Phoma stem canker disease on oilseed rape (Brassica napus) in China is caused by Leptosphaeria biglobosa ‘brassicae’

This document is the Accepted Manuscript version of the following article: Ze Liu, Akinwunmi O. Latunde-Dada, Avice M. Hall, Bruce D. L. Fitt, ‘Phoma stem canker disease on oilseed rape (Brassica napus) in China is caused by Leptosphaeria biglobosa ‘brassicae’’, European Journal of Plant Pathology, Vol. 140(4): 841-857, December 2014. The final publication is available at Springer via: http://dx.doi.org/10.1007/s10658-014-0513-7 © Koninklijke Nederlandse Planteziektenkundige Vereniging 2014Phoma stem canker of oilseed rape (Brassica napus) is a globally important disease that is caused by the sibling ascomycete species Leptosphaeria maculans and L. biglobosa. Sixty fungal isolates obtained from oilseed rape stems with phoma stem canker disease symptoms collected from four provinces in China in 1999, 2005 and 2006 were all identified as Leptosphaeria biglobosa, not L. maculans, by PCR diagnostics based on species-specific primers. There were no differences in cultural characteristics (e.g. pigmentation and in vitro growth) between these L. biglobosa isolates from China and those of 37 proven L. biglobosa isolates from Europe or Canada. In studies using amplified fragment length polymorphism (AFLP) markers, Chinese L. biglobosa populations were genetically more similar to European L. biglobosa populations than to the more diverse Canadian L. biglobosa populations. Sequencing of gene fragments of β-tubulin, actin and the internal transcribed spacer (ITS) region of rDNA from L. biglobosa isolates from China, Europe, Australia and Canada showed a closer taxonomic similarity of Chinese L. biglobosa to the European L. biglobosa ‘brassicae’ than to Canadian L. biglobosa ‘canadensis’ or to the Australian L. biglobosa ‘occiaustralensis’ or ‘australensis’ subclades. These results suggest that the Chinese L. biglobosa population in this study is in the same subclade as European L. biglobosa ‘brassicae’ populationsPeer reviewe

Effector diversification within compartments of the Leptosphaeria maculans genome affected by Repeat-Induced Point mutations

Fungi are of primary ecological, biotechnological and economic importance. Many fundamental biological processes that are shared by animals and fungi are studied in fungi due to their experimental tractability. Many fungi are pathogens or mutualists and are model systems to analyse effector genes and their mechanisms of diversification. In this study, we report the genome sequence of the phytopathogenic ascomycete Leptosphaeria maculans and characterize its repertoire of protein effectors. The L. maculans genome has an unusual bipartite structure with alternating distinct guanine and cytosine-equilibrated and adenine and thymine (AT)-rich blocks of homogenous nucleotide composition. The AT-rich blocks comprise one-third of the genome and contain effector genes and families of transposable elements, both of which are affected by repeat-induced point mutation, a fungal-specific genome defence mechanism. This genomic environment for effectors promotes rapid sequence diversification and underpins the evolutionary potential of the fungus to adapt rapidly to novel host-derived constraints

Regulation of proteinaceous effector expression in phytopathogenic fungi

Effectors are molecules used by microbial pathogens to facilitate infection via effector-triggered susceptibility or tissue necrosis in their host. Much research has been focussed on the identification and elucidating the function of fungal effectors during plant pathogenesis. By comparison, knowledge of how phytopathogenic fungi regulate the expression of effector genes has been lagging. Several recent studies have illustrated the role of various transcription factors, chromosome-based control, effector epistasis, and mobilisation of endosomes within the fungal hyphae in regulating effector expression and virulence on the host plant. Improved knowledge of effector regulation is likely to assist in improving novel crop protection strategies

Du polymorphisme au complexe d'espèces : combien d'agents pathogènes sont impliqués dans la nécrose du collet du colza ?

La nécrose du collet des crucifères, ou «phoma du colza», est une maladie qui a subi une extension considérable à la suite de l'intensification de la culture du colza en Europe, au Canada ou en Australie. Phoma lingam (Tode ex Fr) Desm est reconnu depuis de nombreuses années comme étant l'agent responsable de la maladie. La forme sexuée du parasite, phase quasi systématique du cycle, a été identifiée dans les années 1960 à Leptosphaeria maculans (Desm) Ces et de Not. Dès 1927, 2 groupes de souches isolées de Brassica cultivées sont distingués, selon des critères pathologiques et culturaux d'une fiabilité limitée. Sur la base de ces critères, on a considéré que l'on était en présence de 2 pathotypes : «pathogène» et «faiblement pathogène». Le développement, au début des années 1990, de méthodes fines de caractérisation (isozymes, analyses moléculaires, etc) a démontré une distance génétique importante entre les 2 groupes de souches, suggérant qu'il pouvait s'agir de 2 espèces distinctes. Des travaux récents montrent par ailleurs que les souches «faiblement pathogènes» représentent, en elles-mêmes, une entité complexe regroupant au moins 3 groupes de souches génétiquement distantes. Des observations préliminaires suggèrent en outre que le type prépondérant serait différent en Australie, en Europe ou au Canada. Enfin, la présence de souches proches mais encore très peu étudiées, parasitant les crucifères adventices accroît la complexité du pathosystème. La discrimination originelle en 2 «pathotypes» a été fortement préjudiciable au développement d'analyses plus poussées des populations en présence et de leur interaction avec les crucifères. Par conséquent, une cartographie de l'occurrence des différents types de souches, une réévaluation de leur symptomatologie respective et une analyse de l'incidence des souches issues de crucifères adventices sur les cultures de colza sont nécessaires pour une meilleure appréciation de l'importance de chaque groupe de souches dans la maladie, et un meilleur contrôle de celle-ci.How many pathogens do cause blackleg disease in crucifers? Blackleg in crucifers has become a disease of international importance following the tremendous increase in oilseed rape acreage in Europe, Canada and Australia. The causative agent of the disease has been described as Leptosphaeria maculans (Desm) Ces et de Not, an ascomycete whose anamorph is Phoma lingam (Tode ex Fr) Desm. The pathogen is known to be highly polymorphic in cultural characteristics and pathogenicity. Isolates obtained from Brassica sp in the early 1920s were distinguished as belonging to either a 'pathogenic' pathotype or a 'weakly pathogenic' pathotype. However, the reliability of the morphological, and even the pathogenic, criteria for a discrimination of isolates has been questioned. 'Modern methods' for the study of the polymorphism of fungi, ie isozymic and molecular techniques, usually allow an unequivocal discrimination of the 2 groups. Moreover, these techniques recently demonstrated an important genetic distance between the 2 groups of isolates, suggesting that they may actually belong to 2 distinct species. The 'weakly pathogenic' group can be further separated into 3 genetically distinct subgroups. Moreover, it has been suggested that the prevalent subgroup could be different in Europe, Australia or America. Finally, 4 other L maculans, pathogenic to different species of cruciferous weeds have been described in Canada. Since only pathogenic and morphological criteria were studied, the relationship of these additional L maculans to the 'pathogenic' group or 'weakly pathogenic' groups obtained from Brassica is still unknown. The original discrimination into 2 pathotypes has been highly prejudicial to the understanding and study of this pathosystem. As a consequence, the pathogenicity of each group, or subgroup of isolate to crucifers and its involvement in the blackleg disease has to be reconsidered