Evaluation of a Novel Molecular Marker Associated with the Tan Spot Disease Response in Wheat

Pao Theen See,Caroline S Moffat

doi:10.3390/agriculture11060513

Abstract

After nearly 40 years of DNA molecular marker development in plant breeding, the wheat research community has amassed an extensive collection of molecular markers which have been widely and successfully used for selection of agronomic, physiological and disease resistance traits in wheat breeding programs. Tan spot is a major fungal disease of wheat and a significant global economic challenge and is caused by the necrotrophic fungal pathogen Pyrenophora tritici-repentis (Ptr). Here, the potential for using a PCR-based marker (Ta1AS3422) present on the short arm of wheat chromosome 1A, was evaluated for effectiveness in distinguishing tan spot disease susceptibility. The marker was initially screened against 40 commercial Australian hexaploid wheat varieties, and those that amplified the marker had an overall lower disease score (2.8 ± 0.7 for seedlings and 2.4 ± 0.4 for plants at the tillering stage), compared to those lacking the marker which exhibited a higher disease score (3.6 ± 0.8 for both growth stages). The potential of Ta1AS3422 as a marker for the tan spot disease response was further assessed against a panel of 100 commercial Australian hexaploid wheat varieties. A significant association was observed between marker absence/presence and tan spot disease rating (Pearson’s chi-squared test, χ2 (6) = 20.53, p = 0.002), with absence of Ta1AS3422 associated with susceptibility. This simple and cost-effective PCR-based marker may be useful for varietal improvement against tan spot, although further work is required to validate its effectiveness.

Highlights

Tan spot, known as yellow spot, is a major disease of wheat globally and poses a significant economic challenge
In Australia, the implementation of ToxA-assisted screening by the wheat breeding industry to actively select against varieties possessing the Tsn1 gene, has seen the area sown to ToxA sensitive wheat varieties reduced by approximately 1.4 million hectares from 37.5% in the 2009/10 season to 8.3% by the 2015/16 season [3,6] with a concurrent shift away from varieties that are very susceptible to tan spot towards more moderate resistance
Due to the potent necrosis induced by the prevalent Pyrenophora tritici-repentis (Ptr) effector ToxA, ToxA insensitive varieties (Tsn1) are favoured in Australia [3]

Summary

Introduction

Known as yellow (leaf) spot, is a major disease of wheat globally and poses a significant economic challenge. In the tan spot pathosystem, three Ptr NEs have been described (ToxA, ToxB and ToxC), of which ToxA is the most well studied and has been shown to be a potent effector that causes distinct necrotic symptoms on wheat varieties possessing the ToxA sensitivity gene, Tsn1 [1]. Mutant Ptr strains lacking the ToxA gene (∆toxa) are still pathogenic and able to induce disease responses [3,4], with prominent chlorosis in specific host backgrounds likely the result of other NEs. In Australia, all Ptr isolates examined to date possess the ToxA gene and lack ToxB, while the prevalence of ToxC in Australian Ptr isolates, while present, has not yet been fully determined [3,5]. In Australia, all commercially grown wheat varieties rated as moderately resistant to moderately susceptible (MRMS) or higher to tan spot are ToxA insensitive [7]

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Conclusion