A conserved hypothetical gene is required but not sufficient for Ptr ToxC production in Pyrenophora tritici-repentis

Authors: SHI, GONGJUN - North Dakota State University; KARIYAWASAM, GAYAN - North Dakota State University; LIU, SANZHEN - Kansas State University; LENG, YUEQING - North Dakota State University; ZHONG, SHAOBIN - North Dakota State University; ALI, SHAUKAT - South Dakota State University; MOOLHUIJZEN, PAULA - Curtin University; MOFFAT, CAROLINE - Curtin University; RASMUSSEN, JACK - North Dakota State University; Friesen, Timothy; Faris, Justin; LIU, ZHAOHUI - North Dakota State University

Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/27/2022
Publication Date: 1/31/2022
Citation: Shi, G., Kariyawasam, G., Liu, S., Leng, Y., Zhong, S., Ali, S., Moolhuijzen, P., Moffat, C., Rasmussen, J., Friesen, T.L., Faris, J.D., Liu, Z. 2022. A conserved hypothetical gene is required but not sufficient for Ptr ToxC production in Pyrenophora tritici-repentis. Molecular Plant-Microbe Interactions. 35(4). https://apsjournals.apsnet.org/doi/10.1094/MPMI-12-21-0299-R.
DOI: https://doi.org/10.1094/MPMI-12-21-0299-R

Interpretive Summary: Tan spot of wheat is an economically important disease wherever wheat is grown. At least three major virulence factors exist, including Ptr ToxA, Ptr ToxB and Ptr ToxC. We currently have a fair understanding of Ptr ToxA and Ptr ToxB, however, information on Ptr ToxC has been severely lacking. Here we make significant progress on understanding the genetics of Ptr ToxC production, a first step in understanding how the tan spot pathogen uses this virulence factor to cause disease that regularly results in significant yield and quality losses for wheat growers. A better understanding of this important virulence factor is critical to wheat pathologists and breeders in making recommendations for control of tan spot of wheat.

Technical Abstract: The fungus Pyrenophora tritici-repentis is the causal agent of tan spot, an important foliar disease of wheat worldwide. The fungal pathogen is known to produce three necrotrophic effectors, namely Ptr ToxA, Ptr ToxB and Ptr ToxC to induce necrosis or chlorosis by interacting with their corresponding host sensitivity genes. Both Ptr ToxA and Ptr ToxB are proteins, and the fungal genes encoding them have been cloned and validated through purification and forward genetics. Ptr ToxC was characterized as a low molecular weight molecule 20 years ago but the gene(s) and mechanisms controlling its production in Ptr are still unknown. Here, we report the genetic mapping, molecular cloning and functional analysis of a fungal gene that is required for Ptr ToxC production. The genetic locus controlling Ptr ToxC, termed as ToxC, was first mapped to a subtelomeric region in Ptr using segregating bi-parental populations, genome sequencing, and association analysis. Additional marker analysis further delimited ToxC to a 173 kb region. The predicted genes in the region were examined for presence/absence polymorphism in different races/isolates, leading to the identification of a single candidate gene. Functional validation showed that this gene was required but not sufficient for the Ptr ToxC production, thus it is designated as ToxC1. ToxC1 encodes a conserved hypothetical protein predicated to be located on the vacuole membrane. The gene is highly expressed during infection and has only one haplotype present in 115 isolates we sequenced. Our work strongly suggests that Ptr ToxC is not a protein and is likely produced through a biosynthetic pathway. The identification of ToxC1 is a major first step toward revealing the Ptr ToxC biosynthetic pathway and studying its molecular interactions with host factors.