A “bulk phenotyping” method was developed to estimate the proportion of Zymoseptoria tritici strains capable of attacking a wheat variety carrying a specific resistance gene against septoria leaf blotch, one of the most damaging diseases to wheat in Europe (Suffert et al., 2024).
Keeping an eye on dangerous diseases that can attack cultivated plants with certain defence genes is important for prolonging the resistance of these plants and enhancing their long-term protection. In the case of Zymoseptoria tritici, the fungal pathogen responsible for septoria tritici blotch (STB), which exhibits a high level of diversity due to sexual reproduction, wheat varieties possessing a resistance gene are more susceptible to attack as the frequency of the corresponding virulence – i.e. the proportion of disease-causing strains – increases in the Z. tritici population. This frequency increases even more rapidly, and with it the breakdown of resistance, when the gene responsible for it is highly used in fields. Virulence monitoring increasingly involves field pathogenomics studies of molecular polymorphisms in pathogen genomes based on high-throughput screening technologies. However, it is not always straightforward to predict virulence phenotypes from these polymorphisms and in planta phenotyping remains necessary. These phenotypic assessments are generally laborious because estimating virulence frequency involves testing the interaction between a lot of strains and plant lines that carry the targeted gene and others that do not carry it. To combat this researchers developed a method for ‘bulk phenotyping on checkerboard microcanopies of wheat near-isogenic lines’ (BPC), without the need for such a strain-by-strain pathogen phenotyping.
Image shows experimental steps in the method for ‘bulk phenotyping’ on checkerboard microcanopies of wheat.
The BPC process involves applying, simultaneously, a mixture of Z. tritici strains onto two types of wheat lines: one with a specific resistance gene and the other without it. To ensure uniformity in the application, both types of plants are arranged in a checkerboard pattern. Then, the infection points (lesions) on each type of plant are counted and compared to understand how effective the resistance gene is against the different strains. Using Stb16q, a wheat resistance gene that has recently become ineffective (broken down) in Europe, researchers established a strong connection between the average number of lesions on each type of wheat plant and the prevalence of virulent strains in the mixture applied. The principle of the BPC method was validated and standard curves at virulence frequencies were established. The potential of this method for virulence monitoring in combination with molecular methods was highlighted, especially its use in estimating frequencies in analytical trials involving wheat varietal mixtures, as recently investigated by Orellana-Torrejon et al. (2022a, 2022b) to which the operationality of the BPC method would have been very helpful… One appreciates all the more this clever experimental implementation of a simple idea, especially considering the extensive time spent on strain-by-strain phenotyping while no molecular method is yet available. The fundamentals in phytopathology still have a future.
Estimating the frequency of virulence against an Stb gene in Zymoseptoria tritici populations by bulk phenotyping on checkerboard microcanopies of wheat near-isogenic lines
TITLE IMAGE: Wheat leaf with Septoria tritici blotch lesions, representative of the symptoms observed.
REFERENCES:
Orellana-Torrejon C, Vidal T, Boixel A-L, Gélisse S, Saint-Jean S, Suffert F. 2022a. Annual dynamics of Zymoseptoria tritici populations in wheat cultivar mixtures: a compromise between the efficiency and durability of a recently broken-down resistance gene? Plant Pathology 71: 289-303
Orellana-Torrejon C, Vidal T, Saint-Jean S, Suffert F. 2022b. The impact of wheat cultivar mixtures on virulence dynamics in Zymoseptoria tritici populations persist after interseason sexual reproduction. Plant Pathology 71: 1537-1549
Suffert F, Le Prieur S, Gélisse S, Dzialo E, Saintenac C, Marcel TC. 2024. Estimating the frequency of virulence against an Stb gene in Zymoseptoria tritici populations by bulk phenotyping on checkerboard microcanopies of wheat NILs. Plant pathology
