Cereal Domestication and Evolution of Branching: Evidence for Soft Selection in the Tb1 Orthologue of Pearl Millet (Pennisetum glaucum [L.] R. Br.)

BACKGROUND: During the Neolithic revolution, early farmers altered plant development to domesticate crops. Similar traits were often selected independently in different wild species; yet the genetic basis of this parallel phenotypic evolution remains elusive. Plant architecture ranks among these target traits composing the domestication syndrome. We focused on the reduction of branching which occurred in several cereals, an adaptation known to rely on the major gene Teosinte-branched1 (Tb1) in maize. We investigate the role of the Tb1 orthologue (Pgtb1) in the domestication of pearl millet (Pennisetum glaucum), an African outcrossing cereal. METHODOLOGY/PRINCIPAL FINDINGS: Gene cloning, expression profiling, QTL mapping and molecular evolution analysis were combined in a comparative approach between pearl millet and maize. Our results in pearl millet support a role for PgTb1 in domestication despite important differences in the genetic basis of branching adaptation in that species compared to maize (e.g. weaker effects of PgTb1). Genetic maps suggest this pattern to be consistent in other cereals with reduced branching (e.g. sorghum, foxtail millet). Moreover, although the adaptive sites underlying domestication were not formerly identified, signatures of selection pointed to putative regulatory regions upstream of both Tb1 orthologues in maize and pearl millet. However, the signature of human selection in the pearl millet Tb1 is much weaker in pearl millet than in maize. CONCLUSIONS/SIGNIFICANCE: Our results suggest that some level of parallel evolution involved at least regions directly upstream of Tb1 for the domestication of pearl millet and maize. This was unanticipated given the multigenic basis of domestication traits and the divergence of wild progenitor species for over 30 million years prior to human selection. We also hypothesized that regular introgression of domestic pearl millet phenotypes by genes from the wild gene pool could explain why the selective sweep in pearl millet is softer than in maize

Wheat receptor-kinase-like protein Stb6 controls gene-for-gene resistance to fungal pathogen Zymoseptoria tritici

Deployment of fast-evolving disease-resistance genes is one of the most successful strategies used by plants to fend off pathogens. In gene-for-gene relationships, most cloned disease-resistance genes encode intracellular nucleotide-binding leucine-rich-repeat proteins (NLRs) recognizing pathogensecreted isolate-specific avirulence (Avr) effectors delivered to the host cytoplasm. This process often triggers a localized hypersensitive response, which halts further disease development. Here we report the map-based cloning of the wheat Stb6 gene and demonstrate that it encodes a conserved wallassociated receptor kinase (WAK)-like protein, which detects the presence of a matching apoplastic effector and confers pathogen resistance without a hypersensitive response. This report demonstrates gene-for-gene disease resistance controlled by this class of proteins in plants. Moreover, Stb6 is, to our knowledge, the first cloned gene specifying resistance to Zymoseptoria tritici, an important foliar fungal pathogen affecting wheat and causing economically damaging septoria tritici blotch (STB) disease

Analyse du processus infectieux de Mycosphaerella graminicola, agent responsable de la septoriose du blé

Le champignon Mycosphaerella graminicola est l agent causal d une des septorioses du blé. Malgré des études histologiques préalables, certains aspects, telle la longue phase asymptomatique post-pénétration, demeuraient mal compris. D autre part, très peu de déterminants moléculaires du pouvoir pathogène étaient connus lors de l initiation de ce travail. Nous avons donc cherché à mieux comprendre le cycle de développement de M. graminicola à travers l étude de mutants résultant soit d une stratégie de mutagenèse insertionnelle aléatoire par plasmide soit d une approche gène-candidat . Deux étapes de criblage, selon des tests mis au point au cours de ce travail, ont abouti à l isolement de onze mutants présentant des altérations significatives de leur pathogénie. Des études microbiologique, cytologique et microscopique plus approfondies menées sur sept de ces onze mutants ont mis en évidence trois phases majeures dans le processus infectieux de M. graminicola. L analyse moléculaire, de par la complexité du profil d intégration du plasmide mutagène, n a pu être menée que sur deux mutants, A18 et D22, pour lesquels les régions flanquant l insertion du plasmide ont pu être clonées. Toutefois, la corrélation entre l insertion du plasmide et le phénotype mutant n a été démontrée que pour D22. Une seconde approche de type gène candidat a permis de cloner et de caractériser le gène MgMK1 de M. graminicola orthologue du gène PMK1 de M. grisea codant une MAP kinase impliquée dans la pathogénie. Cette stratégie gène candidat n étant pas envisageable sur la totalité des gènes de M. graminicola, tant pour des raisons techniques que de ressources, s avère complémentaire de la première.Mycosphaerella graminicola is the causal agent of wheat leaf blotch. At the beginning of this work, very few was known about M; graminicola infection cycle, especially at the molecular level. In order to better understand the infection process of this fungus on its host plant, we developed strategies to generate and study pathogenicity mutants. A first approach was random insertional mutagenesis using a plasmid as mutagen. Using two successive screening steps set up during this work, eleven pathogenicity mutants were recovered. More detailed phenotypic analyses of these mutants allowed to identify three major steps in M. graminicola infection cycle : (1) the development of infection hyphae allowing stomatal penetration, (2) the differenciation of invasive hyphae associated with plant tissue colonization and (3) the formation of reproductive hyphae correlated with the diffenciation of pycnidia. Unfortunately, the molecular analysis of these mutants revealed to be laborious as many of them were too complex to be studied. Sequences flanking the insertion were recovered for two mutants, A18 and D22, but the relationship between the plasmid insertion and the mutant phenotype was demonstrated only for the mutant strain D22. A candidate gene approach.was also developed to investigate the role of MgMK1, a PMK1-like orthologous gene, encoding a MAP kinase. Mutant strains were obtained using gene replacement and shown to be non pathogenic, due to a major alteration of stomatal penetration. Altogether, these results show that the two strategies are complementary to identify and study molecular determinants of pathogenicity in a plant pathogenic fungus.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

La lutte génétique comme levier de protection chez les céréales

National audienc

CARACTERISATION D'ANALOGUES DE GENES DE RESISTANCE CHEZ LE HARICOT COMMUN PHASEOLUS VULGARIS, REGROUPES A DES LOCUS CONTENANT DES SPECIFICITES DE RESISTANCE VIS-A-VIS DU CHAMPIGNON PHYTOPATHOGENE COLLETOTRICHUM LINDEMUTHIANUM

ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF