Exploring soybean cultivar susceptibility to sudden death syndrome: Insights into isoflavone responses and biocontrol potential.

Sudden Death Syndrome (SDS) caused by Fusarium tucumaniae is a significant threat to soybean production in Argentina. This study assessed the susceptibility of SY 3 × 7 and SPS 4 × 4 soybeans cultivars to F. tucumaniae and studied changes in root isoflavone levels after infection. Additionally, the biocontrol potential of plant-growth promoting rhizobacteria (PGPR) against SDS was also examined. Our results demonstrated that the SY 3 × 7 cultivar exhibited higher disease severity and total fresh weight loss than SPS 4 × 4. Both cultivars showed induction of daidzein, glycitein, and genistein in response to infection, with the partially resistant cultivar displaying significantly higher daidzein levels than the susceptible cultivar at 14 days post infection (dpi) (2.74 vs 2.17-fold), declining to a lesser extent at 23 dpi (0.94 vs 0.35-fold, respectively). However, daidzein was not able to inhibit F. tucumaniae growth in in vitro assays probably due to its conversion to an isoflavonoid phytoalexin which would ultimately be an effective fungal inhibitor. Furthermore, the PGPR bacterium Bacillus amyloliquefaciens BNM340 displayed antagonistic activity against F. tucumaniae and reduced SDS symptoms in infected plants. This study sheds light on the varying susceptibility of soybean cultivars to SDS, offers insights into isoflavone responses during infection, and demonstrates the potential of PGPR as a biocontrol strategy for SDS management, providing ways for disease control in soybean production.

Genetic architecture and evolution of the mating type locus in fusaria that cause soybean sudden death syndrome and bean root rot.

Fusarium tucumaniae is the only known sexually reproducing species among the seven closely related fusaria that cause soybean sudden death syndrome (SDS) or bean root rot (BRR). In a previous study, laboratory mating of F. tucumaniae yielded recombinant ascospore progeny but required two mating-compatible strains, indicating that it is heterothallic. To assess the reproductive mode of the other SDS and BRR fusaria, and their potential for mating, whole-genome sequences of two SDS and one BRR pathogen were analyzed to characterize their mating type (MAT) loci. This bioinformatic approach identified a MAT1-1 idiomorph in F. virguliforme NRRL 22292 and MAT1-2 idiomorphs in F. tucumaniae NRRL 34546 and F. azukicola NRRL 54364. Alignments of the MAT loci were used to design PCR primers within the conserved regions of the flanking genes APN1 and SLA2, which enabled primer walking to obtain nearly complete sequences of the MAT region for six MAT1-1 and five MAT1-2 SDS/BRR fusaria. As expected, sequences of the highly divergent 4.7 kb MAT1-1 and 3.7 kb MAT1-2 idiomorphs were unalignable. However, sequences of the respective idiomorphs and those that flank MAT1-1 and MAT1-2 were highly conserved. In addition to three genes at MAT1-1 (MAT1-1-1, MAT1-1-2, MAT1-1-3) and two at MAT1-2 (MAT1-2-1, MAT1-2-3), the MAT loci of the SDS/BRR fusaria also include a putative gene predicted to encode for a 252 amino acid protein of unknown function. Alignments of the MAT1-1-3 and MAT1-2-1 sequences were used to design a multiplex PCR assay for the MAT loci. This assay was used to screen DNA from 439 SDS/BRR isolates, which revealed that each isolate possessed MAT1-1 or MAT1-2, consistent with heterothallism. Both idiomorphs were represented among isolates of F. azukicola, F. brasiliense, F. phaseoli and F. tucumaniae, whereas isolates of F. virguliforme and F. cuneirostrum were only MAT1-1 and F. crassistipitatum were only MAT1-2. Finally, nucleotide sequence data from the RPB1 and RPB2 genes were used to date the origin of the SDS/BRR group, which was estimated to have occurred about 0.75 Mya (95% HPD interval: 0.27, 1.68) in the mid-Pleistocene, long before the domestication of the common bean or soybean.

One fungus, one name: defining the genus Fusarium in a scientifically robust way that preserves longstanding use.

In this letter, we advocate recognizing the genus Fusarium as the sole name for a group that includes virtually all Fusarium species of importance in plant pathology, mycotoxicology, medicine, and basic research. This phylogenetically guided circumscription will free scientists from any obligation to use other genus names, including teleomorphs, for species nested within this clade, and preserve the application of the name Fusarium in the way it has been used for almost a century. Due to recent changes in the International Code of Nomenclature for algae, fungi, and plants, this is an urgent matter that requires community attention. The alternative is to break the longstanding concept of Fusarium into nine or more genera, and remove important taxa such as those in the F. solani species complex from the genus, a move we believe is unnecessary. Here we present taxonomic and nomenclatural proposals that will preserve established research connections and facilitate communication within and between research communities, and at the same time support strong scientific principles and good taxonomic practice.

Fusarium azukicola sp. nov., an exotic azuki bean root-rot pathogen in Hokkaido, Japan.

We report on the phenotypic, molecular phylogenetic and pathogenic characterization of a novel azuki bean (Vigna angularis) root-rot (BRR) pathogen from Hokkaido, Japan, which formally is described herein as Fusarium azukicola. This species can be distinguished phenotypically from the other Phaseolus/Vigna BRR and soybean sudden-death syndrome (SDS) pathogens by the production of wider and longer four-septate conidia cultured on SNA. Molecular phylogenetic analyses of four anonymous intergenic loci, a portion of the translation elongation factor (EF-1α) gene and the nuclear ribosomal intergenic spacer region (IGS rDNA) strongly support the genealogical exclusivity of F. azukicola with respect to the other soybean SDS and BRR pathogens within Clade 2 of the F. solani species complex (FSSC). Evolutionary relationships of F. azukicola to other members of the SDS-BRR clade, however, are unresolved by phylogenetic analyses of the individual and combined datasets, with the exception of the IGS rDNA partition, which strongly supports it as a sister of the soybean SDS pathogen F. brasiliense. A multilocus genotyping assay is updated to include primer probes that successfully distinguish F. azukicola from the other soybean SDS and BRR pathogens. Results of a pathogenicity experiment reveal that the F. azukicola isolates are able to induce root-rot symptoms on azuki bean, mung bean (Vigna radiata), kidney bean (Phaseolus vulgaris) and soybean (Glycine max), as well as typical SDS foliar symptoms on soybean. Our hypothesis is that F. azukicola evolved in South America and was introduced to Hokkaido, Japan, on azuki bean but its possible route of introduction remains unknown.

Soybean sudden death syndrome species diversity within north and South america revealed by multilocus genotyping.

Sudden death syndrome (SDS) of soybean has become a serious constraint to the production of this crop in North and South America. Phenotypic and multilocus molecular phylogenetic analyses, as well as pathogenicity experiments, have demonstrated that four morphologically and phylogenetically distinct fusaria can induce soybean SDS. Published molecular diagnostic assays for the detection and identification of these pathogens have reported these pathogens as F. solani, F. solani f. sp. glycines, or F. solani f. sp. phaseoli, primarily because the species limits of these four pathogens were only recently resolved. In light of the recent discovery that soybean SDS and Phaseolus and mung bean root rot (BRR) are caused by four and two distinct species, respectively, multilocus DNA sequence analyses were conducted to assess whether any of the published molecular diagnostic assays were species-specific. Comparative DNA sequence analyses of the soybean SDS and BRR pathogens revealed that highly conserved regions of three loci were used in the design of these assays, and therefore none were species-specific based on our current understanding of species limits within the SDS-BRR clade. Prompted by this finding, we developed a high-throughput multilocus genotyping (MLGT) assay which accurately differentiated the soybean SDS and two closely related Phaseolus and mung BRR pathogens based on nucleotide polymorphism within the nuclear ribosomal intergenic spacer region rDNA and two anonymous intergenic regions designated locus 51 and 96. The single-well diagnostic assay, employing flow cytometry and a novel fluorescent microsphere array, was validated by independent multilocus molecular phylogenetic analysis of a 65 isolate design panel. The MLGT assay was used to reproducibly type a total of 262 soybean SDS and 9 BRR pathogens. The validated MLGT array provides a unique molecular diagnostic for the accurate identification and molecular surveillance of these economically important plant pathogens.