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

Contrasted Patterns of Molecular Evolution in Dominant and Recessive Self-Incompatibility Haplotypes in Arabidopsis

Self-incompatibility has been considered by geneticists a model system for reproductive biology and balancing selection, but our understanding of the genetic basis and evolution of this molecular lock-and-key system has remained limited by the extreme level of sequence divergence among haplotypes, resulting in a lack of appropriate genomic sequences. In this study, we report and analyze the full sequence of eleven distinct haplotypes of the self-incompatibility locus (S-locus) in two closely related Arabidopsis species, obtained from individual BAC libraries. We use this extensive dataset to highlight sharply contrasted patterns of molecular evolution of each of the two genes controlling self-incompatibility themselves, as well as of the genomic region surrounding them. We find strong collinearity of the flanking regions among haplotypes on each side of the S-locus together with high levels of sequence similarity. In contrast, the S-locus region itself shows spectacularly deep gene genealogies, high variability in size and gene organization, as well as complete absence of sequence similarity in intergenic sequences and striking accumulation of transposable elements. Of particular interest, we demonstrate that dominant and recessive S-haplotypes experience sharply contrasted patterns of molecular evolution. Indeed, dominant haplotypes exhibit larger size and a much higher density of transposable elements, being matched only by that in the centromere. Overall, these properties highlight that the S-locus presents many striking similarities with other regions involved in the determination of mating-types, such as sex chromosomes in animals or in plants, or the mating-type locus in fungi and green algae

Actors of ROS Homeostasis in Stigmatic Cells Essential for Plant Reproduction

International audienceReactive oxygen species (ROS) play important roles during development and responses to external stimuli. In Brassicaceae, the stigma epidermis accumulates a large amount of ROS. Moreover, regulating the stigmatic ROS status is crucial for Self-incompatibility (SI) mechanisms, to ensure self-pollen rejection while promoting compatible pollen. Here, scanning our transcriptomic data in light of recent advances in the Brassicaceae SI system, we identified Class III peroxidases that are highly expressed in mature stigma and might regulate stigma ROS homeostasis. We also found two Receptor Like Kinases from the CYSTEINE-RICH RECEPTOR-LIKE KINASES family (CRK31 and CRK41) strongly upregulated upon incompatible pollination. We proposed that these two CRKs might be part of the ROS-mediated SI response and serve to connect pollen recognition and ROS accumulation

Feeling the pressure: A mechanical tale of the pollen tube journey through the pistil

International audienceThe pollen tube grows inside the pistil to carry the male gametes to the ovule. During this journey, it invades diverse tissues and must sense and adapt to abrupt changes in its mechanical environment (mechanical transitions). In this issue, a study presents a Receptorlike-kinase/Rho-GTPase module that regulates such transition

Contrôle de la fécondation par des mécanismes d'auto-incompatibilité

International audienceFlowering plants (angiosperms) are the most prevalent and evolutionarily advanced group of plants. Reproductive strategies that promote cross-fertilization have played an essential role in the success of angiosperms as they contribute to genetic variability among plant species. A major genetic barrier to self-fertilization is self-incompatibility (SI), which allows female reproductive cells to discriminate between self- and non-self pollen and specifically reject self-pollen. In this review, we describe three SI mechanisms showing that different flowering plant families use distinct molecules for recognition of self as well as diverse biochemical pathways to arrest pollen tube growth

Actors of ROS Homeostasis in Stigmatic Cells Essential for Plant Reproduction

International audienceReactive oxygen species (ROS) play important roles during development and responses to external stimuli. In Brassicaceae, the stigma epidermis accumulates a large amount of ROS. Moreover, regulating the stigmatic ROS status is crucial for Self-incompatibility (SI) mechanisms, to ensure self-pollen rejection while promoting compatible pollen. Here, scanning our transcriptomic data in light of recent advances in the Brassicaceae SI system, we identified Class III peroxidases that are highly expressed in mature stigma and might regulate stigma ROS homeostasis. We also found two Receptor Like Kinases from the CYSTEINE-RICH RECEPTOR-LIKE KINASES family (CRK31 and CRK41) strongly upregulated upon incompatible pollination. We proposed that these two CRKs might be part of the ROS-mediated SI response and serve to connect pollen recognition and ROS accumulation

Control of fertilization by self-incompatibility mechanisms

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Characterization of a tRNALys(CUU) gene located in the opposite orientation upstream of a ZmFer2 ferritin gene in the maize nuclear genome

International audienceThe first evidence for a plant tRNA(Lys)(CUU) gene is reported. This gene is found closely linked 400 bp upstream, and on the complementary strand, of a ZmFer2 ferritin gene in the maize nuclear genome. Southern blot analysis indicates that this tRNA(Lys) is a member of a multigene family. This gene does not contain any intron, and exhibits classical intragenic regulatory elements found in eukaryotic tRNA genes (A and B boxes). Moreover, 5' and 3'-flanking sequences display typical features found in nuclear encoded tRNAs. The deduced mature tRNA sequence is almost identical to the sequence of a cytoplasmic tRNA(Lys)(CUU) from wheat germ. The maize tRNA(Lys) gene is expressed in vivo in maize and in transgenic tobacco, as shown by RT-PCR analysis

When no means no: guide to Brassicaceae self-incompatibility

International audienceMore than half of the flowering plants have a sophisticated mechanism for self-pollen rejection, named self-incompatibility (SI). In Brassicaceae, recognition specificity is achieved by the interaction of the stigmatic S-RECEPTOR KINASE (SRK) and its ligand S-LOCUS CYSTEINE-RICH PROTEIN (SCR). Recent years have seen significant advances in understanding the SI response. Progress has been made on elucidating the regulation and function of proteins that act as either molecular partners of SRK or modulators of SI. Thus, modules controlling the specificity of the central receptor-ligand interaction have been identified on both SRK and SCR proteins. A role for intracellular protein trafficking in SI has also been demonstrated. Here, we integrate the novel findings into the existing model to present the current understanding of SI signaling