Chang, H.-X., Domier, L. L., Radwan, O., Yendrek, C. R., Hudson, M. E., and Hartman, G. 2016. Identification of multiple phytotoxins produced by Fusarium virguliforme including a phytotoxic effector (FvNIS1) associated with soybean sudden death syndrome foliar symptoms. Molecular Plant-Microbe Interactions 96:96-108.
Sudden death syndrome (SDS) of soybean is caused by a soilborne pathogen, Fusarium virguliforme. Phytotoxins produced by F. virguliforme are translocated from infected roots to leaves, in which they cause SDS foliar symptoms. In this study, additional putative phytotoxins of F. virguliforme were identified, including three secondary metabolites and 11 effectors. While citrinin, fusaric acid, and radicicol induced foliar chlorosis and wilting, Soybean mosaic virus (SMV)-mediated overexpression of F. virguliforme necrosis-inducing secreted protein 1 (FvNIS1) induced SDS foliar symptoms that mimicked the development of foliar symptoms in the field. The expression level of fvnis1 remained steady over time, although foliar symptoms were delayed compared with the expression levels. SMV::FvNIS1 also displayed genotype-specific toxicity to which 75 of 80 soybean cultivars were susceptible. Genome-wide association mapping further identified three single nucleotide polymorphisms at two loci, where three leucine-rich repeat receptor-like protein kinase (LRR-RLK) genes were found. Culture filtrates of fvnis1 knockout mutants displayed a mild reduction in phytotoxicity, indicating that FvNIS1 is one of the phytotoxins responsible for SDS foliar symptoms and may contribute to the quantitative susceptibility of soybean by interacting with the LRR-RLK genes.
Chang, H.-X., Yendrek, C. R., Caetano-Anollés, G., and Hartman, G. 2016. Genomic characterization of plant cell wall degrading enzymes and in silico analysis of xylanses and polygalacturonases of Fusarium virguliforme. BMC Microbiology 16:147 DOI: 10.1186/s12866-016-0761-0.
Plant cell wall degrading enzymes (PCWDEs) are a subset of carbohydrate-active enzymes (CAZy) produced by plant pathogens to degrade plant cell walls. To counteract PCWDEs, plants release PCWDEs inhibitor proteins (PIPs) to reduce their impact. Several transgenic plants expressing exogenous PIPs that interact with fungal glycoside hydrolase (GH)11-type xylanases or GH28-type polygalacturonase (PG) have been shown to enhance disease resistance. However, many plant pathogenic Fusarium species were reported to escape PIPs inhibition. Fusarium virguliforme is a soilborne pathogen that causes soybean sudden death syndrome (SDS). Although the genome of F. virguliformewas sequenced, there were limited studies focused on the PCWDEs of F. virguliforme. Our goal was to understand the genomic CAZy structure of F. viguliforme, and determine if exogenous PIPs could be theoretically used in soybean to enhance resistance against F. virguliforme.
F. virguliforme produces diverse CAZy to degrade cellulose and pectin, similar to other necrotorphic and hemibiotrophic plant pathogenic fungi. However, some common CAZy of plant pathogenic fungi that catalyze hemicellulose, such as GH29, GH30, GH44, GH54, GH62, and GH67, were deficient in F. virguliforme. While the absence of these CAZy families might be complemented by other hemicellulases, F. virguliforme contained unique families including GH131, polysaccharide lyase (PL) 9, PL20, and PL22 that were not reported in other plant pathogenic fungi or oomycetes. Sequence analysis revealed two GH11 xylanases of F. virguliforme, FvXyn11A and FvXyn11B, have conserved residues that allow xylanase inhibitor protein I (XIP-I) binding. Structural modeling suggested that FvXyn11A and FvXyn11B could be blocked by XIP-I that serves as good candidate for developing transgenic soybeans. In contrast, one GH28 PG, FvPG2, contains an amino acid substitution that is potentially incompatible with the bean polygalacturonase-inhibitor protein II (PvPGIP2).
Identification and annotation of CAZy provided advanced understanding of genomic composition of PCWDEs in F. virguliforme. Sequence and structural analyses of FvXyn11A and FvXyn11B suggested both xylanases were conserved in residues that allow XIP-I inhibition, and expression of both xylanases were detected during soybean roots infection. We postulate that a transgenic soybean expressing wheat XIP-I may be useful for developing root rot resistance to F. virguliforme.
Chang, H.-X., Brown, P., Lipka, A., Domier, L. L., and Hartman, G. L. 2016. Genome-wide association and genomic prediction identifies associated loci and predicts the sensitivity of Tobacco ringspot virus in soybean plant introductions. BMC Genomics 17:153:DOI 10.1186/s12864-12016-12487-12867.
Genome-wide association study (GWAS) is a useful tool for detecting and characterizing traits of interest including those associated with disease resistance in soybean. The availability of 50,000 single nucleotide polymorphism (SNP) markers (SoySNP50K iSelect BeadChip; www.soybase.org
) on 19,652 soybean and wild soybean plant introductions (PIs) in the USDA Soybean Germplasm Collection allows for fast and robust identification of loci associated with a desired phenotype. By using a genome-wide marker set to predict phenotypic values, genomic prediction for phenotype-unknown but genotype-determined PIs has become possible. The goal of this study was to describe the genetic architecture associated with sensitivity to Tobacco ringspot virus
(TRSV) infection in the USDA Soybean Germplasm Collection.
TRSV-induced disease sensitivities of the 697 soybean PIs were rated on a one to five scale with plants rated as one exhibiting mild symptoms and plants rated as five displaying terminal bud necrosis (i.e., bud blight). The GWAS identified a single locus on soybean chromosome 2 strongly associated with TRSV sensitivity. Cross-validation showed a correlation of 0.55 (P < 0.01) to TRSV sensitivity without including the most significant SNP marker from the GWAS as a covariate, which was a better estimation compared to the mean separation by using significant SNPs. The genomic estimated breeding values for the remaining 18,955 unscreened soybean PIs in the USDA Soybean Germplasm Collection were obtained using the GAPIT R package. To evaluate the prediction accuracy, an additional 55 soybean accessions were evaluated for sensitivity to TRSV, which resulted in a correlation of 0.67 (P < 0.01) between actual and predicted severities.
A single locus responsible for TRSV sensitivity in soybean was identified on chromosome 2. Two leucine-rich repeat receptor-like kinase genes were located near the locus and may control sensitivity of soybean to TRSV infection. Furthermore, a comprehensive genomic prediction for TRSV sensitivity for all accessions in the USDA Soybean Germplasm Collection was completed.
Hartman, G. L., Chang, H.-X., and Leandro, L. F. 2015. Research advances and management of soybean sudden death syndrome. Crop Protection 73:60-66.
Fusarium virguliforme causes soybean sudden death syndrome (SDS) in the United States. The disease was first observed in Arkansas in 1971, and since has been reported in most soybean-producing states, with a general movement from the southern to the northern states. In addition to F. virguliforme, three other species, Fusarium brasiliense, Fusarium crassistipitatum, and Fusarium tucumaniae, have been reported to cause SDS in South America. Yield losses caused by F. virguliforme range from slight to 100%. Severely infected plants often have increased flower and pod abortion, reduced seed size, increased defoliation, and prematurely senescence. Foliar symptoms observed in the field are most noticeable from mid to late reproductive growth stages. To manage SDS, research on crop rotations, soil types, tillage practices, seed treatments, and the development and utilization of host resistance has been investigated. This review focuses on what is known about F. virguliforme, the management of SDS in the United States, and how genetic engineering along with other traditional management options may be needed as integrated approaches to manage SDS.
Chang, H-X, Miller, L. A., Hartman, G. L. 2014. Melanin-independent accumulation of turgor pressure in appressoria of Phakopsora pachyrhizi. Phytopathology 104:977-984.
Appressoria of some plant pathogenic fungi accumulate turgor pressure that produces a mechanical force enabling the direct penetration of hyphae through the epidermis. Melanin has been reported to function as an impermeable barrier to osmolytes, which allow appressoria to accumulate high turgor pressure. Deficiency of melanin in appressoria has been shown to reduce turgor pressure and compromise the infection process. In Phakopsora pachyrhizi, the soybean rust pathogen, the appressoria are transparent. Our objective was to determine if melanin inhibitors would alter appressorial turgor pressure and if a melanin layer would form specifically between the appressorial cell wall and plasma membrane. We used two melanin biosynthesis inhibitors and found that these melanin inhibitors did not reduce turgor pressure or compromise the infection process. In addition, the turgor pressure of P. pachyrhizi appressoria ranged from 5 to 6 MPa based on extracellular osmolytes used to simulate different osmotic pressures. Transmission electron microscopy also showed the absence of a melanin layer between the appressorial cell wall and plasma membrane. This is the first report showing that turgor pressure accumulation of P. pachyrhizi appressoria was independent of melanin.