Fabrice Dessaint

A combined histology and transcriptome analysis unravels novel questions on Medicago truncatula seed coat

The seed coat is involved in the determination of seed quality traits such as seed size, seed composition, seed permeability, and hormonal regulation. Understanding seed coat structure is therefore a prerequisite to deciphering the genetic mechanisms that govern seed coat functions. By combining histological and transcriptomic data analyses, cellular and molecular events occurring during Medicago truncatula seed coat development were dissected in order to relate structure to function and pinpoint target genes potentially involved in seed coat traits controlling final seed quality traits. The analyses revealed the complexity of the seed coat transcriptome, which contains >30 000 genes. In parallel, a set of genes showing a preferential expression in seed coat that may be involved in more specific functions was identified. The study describes how seed coat anatomy and morphological changes affect final seed quality such as seed size, seed composition, seed permeability, and hormonal regulation. Putative regulator genes of different processes have been identified as potential candidates for further functional genomic studies to improve agronomical seed traits. The study also raises new questions concerning the implication of seed coat endopolyploidy in cell expansion and the participation of the seed coat in de novo abscisic acid biosynthesis at early seed filling.

Biogenesis of protein bodies during vicilin accumulation in Medicago truncatula immature seeds

BackgroundGrain legumes play a worldwide role as a source of plant proteins for feed and food. In the model legume Medicago truncatula, the organisation of protein storage vacuoles (PSV) in maturing seeds remains unknown.FindingsThe sub-cellular events accompanying the accumulation of vicilin (globulin7S) were analysed during seed mid-maturation. Immuno-detection of vicilin in light microscopy, allowed a semi-quantitative assessment of the protein body complement. The identified populations of vicilin-containing protein bodies are distinguished by their number and size which allowed to propose a model of their biogenesis. Two distributions were detected, enabling a separation of their processing at early and mid maturation stages. The largest protein bodies, at 16 and 20 days after pollination (DAP), were formed by the fusion of small bodies. They have probably attained their final size and correspond to mature vicilin aggregations. Electron microscopic observations revealed the association of the dense protein bodies with rough endoplasmic reticulum. The presence of a ribosome layer surrounding protein bodies, would support an endoplasmic reticulum–vacuole trafficking pathway.ConclusionsThe stastistic analysis may be useful for screening mutations of candidate genes governing protein content. The definitive evidence for an ER-storage vacuole pathway corresponds to a challenge, for the storage of post-translationally unstable proteins. It was proposed for the accumulation of one class of storage protein, the vicilins. This alternative pathway is a matter of controversy in dicotyledonous seeds.

L’ambroisie : Chronique de l’extension d’un « polluant biologique » en France

L’ambroisie à feuilles d’armoise (Ambrosia artemisiifolia L.) est une astéracée originaire du continent américain. Citée parmi les espèces envahissantes [1], son extension en France semble récente, mais les problèmes de santé publique posés par son expansion géographique sont de plus en plus inquiétants [2]. Naturalisée en France au milieu du XIXe siècle, elle est restée relativement peu connue des allergologues jusqu’au milieu des années 1960. En 1966, R. Touraine et al. [3] détaillent son rôle dans un certain nombre de pollinoses de la région lyonnaise et signalent les risques pour la santé publique de son extension géographique. En effet, le pollen de l’ambroisie est à l’origine de très fortes allergies (pollinose, rhinite saisonnière) dont les symptômes sont, par ordre décroissant de fréquence, des rhinites, des conjonctivites, de l’asthme et, plus rarement, de l’eczéma et de l’urticaire [4]. Localisée principalement dans la région Rhône-Alpes, elle est considérée comme un véritable « polluant biologique » et fait l’objet d’arrêtés d’éradication dans plusieurs départements (Rhône, Drôme, Vaucluse, Ardèche1...). Des campagnes d’arrachage sont même organisées depuis plusieurs années par le Conseil Régional Rhône-Alpes pour limiter les émissions de pollen. Biologie et écologie de la plante

Local and regional changes in taxonomic and functional diversity of arable weed communities in Burgundy (France) between the 1970s and the 2000s

Abstract Human activities lead to a process of homogenization of biotas in which specialist species are increasingly replaced by common and widespread species. Using a 30-year diachronic record of arable weed communities, we tested this hypothesis by quantifying changes in α- and β-diversity, using both taxonomic and functional diversity and by partitioning β-diversity into species replacement and richness differences. Arable weed communities were sampled in the same 158 fields of the Côte-d’Or region (northeastern France) between the 1970s and the 2000s. For each period, each field was characterized by crop types, soil characteristics and a High Nature Value (HNV) farmland index based on agricultural intensification at the landscape level. At the field scale, we observed a loss of 46% and 38% in α-taxonomic and functional diversity, respectively, which was in accordance with the decrease in the HNV farmland index over the same period. At the regional scale, there was an increase of 15% and 21% in β-taxonomic and functional diversity (across fields), respectively. Crop type and soil characteristics explained similar levels of variation in species replacement, and crop type explained much larger richness differences in the 2000s suggesting that crop and associated practices may exert a high filtering effect. Our results also highlighted a marked decline of common weeds; a process that is far from being counterbalanced by the few colonizing weeds. Rather than to biotic homogenization, this pattern of loss has led to a higher differentiation of arable weed communities. This could correspond to a fragmentation of suitable habitats for species that depend on weeds. This pattern was associated with a decrease of species richness per field; the loss of common species and their associated functions may be of greater significance for agroecosystem functioning.

Is induction ability of seed germination of Phelipanche ramosa phylogenetically structured among hosts? A case study on Fabaceae species

Phelipancheramosa is a major root-holoparasitic damaging weed characterized by a broad host range, including numerous Fabaceae species. In France, the agricultural threat posed by P. ramosa has increased over two decades due to the appearance of a genetically differentiated pathovar presenting a clear host specificity for oilseed rape. The new pathovar has led to a massive expansion of P. ramosa in oilseed rape fields. The germination rate of P. ramosa seeds is currently known to vary among P. ramosa pathovars and host species. However, only a few studies have investigated whether phylogenetic relatedness among potential host species is a predictor of the ability of these species to induce the seed germination of parasitic weeds by testing for phylogenetic signal. We focused on a set of 12 Fabaceae species and we assessed the rate of induction of seed germination by these species for two pathovars based on in vitro co-cultivation experiments. All Fabaceae species tested induced the germination of P. ramosa seeds. The germination rate of P. ramosa seeds varied between Fabaceae species and tribes studied, while pathovars appeared non-influential. Considering oilseed rape as a reference species, we also highlighted a significant phylogenetic signal. Phylogenetically related species therefore showed more similar rates of induction of seed germination than species drawn at random from a phylogenetic tree. In in vitro conditions, only Lotus corniculatus induced a significantly higher germination rate than oilseed rape, and could potentially be used as a catch crop after confirmation of these results under field conditions.