Pyrenophora tritici–repentis in Tunisia: Race Structure and Effector Genes

Abstract

Tan spot is a destructive foliar wheat disease worldwide and caused by the ascomycete fungus Pyrenophora tritici-repentis (Ptr); it has become more frequent in Tunisia over the last decade. In this study, the virulence of 73 single-spore isolates, collected from durum and bread wheat fields during 2017-2018 growing season, was evaluated on four differential wheat genotypes. This was followed by polymerase chain reaction tests with specific primers for the effector genes ToxA, ToxB, and toxb (ToxB-homolog). Sequence analysis to validate the identity of the amplified genes was followed, and ToxA amplicons from a subset of 22 isolates were analyzed to determine its haplotype identity. Ptr isolates from Tunisia were grouped in races 2, 4, 5, and 7, and 44% of the tested isolates did not fit under any known race, and were denoted here as atypical. These atypical isolates induced the same symptoms as race 7 isolates, extensive necrosis, and chlorosis on susceptible genotypes, but lacked the ToxA gene. ToxA is the only identified necrosisinducing effector in Ptr, and was amplified in 51% of tested isolates, and shared identical sequence to previously identified haplotype (H15). ToxB and its homolog toxb were present in 97% and 93% of tested isolates, respectively. Ptr in Tunisia lacked Ptr ToxC activity, and none of the tested isolates induced the specific symptoms of that effector. Race 7 and the atypical isolates dominated the Tunisian Ptr population, while races 2, 4, and 5 were found at low percentages. In conclusion, ToxB and its homolog were the most dominant genes in Ptr from Tunisia, and the majority of the isolates induced necrosis and chlorosis on Ptr ToxA and Ptr ToxB susceptible wheat genotypes. However, only about half of that necrosis can be attributed to ToxA presence, this result necessitates further research to investigate the prevalence of additional necrotic effector(s). Terminology: in this paper, Pyrenophora tritici-repentis abbreviated as Ptr, the effectors are referred to by Ptr ToxA, Ptr ToxB and Ptr ToxC, and the genes coding for them are written in italic as ToxA, ToxB, and ToxC, respectively.

Determining the presence of host specific toxin genes, ToxA and ToxB, in New Zealand Pyrenophora tritici-repentis isolates, and susceptibility of wheat cultivars

“…Further, races 1 and 2 are reported to be the predominant P. tritici-repentis races present in wheat growing areas in Canada (Lamari et al 1998;Lamari et al 2005;Aboukhaddour et al 2013), USA (Friesen et al 2005), Baltic States and Romania (Abdullah et al 2017) and South America (Gamba et al 2012;Bertagnolli et al 2019). In contrast, in North Africa, P. tritici-repentis populations in Morocco and Tunisia were reported to be mostly positive for ToxB (Gamba et al 2017;Kamel et al 2019). Kamel et al (2019) reported that whilst ToxA was only detected in 51% of isolates, ToxB was amplified in 97% of the population.…”

“…In contrast, in North Africa, P. tritici-repentis populations in Morocco and Tunisia were reported to be mostly positive for ToxB (Gamba et al 2017;Kamel et al 2019). Kamel et al (2019) reported that whilst ToxA was only detected in 51% of isolates, ToxB was amplified in 97% of the population. The New Zealand populations of P. tritici-repentis screened in this study were isolated from symptomatic wheat leaves, with both necrosis and extensive chlorosis observed in the field indicating the isolates were capable of inducing chlorosis on all wheat cultivars sampled (Weith 2015).…”

Sensitivity genes in wheat and corresponding effector genes in necrotrophs exhibiting inverse gene-for-gene relationship

“…PCR was also performed to examine the frequencies of each of the three genes (ToxA, ToxB and toxB), but not for ToxC, which has not been cloned and sequenced, so that primers could not be designed for this gene. ToxA was present in 37 (51%) isolates, ToxB was present in 71 (97%) isolates with its homolog toxB present in 68 (93%) isolates (Kamel et al 2019). A subset of 22 isolates was also subjected to sequencing resolving sequence polymorphism in ToxA and ToxB/toxB (Kamel et al 2019).…”

“…ToxA was present in 37 (51%) isolates, ToxB was present in 71 (97%) isolates with its homolog toxB present in 68 (93%) isolates (Kamel et al 2019). A subset of 22 isolates was also subjected to sequencing resolving sequence polymorphism in ToxA and ToxB/toxB (Kamel et al 2019). (ii) In Australia, races 1 and 2 are the most predominant races, although races 5-8 are also common, which were rarely found in the Americas.…”

“…(v) In the centre of origin of wheat (Middle East, North Africa, and the Caucasus regions), the situation was rather complex, with the occurrence of all the eight races and all the three effectors. (vi) In Western Canada, race composition varied, when distribution of 144 Ptr isolates was examined in durum wheat, bread wheat and grasses; R1 and R2 were common in wheat and occurred in equal frequencies in durum wheat; R1 was twice as frequent as R2 on winter bread wheat; R3 was recovered only from durum wheat and R4 was the only race recovered from grasses (Wet et al 2021) (for a Review and References, see Kamel et al 2019).…”