From passive to informed:Mechanical mechanisms of seed dispersal

Plant dispersal mechanisms rely on anatomical and morphological adaptations for the use of physical or biological dispersal vectors. Recently, studies of interactions between the dispersal unit and physical environment have uncovered fluid dynamic mechanisms of seed flight, protective measures against fire, and release mechanisms of explosive dispersers. Although environmental conditions generally dictate dispersal distances, plants are not purely passive players in these processes. Evidence suggests that some plants may enact informed dispersal, where dispersal‐related traits are modified according to the environment. This can occur via developmental regulation, but also on shorter timescales via structural remodelling in relation to water availability and temperature. Linking interactions between dispersal mechanisms and environmental conditions will be essential to fully understand population dynamics and distributions

A complex interplay of tandem- and whole genome duplication drives expansion of the L-type lectin receptor kinase gene family in the brassicaceae

The comparative analysis of plant gene families in a phylogenetic framework has greatly accelerated due to advances in next generation sequencing. In this study, we provide an evolutionary analysis of the L-type lectin receptor kinase and L-type lectin domain proteins (L-type LecRKs and LLPs) that are considered as components in plant immunity, in the plant family Brassicaceae and related outgroups. We combine several lines of evidence provided by sequence homology, HMM-driven protein domain annotation, phylogenetic analysis and gene synteny for large-scale identification of L-type LecRK and LLP genes within nine core-eudicot genomes. We show that both polyploidy and local duplication events (tandem duplication and gene transposition duplication) have played a major role in L-type LecRK and LLP gene family expansion in the Brassicaceae. We also find significant differences in rates of molecular evolution based on the mode of duplication. Additionally, we show that LLPs share a common evolutionary origin with L-type LecRKs and provide a consistent gene family nomenclature. Finally, we demonstrate that the largest and most diverse L-type LecRK clades are lineage-specific. Our evolutionary analyses of these plant immune components provide a framework to support future plant resistance breeding

Seed coat development in explosively dispersed seeds of Cardamine hirsuta

BACKGROUND AND AIMS: Seeds are dispersed by explosive coiling of the fruit valves in Cardamine hirsuta. This rapid coiling launches the small seeds on ballistic trajectories to spread over a 2 m radius around the parent plant. The seed surface interacts with both the coiling fruit valve during launch and subsequently with the air during flight. We aim to identify features of the seed surface that may contribute to these interactions by characterizing seed coat differentiation. METHODS: Differentiation of the outermost seed coat layers from the outer integuments of the ovule involves dramatic cellular changes that we characterize in detail at the light and electron microscopical level including immunofluorescence and immunogold labelling. KEY RESULTS: We found that the two outer integument (oi) layers of the seed coat contributed differently to the topography of the seed surface in the explosively dispersed seeds of C. hirsuta vs. the related species Arabidopsis thaliana where seed dispersal is non-explosive. The surface of A. thaliana seeds is shaped by the columella and the anticlinal cell walls of the epidermal oi2 layer. In contrast, the surface of C. hirsuta seeds is shaped by a network of prominent ridges formed by the anticlinal walls of asymmetrically thickened cells of the sub-epidermal oi1 layer, especially at the seed margin. Both the oi2 and oi1 cell layers in C. hirsuta seeds are characterized by specialized, pectin-rich cell walls that are deposited asymmetrically in the cell. CONCLUSIONS: The two outermost seed coat layers in C. hirsuta have distinct properties: the sub-epidermal oi1 layer determines the topography of the seed surface, while the epidermal oi2 layer accumulates mucilage. These properties are influenced by polar deposition of distinct pectin polysaccharides in the cell wall. Although the ridged seed surface formed by oi1 cell walls is associated with ballistic dispersal in C. hirsuta, it is not restricted to explosively dispersed seeds in the Brassicaceae

Variabilité naturelle des mucilages des graines de Brassicaceae : étude comparative inter et intra espèces

La myxospermie et la myxocarpie (regroupées sous le terme myxodiasporie) désignent respectivement la capacité d'une graine ou d'un fruit à extruder du mucilage polysaccharidique lors de l'imbibition. Les espèces myxodiasporiques sont largement répandues parmi les plantes à fleurs, mais sont souvent étroitement apparentées aux espèces non myxodiasporiques. Cette répartition rend mystérieuse l'histoire évolutive de la myxodiasporie : Est-ce un trait ancestral qui s'est perdu plusieurs fois chez les espèces non myxodiasporiques? Ou est-ce une fonctionnalité qui est apparue plusieurs fois indépendamment chez les espèces myxodiasporiques? De plus, les rôles écologiques de la myxodiasporie sont très divers et semblent dépendre de l'espèce. La génétique contrôlant cette caractéristique est encore inégalement connue, en dehors de l'espèce modèle Arabidopsis thaliana. En effet, cette plante modèle est une Brassicaceae myxospermique qui a été bien caractérisée pour la morphologie, le développement, la composition et la génétique de son mucilage de graine et de ses cellules sécrétrices de mucilage (CSM). Cette thèse met en lumière l'évolution, la génétique et l'écologie de la myxospermie dans la famille des Brassicaceae au niveau inter- et intra-espèce. Au niveau inter-espèces, les graines et leurs cellules épidermiques d'une trentaine d'espèces de Brassicaceae ont été comparées à celles d'A. thaliana pour étudier l'évolution de la myxospermie dans cette famille. Chaque espèce de Brassicaceae a été soigneusement phénotypée à l'échelle macroscopique (graine entière) et microscopique (CSM) avant et après imbibition. De plus, les orthologues de chaque gène d'A. thaliana connus pour être impliqués dans la myxospermie ont été recherchés dans les génomes disponibles des espèces étudiées. La disponibilité de données transcriptomiques pour quelques espèces ont permis d'approfondir l'étude de ces gènes orthologues. Dans la famille des Brassicaceae, une grande diversité de structures de mucilages a été observée à des niveaux macro et même microscopiques. D'après les données morphologiques, il semble y avoir une origine ancestrale du mucilage chez les Brassicaceae, mais elle est très différente de celle d'A. thaliana. Malgré les morphologies contrastées observées, les acteurs génétiques candidats n'étaient pas corrélés avec le phénotype. Une grande majorité des gènes orthologues de ceux impliqués dans la myxospermie d'A. thaliana étaient présents et partiellement exprimés même pour les espèces non myxospermiques. Au niveau intra-espèce, 166 populations naturelles d'A. thaliana ont été analysées pour leur taille de mucilage adhérent et non adhérent. Ces populations sont originaires d'une région du sud de la France et possèdent des données génétiques (fréquences des polymorphismes mono-nucléotidiques (SNP)) et environnementales associées. L'association de leur phénotype de mucilage aux fréquences des SNP a permis de mettre en évidence de nouveaux gènes candidats prometteurs impliqués dans les deux couches du mucilage d'A. thaliana. De plus, des corrélations intéressantes avec les données environnementales orientent vers de nouveaux rôles écologiques putatifs à découvrir pour le mucilage d'A. thaliana.The myxospermy and myxocarpy (grouped under the term myxodiaspory) designates the ability of a seed or a fruit to extrude mucilage upon imbibition, respectively. Myxodiasporous species are widely spread among flowering plants but are often closely related to non-myxodiasporous species. This repartition makes mysterious the evolutionary scenario of myxosdiaspory: Is it an ancestral trait which was lost several times in non-myxodiasporous species? Or is it a feature that appeared several times independently in myxodiasporous species? Additionally, the ecological roles of myxodiaspory are very diverse and seem to be species-dependent. Also, the genetics controlling this feature is still inequally known except for the model plant Arabidopsis thaliana. Indeed, this myxospermous Brassicaceae species has been well characterized for the morphology, development, composition, and genetics of its seed mucilage and mucilage secretory cells (MSCs). This thesis sheds light on the evolution, genetics, and ecology of myxospermy in the Brassicaceae family at both the inter- and intra- species level. At the inter-species level, the seeds and their epidermal cells from about thirty Brassicaceae species were compared to those of A. thaliana to investigate the myxospermy evolution in this family. Every Brassicaceae species was thoroughly phenotyped at the macroscopic (whole seed) and microscopic (MSCs) scale before and after imbibition. Additionally, ortholog from every A. thaliana gene known to be implicated in myxospermy was tracked in the available genomes of the studied species. The availability of transcriptomic data for few species allowed further investigation of these orthologous genes. Among the Brassicaceae family, a wide diversity of mucilage structures was observed at the macro- and microscopic level. Based on morphological data, the origin of myxospermy in Brassicaceae appears ancestral, but the trait is very different from that of A. thaliana. Despite the observed contrasted morphologies, the putative genetic actors were not correlated with the phenotype. A large majority of orthologous genes to those related to myxospermy in A. thaliana were present and partially expressed even in non-myxospermous species. At the intra-species level, 166 natural populations of A. thaliana were analysed for their adherent and non-adherent mucilage size. These populations come from a South France region and have available associated genetic (single nucleotide polymorphisms (SNPs) frequencies) and environmental data. The association of their mucilage phenotype with the SNPs frequencies highlights promising new candidate genes putatively implicated in both mucilage layers of A. thaliana. In addition, interesting correlations with environmental data point to new putative ecological roles to be discovered for the mucilage of A. thaliana