Genetic diversity and population structure in French populations of Mycosphaerella graminicola.

Mycosphaerella graminicola populations were examined in France with microsatellite markers and PCR-SSCP analysis of partial actin and ß-tubulin encoding sequences. A total of 363 isolates was sampled in 2005 from 17 provinces, and genotypes from corresponding strains were characterized. Unique haplotypes comprised 84% of the population, and gene diversity was high nationwide (0.70) and locally. A moderate genetic differentiation (G(ST) = 0.18) was found and indicated that in France the M. graminicola population was more structured than in other previously studied European countries. Bayesian structure analysis identified three genetic clusters distributed among the 17 provinces. Our results highlighted the potential for the adaptation of the fungus to local conditions, leading to genetic clusters among the French population of the fungus as well as genotype flow between regional clusters.

Mating type idiomorphs from a French population of the wheat pathogen Mycosphaerella graminicola: widespread equal distribution and low but distinct levels of molecular polymorphism.

Septoria tritici blotch caused by the heterothallic ascomycete Mycosphaerella graminicola is currently the most frequent and the most economically damaging disease on wheat worldwide. Five hundred and ten strains of this fungus were sampled from 16 geographical locations representing the major wheat producing areas in France. Multiplex PCR amplification, PCR-RFLP-SSCP screening and sequencing of parts of mating type encoding sequences were performed in order to assess the distribution and molecular polymorphism of the mating type idiomorphs. The two idiomorphs were scored at similar frequencies within all sampled locations. Both mating types were also identified at the leaf spatial scale, on 42% of leaves from which two or three strains were isolated. No correlation was found between distribution of mating types and either host cultivars from which the sampling was carried out or in vitro colony phenotypes observed during the culture of strains on potato dextrose agar (PDA) medium. PCR-RFLP-SSCP assay highlighted only one MAT1-1 strain exhibiting a profile distinct from all other MAT1-1 strains, whereas ten MAT1-2 strains (among which two and four with same profiles, respectively) showed profiles differing from the other MAT1-2 strains. Sequencing revealed that all polymorphisms corresponded to single nucleotide variations and all strains displaying the same single strand conformation polymorphism (SSCP) profiles showed identical nucleotide sequences, thereby confirming the high sensitivity of SSCP. Only two out of the disclosed nucleotide variations were nonsynonymous. This study strongly suggests a large potential for sexual reproduction in the French population of M. graminicola and reports a high conservation of mating type sequences in the fungus at both nucleotide and population levels, with a great difference in molecular variability between the two idiomorphs.

Mechanisms of resistance and tolerance to Mycosphaerella graminicola in wheat.

The aim of this study was to investigate the infection process of M. graminicola and the defence mechanisms related to active oxygen species (AOS) in five French wheat cultivars. These cultivars exhibited various resistant levels to M. graminicola infection: Maxyl, Caphorn and Gen11 are susceptible cultivars, whereas Capnor and Gen23 show high levels of quantitative resistances. In addition, Capnor, Gen23 and Gen11 are tolerant cultivars, i.e., their yield performance was less affected by infection compared to non-tolerant cultivars. Cultivars were inoculated with the IPO323 reference M. graminicola strain. First wheat leaves were collected 3, 5, 7, 9, 11, 13, 15, 17, 19, and 21 days after inoculation. The cytological and antioxidant response of the cultivars were both studied over the whole time course. Although infection occurred mainly through stomata, direct penetration attempts were also scored. Moreover, papilla formation turned out to be very rare. Assays for changes in peroxydase (PO), glutathione-S-transferase (GST) and lipoxygenase (LOX) activities allowed us to compare their levels in the five French wheat cultivars regarding to their resistance and/or tolerance towards M. graminicola infection. PO and GST were correlated to necrosis probably as a consequence of detoxification and LOX was related to some of the germination process steps. We also showed that significant differences for several biochemical parameters exist between the studied cultivars in non inoculated conditions but these differences were less important in the presence of the fungus.