Microscopic, Biochemical, and Molecular Comparisons of Moderately Resistant and Susceptible Populus Genotypes Inoculated with Sphaerulina musiva.

Sphaerulina musiva, the causal agent of Septoria leaf spot and stem canker, is responsible for mortality and yield loss in Populus plantations. However, little is known about the mode of infection and the mechanisms of resistance in this pathosystem. To characterize these phenomena, microscopic, biochemical, and transcriptome comparisons were performed between leaves of moderately resistant and susceptible genotypes of Populus inoculated with S. musiva conidia. Using scanning electron, cryofracture, and laser-scanning confocal microscopy, the infection and colonization of Populus leaves by S. musiva were examined across five time points (48 h, 96 h, 1 week, 2 weeks, and 3 weeks). The infection process was similar regardless of the host genotype. Differences in host colonization between susceptible and moderately resistant genotypes were apparent by 1 week postinoculation. However, the germination of conidia was greater on the susceptible than on the moderately resistant genotype (P < 0.008). Diaminobenzidine staining, a measure of hydrogen peroxide accumulation, was different (P < 0.001) between the host genotypes by 2 weeks postinoculation. Transcriptome differences between genotypes indicated that the speed and amplitude of the defense response were faster and more extensive in the moderately resistant genotype. Changes in gene expression support the microscopic and biochemical observations.

A Culture-Independent PCR-Based Assay to Detect the Root Rot Pathogen Fusarium solani Species Complex 11 from Soybean Roots and Soil.

Fusarium solani species complex (FSSC) 11 is the primary phylogenetic species of FSSC causing root rot in soybean in the north-central United States. A polymerase chain reaction (PCR)-based assay was developed to identify and differentiate FSSC 11 from the less aggressive FSSC 5 and other Fusarium and Pythium spp. associated with soybean roots. The primer set FSSC11-F and FSSC11-R designed from the RNA polymerase second largest subunit gene yielded the expected amplicon of about 900 bp with DNA from all 22 FSSC 11 isolates tested in PCR. However, it did not produce an amplicon with DNA from 29 isolates of FSSC 5, seven other Fusarium spp., three Pythium spp., and soybean tested in PCR. Furthermore, the primer set successfully detected FSSC 11 from a DNA mixture containing the DNA of FSSC 11, FSSC 5, other Fusarium spp., and soybean. The primer set also detected FSSC 11 from both soil and soybean roots. Additionally, the prevalence of FSSC 11 in soybean roots was determined in five fields in North Dakota by both a culture-independent PCR approach with FSSC11-F and FSSC11-R and a culture-dependent approach. Results from both the culture-dependent and culture-independent approaches with FSSC11-F and FSSC11-R were consistent and revealed the presence of the FSSC 11 in three of five fields sampled.

The Sclerotinia sclerotiorum mating type locus (MAT) contains a 3.6-kb region that is inverted in every meiotic generation.

Sclerotinia sclerotiorum is a fungal plant pathogen and the causal agent of lettuce drop, an economically important disease of California lettuce. The structure of the S. sclerotiorum mating type locus MAT has previously been reported and consists of two idiomorphs that are fused end-to-end as in other homothallics. We investigated the diversity of S. sclerotiorum MAT using a total of 283 isolates from multiple hosts and locations, and identified a novel MAT allele that differed by a 3.6-kb inversion and was designated Inv+, as opposed to the previously known S. sclerotiorum MAT that lacked the inversion and was Inv-. The inversion affected three of the four MAT genes: MAT1-2-1 and MAT1-2-4 were inverted and MAT1-1-1 was truncated at the 3'-end. Expression of MAT genes differed between Inv+ and Inv- isolates. In Inv+ isolates, only one of the three MAT1-2-1 transcript variants of Inv- isolates was detected, and the alpha1 domain of Inv+ MAT1-1-1 transcripts was truncated. Both Inv- and Inv+ isolates were self-fertile, and the inversion segregated in a 1∶1 ratio regardless of whether the parent was Inv- or Inv+. This suggested the involvement of a highly regulated process in maintaining equal proportions of Inv- and Inv+, likely associated with the sexual state. The MAT inversion region, defined as the 3.6-kb MAT inversion in Inv+ isolates and the homologous region of Inv- isolates, was flanked by a 250-bp inverted repeat on either side. The 250-bp inverted repeat was a partial MAT1-1-1 that through mediation of loop formation and crossing over, may be involved in the inversion process. Inv+ isolates were widespread, and in California and Nebraska constituted half of the isolates examined. We speculate that a similar inversion region may be involved in mating type switching in the filamentous ascomycetes Chromocrea spinulosa, Sclerotinia trifoliorum and in certain Ceratocystis species.