Systematic gene deletion and functional characterization of histidine kinase phosphorelay receptors (HKRs) in the human pathogenic fungus Aspergillus fumigatus.

Histidine kinase receptors (HKRs) appear to be a common strategy for model and pathogenic fungi to sense and respond to environmental stresses. In the human pathogen Aspergillus fumigatus, which is responsible for invasive aspergillosis, 13 genes potentially encoding HKRs have been identified. Until now, only three HKRs have been functionally characterized. The aim of this study was to perform the systematic invalidation of A. fumigatus HKR genes and the careful phenotypic characterization of the relevant mutants. This study notably allowed to gain new important insights into the role of HKRs in physiology of A. fumigatus. Actually, we showed that (i) NikA/TcsC could be involved in the cell wall integrity pathway, (ii) Fhk6 and PhkA were involved in the regulation of the "fluffy" developmental program, (iii) PhkB could participate in the regulation of conidiation and (iv) PhkA was implied in the resistance of oxidative stresses.

Migration patterns and changes in population biology associated with the worldwide spread of the oilseed rape pathogen Leptosphaeria maculans.

Pathogen introductions into novel areas can lead to the emergence of new fungal diseases of plants. Understanding the origin, introduction pathways, possible changes in reproductive system and population size of fungal pathogens is essential in devising an integrated strategy for the control of these diseases. We used minisatellite markers to infer the worldwide invasion history of the fungal plant pathogen Leptosphaeria maculans, which causes stem canker (blackleg) of oilseed and vegetable brassicas. Clustering analyses partitioned genotypes into distinct populations corresponding to major geographic regions, along with two differentiated populations in Western Canada. Comparison of invasion scenarios using Approximate Bayesian Computation suggested an origin of the pathogen in the USA, the region where epidemics were first recorded, and independent introductions from there over the last few decades into Eastern Canada (Ontario), Europe and Australia. The population in Western Canada appeared to be founded from a source in Ontario and the population in Chile resulted from an admixture between multiple sources. A bottleneck was inferred for the introduction into Western Canada but not into Europe, Ontario or Australia. Clonality appeared high in Western Canada, possibly because environmental conditions there were less conducive to sexual reproduction. Leptosphaeria maculans is a model invasive pathogen with contrasting features in different regions: shallow population structure, high genetic variability and regular sexual recombination in some regions, by comparison with reduced genetic variability, high rates of asexual multiplication, strong population structure or admixture in others.

First Report of Leptosphaeria biglobosa (Blackleg) on Brassica oleracea (Cabbage) in Mexico.

Broccoli (Brassica oleracea var. italica), cauliflower (B. oleracea var. botrytis), and cabbage (B. oleracea var. capitata) have been grown in central Mexico since 1970, with 21,000 ha cropped in 2001. In contrast, areas grown with oilseed rape (B. napus) are very limited in Mexico (<8,000 ha). Blackleg, a destructive disease of B. napus in most parts of the world, was first observed in Mexico in Zacatecas and Aguascalientes in 1988 on B. oleracea, causing as much as 70% yield loss. A species complex of two closely related Dothideomycete species, Leptosphaeria maculans and L. biglobosa, is associated with this disease of crucifers (1), but leaf symptoms on susceptible plants are different, with L. maculans typically causing >15-mm pale gray lesions with numerous pycnidia, whereas L. biglobosa causes dark and smaller lesions only containing a few pycnidia. Having a similar epidemiology, both species can be present on the same plants at the same time, and symptom confusion can occur as a function of the physiological condition of the plant or expression of plant resistance responses. A total of 209 isolates from symptomatic B. oleracea leaves were collected from three fields in central states of Mexico (58 to 71 isolates per location). All leaves showed similar symptoms, including a 10- to 15-mm tissue collapse with an occasional dark margin. Cotyledons of seven B. napus differentials were inoculated with conidia of all the isolates as described by Dilmaghani et al. (1). Two hundred isolates caused tissue collapse typical of L. maculans. However, nine obtained from white cabbage in a single location in Aguascalientes caused <5-mm dark lesions. When inoculated onto cotyledons of three B. oleracea genotypes commonly grown in Mexico (cvs. Domador, Monaco, and Iron Man), the nine isolates caused a range of symptoms characterized by tissue collapse (maximum 10 to 15 mm), showing the presence of patches of black necrotic spots within the collapse. The occasional presence of a few pycnidia allowed us to reisolate the fungus for molecular identification. ITS1-5.8S-ITS2, (internal transcribed spacers and 5.8S rDNA), actin, and ß-tubulin sequences were obtained as described previously (4). Multiple gene genealogies based on these sequence data showed two subclades of L. biglobosa: L. biglobosa 'occiaustralensis' (one isolate; ITS [AM410082], actin [AM410084], and ß-tubulin [AM410083]) and L. biglobosa 'canadensis' (eight isolates; ITS [AJ550868], actin [AY748956], and ß-tubulin [AY749004]) (3,4), which were previously described on B. napus in the United States, Canada, and Chile. To our knowledge, this is the first report of L. biglobosa in Mexico. Previously, this species has only been reported once on B. oleracea without discrimination into subclades (2). In the Aguascalientes sampling, 24% of the isolates were L. biglobosa, similar to Canadian locations where this species is still common as compared with L. maculans (1). The large proportion of sampled L. biglobosa 'canadensis', highlights the prevalence of this subclade throughout the American continent (1). References: (1) A. Dilmaghani et al. Plant Pathol. 58:1044, 2009. (2) E. Koch et al. Mol. Plant-Microbe Interact. 4:341, 1991. (3) E. Mendes-Pereira et al. Mycol Res. 107:1287, 2003. (4) L. Vincenot et al. Phytopathology 98:321, 2008.