Characterization of a host-specific toxic activity produced by Bipolaris cookei, causal agent of Target Leaf Spot of Sorghum

Authors: SAMIRA, ROZALYNNE - North Carolina State University; LOPEZ, LUIS - North Carolina State University; Holland, Jim - Jim; Balint-Kurti, Peter

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/11/2023
Publication Date: 9/1/2023
Citation: Samira, R., Lopez, L.F., Holland, J.B., Balint Kurti, P.J. 2023. Characterization of a host-specific toxic activity produced by Bipolaris cookei, causal agent of Target Leaf Spot of Sorghum. Phytopathology. 113(7):1301-1306. https://doi.org/10.1094/PHYTO-11-22-0427-R.
DOI: https://doi.org/10.1094/PHYTO-11-22-0427-R

Interpretive Summary: Target Leaf Spot (TLS) is an important disease of Sorghum. The fungus that causes TLS is Bipolaris cookei. We grew B. cookei in culture for more the two months and filtered out all the fungal material , to produce the ‘culture filtrate’ (CF). We discovered that, when infiltrated in leaves, this CF is toxic to some lines of sorghum but not to others. We further showed that lines that are susceptible to TLS are more likely to be sensitive to the B. cookei CF. This is strong evidence that the toxic activity in the CF is important in the pathogenesis process of the fungus on sorghum. We identified two regions of the genome most associated with variation in sensitivity to the CF. We showed that the toxic activity of the CF was light-sensitive. We also showed that digestion of the proteins in the CF did not affect the toxic activity of the CF, suggesting that the toxin was not a protein.

Technical Abstract: Target leaf spot (TLS) of sorghum, caused by the necrotrophic fungus Bipolaris cookei, can cause severe yield loss in many parts of the world. We grew B. cookei in liquid culture and observed that the resulting culture filtrate (CF) was differentially toxic when infiltrated into leaves of a population of 288 diverse sorghum lines. In this population we found a significant correlation between high CF sensitivity and susceptibility to TLS. This suggests that the toxin produced in culture may play a role in the pathogenicity of the B. cookei in the field. We demonstrated that the toxic activity is light-sensitive and, surprisingly, is insensitive to pronase, suggesting that it is not proteinaceous. We identified the two sorghum genetic loci most associated with the response to CF in this population. Screening seedlings with B. cookei CF could be a useful approach for prescreening germplasm for TLS resistance.