Foliar pathogens face heterogeneous environments depending on the maturity of leaves they interact with. In particular, nutrient availability as well as defense levels may vary significantly, with opposing effects on the success of infection. The present study tested which of these factors have a dominant effect on the pathogen’s development. Poplar leaf disks of eight maturity levels were inoculated with the poplar rust fungus Melampsora larici-populina using an innovative single-spore inoculation procedure. A set of quantitative fungal traits (infection efficiency, latent period, uredinia size, mycelium quantity, sporulation rate, sporulation capacity, and spore volume) was measured on each infected leaf disk. Uninfected parts of the leaves were analyzed for their nutrient (sugars, total C and N) and defense compounds (phenolics) content. We found that M. larici-populina is more aggressive on more mature leaves as indicated by wider uredinia and a higher sporulation rate. Other traits varied independently from each other without a consistent pattern. None of the pathogen traits correlated with leaf sugar, total C, or total N content. In contrast, phenolic contents (flavonols, hydroxycinnamic acid esters, and salicinoids) were negatively correlated with uredinia size and sporulation rate. The pathogen’s fitness appeared to be more constrained by the constitutive plant defense level than limited by nutrient availability, as evident in the decrease in sporulation.
The poplar rust fungus Melampsora larici-populina causes significant yield reduction and severe economic losses in commercial poplar plantations. After several decades of breeding for qualitative resistance and subsequent breakdown of the released resistance genes, breeders now focus on quantitative resistance, perceived to be more durable. But quantitative resistance also can be challenged by an increase of aggressiveness in the pathogen. Thus, it is of primary importance to better understand the genetic architecture of aggressiveness traits. To this aim, our goal is to build a genetic linkage map for M. larici-populina in order to map quantitative trait loci related to aggressiveness. First, a large progeny of M. larici-populina was generated through selfing of the reference strain 98AG31 (which genome sequence is available) on larch plants, the alternate host of the poplar rust fungus. The progeny's meiotic origin was validated through a segregation analysis of 115 offspring with 14 polymorphic microsatellite markers, of which 12 segregated in the expected 1:2:1 Mendelian ratio. A microsatellite-based linkage disequilibrium analysis allowed us to identify one potential linkage group comprising two scaffolds. The whole genome of a subset of 47 offspring was resequenced using the Illumina HiSeq 2000 technology at a mean sequencing depth of 6X. The reads were mapped onto the reference genome of the parental strain and 144,566 SNPs were identified across the genome. Analysis of distribution and polymorphism of the SNPs along the genome led to the identification of 2580 recombination blocks. A second linkage disequilibrium analysis, using the recombination blocks as markers, allowed us to group 81 scaffolds into 23 potential linkage groups. These preliminary results showed that a high-density linkage map could be constructed by using high-quality SNPs based on low-coverage resequencing of a larger number of M. larici-populina offspring.
Les Fusarium pathogènes des plantes cultivées dont Fusarium oxysporum f. sp. elaeidis agent de la fusariose du palmier à huile, sont des espèces de champignons phytopathogènes rencontrées dans les sols cultivés aussi bien des régions tempérées que des régions tropicales. Ils s'attaquent aux cultures et engendrent des dégâts économiques conséquents. Cette synthèse fait le point des études menées sur les effets des facteurs de l'environnement sur le développement et le potentiel infectieux des espèces pathogènes du genre Fusarium. Les variables climatiques à savoir les précipitations, l'humidité du sol et de l'air, l'ensoleillement et la température sont autant de facteurs impactant le développement des Fusarium. La salinité de l'air et du sol agit de façon directe ou indirecte sur leur biologie et leur cycle de vie. A forte concentration, le chlorure de sodium stimule certaines espèces de Fusarium alors qu'il entraine une forte réduction de la densité d'autres espèces. De plus, la composition chimique, la communauté microbienne ainsi que le pH du sol modulent également leur développement. Enfin les éléments minéraux tels que le Fer, le Manganèse, le Zinc, le Calcium, le Potassium, le Sodium et le Magnésium peuvent induire des mécanismes de résistance chez les plantes en réponse aux attaques causées par les Fusarium. Cependant, il faut noter que les effets des facteurs climatiques peuvent être bénéfiques ou non en fonction des espèces. Il en est de même pour le pH, les communautés microbiennes, les éléments minéraux et les sels notamment le NaCl contenus dans les sols dont nous constatons que les effets diffèrent d'une espèce à l'autre. Ces résultats ne sont donc pas extrapolables au couple Fusarium oxysporum f. sp. elaeidis-palmier à huile. Par conséquent, la collecte des données pouvant permettre l'identification des différents facteurs agissant sur Fusarium oxysporum f. sp. elaeidis devient pertinente. De plus, ces connaissances contribueraient à optimiser les tests de screening pour la sélection du matériel tolérant et permettraient une meilleure gestion intégrée de la fusariose en plantation.
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