KATANIN-dependent mechanical properties of the stigmatic cell wall mediate the pollen tube path in Arabidopsis.

Successful fertilization in angiosperms depends on the proper trajectory of pollen tubes through the pistil tissues to reach the ovules. Pollen tubes first grow within the cell wall of the papilla cells, applying pressure to the cell. Mechanical forces are known to play a major role in plant cell shape by controlling the orientation of cortical microtubules (CMTs), which in turn mediate deposition of cellulose microfibrils (CMFs). Here, by combining imaging, genetic and chemical approaches, we show that isotropic reorientation of CMTs and CMFs in aged Col-0 and katanin1-5 (ktn1-5) papilla cells is accompanied by a tendency of pollen tubes to coil around the papillae. We show that this coiled phenotype is associated with specific mechanical properties of the cell walls that provide less resistance to pollen tube growth. Our results reveal an unexpected role for KTN1 in pollen tube guidance on the stigma by ensuring mechanical anisotropy of the papilla cell wall

Dissection des premières étapes de la reproduction chez Arabidopsis par imagerie cellulaire en temps réel

La spécificité cellulaire, la croissance et le développement des organismes pluricellulaires dépendent d’une communication qui s’établit soit avec le milieu extérieur soit entre les différents tissus qui les constituent. Les informations issues de signaux extracellulaires sont perçues par la cellule et traduits par des réactions intracellulaires. La reproduction sexuée chez les plantes est un exemple de communication cellulaire. Une communication s’établit entre les partenaires mâles et femelles dès le dépôt du grain de pollen (contenant les gamètes mâles) à l’extrémité de l’organe femelle appelé stigmate et composé de nombreuses cellules épidermiques, les papilles stigmatiques. Chez certaines espèces, les organes mâles et femelles sont proches, enfermés dans la fleur, ce qui favorise le dépôt du pollen sur son propre stigmate (autopollinisation). Afin d’empêcher la consanguinité et favoriser le brassage génétique, certaines espèces sont capables de rejeter leur propre pollen, on parle alors de réponse auto-incompatible et d’espèces auto-incompatibles. Chez les Brassicacées, ce rejet dépend d’une réaction récepteur-ligand qui intervient dès le dépôt du pollen sur les papilles stigmatiques. L’autopollen est bloqué à la surface du stigmate alors qu’un pollen génétiquement différent de la femelle sera accepté et produira une descendance. Arabidopsis thaliana, appartenant à la famille des Brassicacées, a perdu au cours de l’évolution cette capacité à rejeter son propre pollen. Il est toutefois possible de réactiver cette réponse en ré-introduisant par transgenèse, un couple récepteur-ligand fonctionnel provenant de l’espèce proche Arabidopsis lyrata, auto-incompatible. L’objectif de ce travail est de disséquer les premières étapes de reconnaissance entre les cellules stigmatiques et les grains de pollen compatibles et incompatibles. Nous avons mis en place un système semi in vivo permettant de suivre, par microscopie confocale, le devenir des grains de pollen à la surface stigmatique, ainsi que de visualiser les mouvements cellulaires ou les modifications structurales qui interviennent dans la cellule femelle lors de l’interaction. Nous avons montré que le blocage du pollen incompatible intervient à plusieurs niveaux, précocement lors de l’activation du grain de pollen ou plus tardivement lors de la pénétration de la surface stigmatique. Nous avons également visualisé des mouvements du cytosquelette d’actine dans la papille stigmatique au niveau du point de contact avec le pollen compatible résultant de la communication cellulaire générée par la rencontre des deux partenaires

Pesticides et environnement: comportement des pesticides dans le sol et risque de contamination

*Article co-redige avec la redaction de Pour la ScienceNational audienc

The AHP6 cytokinin signaling inhibitor mediates an auxin-cytokinin crosstalk that regulates the timing of organ initiation at the shoot apical meristem

Phyllotaxis, the spatio-temporal pattern of organogenesis at the shoot apical meristem, emerges in large part from inhibitory fields consisting in auxin-depleted areas centered on organs. We recently demonstrated the existence of an additional hormone-based inhibitory field generated by Arabidopsis Histidine Phosphotransfer Protein 6 (AHP6), an inhibitor of cytokinin signaling. We have shown that the spatio-temporal distribution of AHP6 in the meristem is essential for optimizing the rhythmicity of organ initiation. Here, we further analyzed AHP6 expression using fluorescent whole mount mRNA in situ hybridization and demonstrate a precise control of AHP6 level and expression domain over time. While we previously showed a regulation of AHP6 directly downstream of auxin, we show here that AHP6 transcription is unlikely influenced by cytokinin distribution in the meristem. Finally, we provide evidence that cytokinins and auxin might act synergistically during organ initiation, providing a plausible explanation for how AHP6 regulates phyllotaxis

The HD-ZIP IV transcription factor OCL4 is necessary for trichome patterning and anther development in maize

International audienceAmong the genes controlling the differentiation and maintenance of epidermal cell fate are members of the HD-ZIP IV class family of plant-specific transcription factors, most of which are specifically expressed in the epidermis of tissues. Here, we report the functional analysis of the maize HD-ZIP IV gene OCL4 (outer cell layer 4) via the phenotypic analysis of two insertional mutants, and of OCL4-RNAi transgenic plants. In all three materials, the macrohairs, one of the three types of trichomes present on adult maize leaf blades, developed ectopically at the margin of juvenile and adult leaves. Consistent with this phenotype, OCL4 is expressed in the epidermis of the leaf blade, with a maximum at the margin of young leaf primordia. Expression of OCL4 in the model plant Arabidopsis under the control of the GLABRA2 (GL2) promoter, a member of the Arabidopsis HD-ZIP IV family involved in trichome differentiation, did not complement the gl2-1 mutant, but instead aggravated its phenotype. The construct also caused a glabrous appearance of rosette leaves in transformed control plants of the Ler ecotype, suggesting that OCL4 inhibits trichome development both in maize and Arabidopsis. Furthermore, insertional mutants showed a partial male sterility that is likely to result from the presence of an extra subepidermal cell layer with endothecium characteristics in the anther wall. Interestingly, the epidermis-specific OCL4 expression in immature anthers was restricted to the region of the anther locule where the extra cell layer differentiated. Taken together these results suggest that OCL4 inhibits trichome development and influences division and/or differentiation of the anther cell wall

The retromer protein VPS29 links cell polarity and organ initiation in plants.

SummaryA key feature of plants (as opposed to animals) is their ability to establish new organs not only during embryogenesis, but also throughout their development. A master regulator of organ initiation in plants is the phytohormone auxin. Auxin acts locally as a morphogen and is directionally transported from cell to cell by polarized auxin efflux carriers, termed PIN-FORMED (PIN) proteins. Here we report that the Arabidopsis ortholog of the yeast and mammalian vacuolar protein sorting 29 (VPS29), a member of the retromer complex, mediates the formation of new axes of development. Furthermore, we show that VPS29 is required for endosome homeostasis, PIN protein cycling, and dynamic PIN1 repolarization during development. We propose a model that links VPS29 function, PIN1 polarity, and organ initiation in plants