Genome-wide analysis of small secreted cysteine-rich proteins identifies candidate effector proteins potentially involved in Fusarium graminearum-wheat interactions

Authors: Lu, Shunwen; Edwards, Michael

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/28/2015
Publication Date: 2/1/2016
Publication URL: http://handle.nal.usda.gov/10113/61824
Citation: Lu, S., Edwards, M.C. 2016. Genome-wide analysis of small secreted cysteine-rich proteins identifies candidate effector proteins potentially involved in Fusarium graminearum-wheat interactions. Phytopathology. 106:166-176.

Interpretive Summary: Fusarium graminearum is the fungus that causes Fusarium head blight (FHB), a devastating disease of wheat crops worldwide. In addition to causing severe yield losses, the fungus also poses a health risk to humans and animals by producing mycotoxins that contaminate food/feed products. How the fungus induces FHB in wheat is still not well understood. The major goals of this study were to identify proteins produced by this fungus that are known as small secreted cysteine-rich proteins (SSCPs), and to determine if any of these proteins might act as "effector" proteins. Effector proteins are a type of small protein that mediates the development of disease in host plants – in this case, FHB. A total of 190 SSCPs were identified from the fungal genome and 25 were confirmed to be actually produced and secreted by the fungus. At least 34 SSCPs were found to be expressed during fungal infection of host plants, and the expression patterns of some SSCPs were found to correlate with the development of FHB symptoms on susceptible wheat plants. This work provides a solid candidate list for SSCP-derived effectors potentially involved in F. graminearum-wheat interactions. Further study of these candidate effectors would aid the discovery of novel pathogenicity/virulence factors in the fungus and ultimately, the molecular keys controlling host resistance to FHB in wheat.

Technical Abstract: Pathogen-derived, small secreted cysteine-rich proteins (SSCPs) are known to be a common source of fungal effectors that trigger resistance or susceptibility in specific host plants. This group of proteins has not been well studied in Fusarium graminearum, the primary cause of Fusarium head blight (FHB), a devastating disease of wheat. We report here a comprehensive analysis of SSCPs encoded in the genome of this fungus and selection of candidate effector proteins through proteomics and sequence/transcriptional analyses. A total of 190 SSCPs were identified in the genome of F. graminearum (isolate PH-1) based on the presence of N-terminal signal peptide sequences, size (<= 200 amino acids), and cysteine content (>= 2%) of the mature proteins. Twenty-five (~13%) SSCPs were confirmed to be true extracellular proteins by nanoscale liquid chromatography-tandem mass spectrometry (nanoLC-MS/MS) analysis of a minimal medium-based in vitro secretome. Sequence analysis suggested that 17 SSCPs harbor conserved functional domains, including two homologous to Ecp2, a known effector produced by the tomato pathogen Cladosporium fulvum. Transcriptional analysis revealed that at least 34 SSCPs (including 23 detected in the in vitro secretome) are expressed in infected wheat heads; about half are up-regulated with expression patterns correlating with the development of FHB. This work provides a solid candidate list for SSCP-derived effectors that may play roles in mediating F. graminearum-wheat interactions. The in vitro secretome-based method presented here also may be applicable for identifying candidate effectors in other ascomycete pathogens of crop plants.