Characterization of virulence in Pyrenophora teres f. maculata

Characterization of virulence in Pyrenophora teres f. maculata

Net blotches are common foliar diseases of barley caused by the fungus Pyrenophora teres. There are two forms of net blotch. The net form net blotch (NFNB), caused by P. teres f. teres (Ptt) and the spot form net blotch (SFNB), caused by P. teres f. maculata (Ptm). Spot form net blotch is a major foliar disease of barley worldwide. In Australia, annual losses to the barley industry caused by SFNB are estimated to be $192 million. Grain quality parameters such as kernel size, bulk density and plumpness are also negatively affected by SFNB. To date seven studies have been conducted to study the virulence/avirulence associated genomic regions in Ptt while only one study has been studied the virulence associated genomic regions in Ptm. It is important to understand the genetics of Ptm virulence to manage SFNB resistance in barley varieties. Most of our current understanding of Ptm-barley interactions relates to the genomic regions involved in resistance of the barley host. Very little is known about the virulence factors of Ptm. This project investigated the genomic regions conferring virulence in Ptm isolates using genome wide association (GWAS) and quantitative trait loci (QTL) mapping. For GWAS, one hundred and twenty-one Ptm isolates from Australia were genotyped and phenotyped across 12 barley genotypes. Ten novel quantitative trait loci (QTL) associated with virulence in Ptm were identified. We also identified 125 candidate genes including, two genes encoding putative effector proteins. For QTL mapping, a bi-parental population was generated by crossing two Australian Ptm isolates. Progeny was genotyped and phenotyped across four barley genotypes. Genetic maps of 12 chromosomes were constructed and two QTL regions were found to be associated with virulence. Eight signal peptides were identified, of which five were small proteins. Five non-ribosomal peptide synthetase clusters were identified. These could have a potential role in the virulence and need further study.

Hybridisation between the two forms of P.teres has previously been documented but appears to be rare in nature. Hybrids may not be correctly identified as they cause symptoms similar to those of the parents or may be avirulent in nature. While rare, the occurrence of hybrids with novel virulences is of concern as they may overcome deployed sources of resistance. Production of hybrids in vitro is relatively easy as indicated by recent papers (Dahanayaka et al., 2021).To study the virulence of hybrids, we made four in-vitro crosses between Ptt and Ptm. The progeny populations were phenotyped across 20 barley genotypes. A total of 62 hybrid-barley interactions were virulent on barley genotypes to which parent isolates had avirulent responses. Twenty-seven progeny expressed greater average infection responses compared to the parental isolates. This study showed that novel virulences can be generated as a result of Ptt and Ptm hybridisation and could be a potential threat to sources of host-plant resistance in commercial varieties.

In summary, this study investigated genomic regions conferring virulence to Ptm. In addition, we also studied the virulence of hybrid isolates. Genomic sequence comparison of hybrids and parents can help us better understand the genetics of pathogenicity and provide insights into the genetic relationship between parents and hybrids.

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