of their evolution, and many important crop plants are current polyploids. The occurrence of 2n gametes (diplogametes) in diploid populations is widely recognised as the major source of polyploid formation. However, limited information is available on the genetic control of diplogamete production. Here, we describe the isolation and characterisation of the first gene, AtPS1 (Arabidopsis thaliana Parallel Spindle 1), implicated in the formation of a high frequency of diplogametes in plants. Atps1 mutants produce diploid male spores, diploid pollen grains, and spontaneous triploid plants in the next generation. Female meiosis is not affected in the mutant. We demonstrated that abnormal spindle orientation at male meiosis II leads to diplogamete formation. Most of the parents heterozygosity is therefore conserved in the Atps1 diploid gametes, which is a key issue for plant breeding. The AtPS1 protein is conserved throughout the plant kingdom and carries domains suggestive of a regulatory function. The isolation of a gene involved in diplogamete production opens the way for new strategies in plant breeding programmes and progress in evolutionary studies.

The sunflower genome provides insights into oil metabolism, flowering and Asterid evolution

The domesticated sunflower, Helianthus annuus L., is a global oil crop that has promise for climate change adaptation, because it can maintain stable yields across a wide variety of environmental conditions, including drought. Even greater resilience is achievable through the mining of resistance alleles from compatible wild sunflower relatives, including numerous extremophile species. Here we report a high-quality reference for the sunflower genome (3.6 gigabases), together with extensive transcriptomic data from vegetative and floral organs. The genome mostly consists of highly similar, related sequences and required single-molecule real-time sequencing technologies for successful assembly. Genome analyses enabled the reconstruction of the evolutionary history of the Asterids, further establishing the existence of a whole-genome triplication at the base of the Asterids II clade and a sunflower-specific whole-genome duplication around 29 million years ago. An integrative approach combining quantitative genetics, expression and diversity data permitted development of comprehensive gene networks for two major breeding traits, flowering time and oil metabolism, and revealed new candidate genes in these networks. We found that the genomic architecture of flowering time has been shaped by the most recent whole-genome duplication, which suggests that ancient paralogues can remain in the same regulatory networks for dozens of millions of years. This genome represents a cornerstone for future research programs aiming to exploit genetic diversity to improve biotic and abiotic stress resistance and oil production, while also considering agricultural constraints and human nutritional needs.

The CCAAT box-binding transcription factor NF-YA1 controls rhizobial infection

Symbiosis between legume plants and soil rhizobia culminates in the formation of a novel root organ, the ‘nodule’, containing bacteria differentiated as facultative nitrogen-fixing organelles. MtNF-YA1 is a Medicago truncatula CCAAT box-binding transcription factor (TF), formerly called HAP2-1, highly expressed in mature nodules and required for nodule meristem function and persistence. Here a role for MtNF-YA1 during early nodule development is demonstrated. Detailed expression analysis based on RNA sequencing, quantitiative real-time PCR (qRT-PCR), as well as promoter–β-glucuronidase (GUS) fusions reveal that MtNF-YA1 is first induced at the onset of symbiotic development during preparation for, and initiation and progression of, symbiotic infection. Moreover, using a new knock-out mutant, Mtnf-ya1-1, it is shown that MtNF-YA1 controls infection thread (IT) progression from initial root infection through colonization of nodule tissues. Extensive confocal and electronic microscopic observations suggest that the bulbous and erratic IT growth phenotypes observed in Mtnf-ya1-1 could be a consequence of the fact that walls of ITs in this mutant are thinner and less coherent than in the wild type. It is proposed that MtNF-YA1 controls rhizobial infection progression by regulating the formation and the wall of ITs.

The enigmatic invasive Spartina densiflora: A history of hybridizations in a polyploidy context

The aim of this study was to explore the origin of the invasive Spartina densiflora by analysing samples from the native region (South America) and from a recently colonized area (California). A combination of various molecular data (chloroplast and nuclear sequences, molecular fingerprint) and ploidy level estimations was used to answer the question whether the reticulate phylogenetic pattern previously detected in S. densiflora was restricted to California, or alternatively, whether a more ancient hybrid origin preceded formation of this species in its native area. We found that this species is heptaploid in both its native and introduced range. Identification of nuclear homeologous sequences indicate that this species has a reticulate origin in its native range, involving a lineage related to the hexaploid clade formed by S. alterniflora, S. foliosa, and S. maritima, and another lineage related to the sub‐Antarctic endemic S. arundinacea that provided the chloroplast genome. The samples from California displayed similar multilocus patterns to the samples from Chile, supporting the hypothesis that this species originated on the southeast American coast (Argentina), from where it eventually spread to the west coast of South America (Chile) first and to the Northern Hemisphere (California) later.

Arabidopsis PASTICCINO2 Is an Antiphosphatase Involved in Regulation of Cyclin-Dependent Kinase A

PASTICCINO2 (PAS2), a member of the protein Tyr phosphatase-like family, is conserved among all eukaryotes and is characterized by a mutated catalytic site. The cellular functions of the Tyr phosphatase-like proteins are still unknown, even if they are essential in yeast and mammals. Here, we demonstrate that PAS2 interacts with a cyclin-dependent kinase (CDK) that is phosphorylated on Tyr and not with its unphosphorylated isoform. Phosphorylation of the conserved regulatory Tyr-15 is involved in the binding of CDK to PAS2. Loss of the PAS2 function dephosphorylated Arabidopsis thaliana CDKA;1 and upregulated its kinase activity. In accordance with its role as a negative regulator of the cell cycle, overexpression of PAS2 slowed down cell division in suspension cell cultures at the G2-to-M transition and early mitosis and inhibited Arabidopsis seedling growth. The latter was accompanied by altered leaf development and accelerated cotyledon senescence. PAS2 was localized in the cytoplasm of dividing cells but moved into the nucleus upon cell differentiation, suggesting that the balance between cell division and differentiation is regulated through the interaction between CDKA;1 and the antiphosphatase PAS2.

A Laser Dissection-RNAseq Analysis Highlights the Activation of Cytokinin Pathways by Nod Factors in the Medicago truncatula Root Epidermis.

Nod factors induce massive reprogramming of gene expression in the root epidermis, including the CRE1 cytokinin pathway which leads to both positive and negative regulation of nodulation. Nod factors (NFs) are lipochitooligosaccharidic signal molecules produced by rhizobia, which play a key role in the rhizobium-legume symbiotic interaction. In this study, we analyzed the gene expression reprogramming induced by purified NF (4 and 24 h of treatment) in the root epidermis of the model legume Medicago truncatula. Tissue-specific transcriptome analysis was achieved by laser-capture microdissection coupled to high-depth RNA sequencing. The expression of 17,191 genes was detected in the epidermis, among which 1,070 were found to be regulated by NF addition, including previously characterized NF-induced marker genes. Many genes exhibited strong levels of transcriptional activation, sometimes only transiently at 4 h, indicating highly dynamic regulation. Expression reprogramming affected a variety of cellular processes, including perception, signaling, regulation of gene expression, as well as cell wall, cytoskeleton, transport, metabolism, and defense, with numerous NF-induced genes never identified before. Strikingly, early epidermal activation of cytokinin (CK) pathways was indicated, based on the induction of CK metabolic and signaling genes, including the CRE1 receptor essential to promote nodulation. These transcriptional activations were independently validated using promoter:β-glucuronidase fusions with the MtCRE1 CK receptor gene and a CK response reporter (TWO COMPONENT SIGNALING SENSOR NEW). A CK pretreatment reduced the NF induction of the EARLY NODULIN11 (ENOD11) symbiotic marker, while a CK-degrading enzyme (CYTOKININ OXIDASE/DEHYDROGENASE3) ectopically expressed in the root epidermis led to increased NF induction of ENOD11 and nodulation. Therefore, CK may play both positive and negative roles in M. truncatula nodulation.

Cupriavidus taiwanensis Bacteroids in Mimosa pudica Indeterminate Nodules Are Not Terminally Differentiated

ABSTRACT The beta-rhizobium Cupriavidus taiwanensis forms indeterminate nodules on Mimosa pudica. C. taiwanensis bacteroids resemble free-living bacteria in terms of genomic DNA content, cell size, membrane permeability, and viability, in contrast to bacteroids in indeterminate nodules of the galegoid clade. Bacteroid differentiation is thus unrelated to nodule ontogeny.

Morphology and anatomy of root nodules of Retama monosperma (L.)Boiss.

Background and aimsIn spite of the importance of Retama species for dune stabilization and re-vegetation and the contribution to the bio-fertilization of semi-arid and arid ecosystems, the symbiotic interaction of Retama species with rhizobia remains largely unstudied. In this paper, we aim to provide the first detailed study on nodule morphology and anatomy of Retama monosperma.MethodsWe collected nodules from coastal areas nearby Oran (Algeria) and studied in detail their anatomy and ultrastructure by light and electron microscopy.ResultsFirst, we confirmed the likely identity of the microsymbiont as B. retamae and found that nodules of R. monosperma belong to the genistoid type of indeterminate nodules. Infection threads, typical for most nodules of legumes, are absent in nodules of R. monosperma and bacterial spread is associated with plant cell division. The nitrogen fixation zone is homogenous with only invaded cells and a network of non-invaded cells found in many nodules, is absent. Moreover, endoreduplication does not take place in bacteroids in nodules of R. monosperma.ConclusionsThe features observed in this study are compared to the morphology and anatomy of nodules of other legumes and the possible consequences for nodule functioning and the mode of infection during the establishment of the interaction are discussed.

Unraveling the early molecular and physiological mechanisms involved in response to phenanthrene exposure

BackgroundHigher plants have to cope with increasing concentrations of pollutants of both natural and anthropogenic origin. Given their capacity to concentrate and metabolize various compounds including pollutants, plants can be used to treat environmental problems - a process called phytoremediation. However, the molecular mechanisms underlying the stabilization, the extraction, the accumulation and partial or complete degradation of pollutants by plants remain poorly understood.ResultsHere, we determined the molecular events involved in the early plant response to phenanthrene, used as a model of polycyclic aromatic hydrocarbons. A transcriptomic and a metabolic analysis strongly suggest that energy availability is the crucial limiting factor leading to high and rapid transcriptional reprogramming that can ultimately lead to death. We show that the accumulation of phenanthrene in leaves inhibits electron transfer and photosynthesis within a few minutes, probably disrupting energy transformation.ConclusionThis kinetic analysis improved the resolution of the transcriptome in the initial plant response to phenanthrene, identifying genes that are involved in primary processes set up to sense and detoxify this pollutant but also in molecular mechanisms used by the plant to cope with such harmful stress. The identification of first events involved in plant response to phenanthrene is a key step in the selection of candidates for further functional characterization, with the prospect of engineering efficient ecological detoxification systems for polycyclic aromatic hydrocarbons.