Abstract
The objective of the present study was to investigate to what extent the macroscopic phenotype of incompatible host–pathogen interactions reflects differences in fungal development and host responses at the histological level. This was done by conventional and advanced microscopic analysis of six wheat near‐isogenic lines differing by individual R genes and inoculated with an avirulent isolate of Puccinia striiformis. Wheat line AvocetYr15 had the lowest macroscopic infection type (IT) 0–1, in which fungal growth was stopped at early stages due to extensive expression of a hypersensitive response (HR) at all time points (4, 8 and 16 days post‐inoculation, dpi) without any sign of haustorial bodies. AvocetYr5 and AvocetYr1 had IT 1, in which most fungal colonies developed secondary hyphae. Many colonies were encased by HR at 16 dpi, more extensively in AvocetYr1 than AvocetYr5. In AvocetYr6 (IT 2), HR was expressed after the formation of secondary hyphae at 4 dpi, after which fungal growth and HR increased simultaneously until most colonies became encased by HR. AvocetYr27 (IT 2–3) and AvocetYr17 (IT 4–5) showed similar fungal growth and HR at 4 dpi, where HR was only weakly expressed in a few host cells. Encasement of secondary and runner hyphae increased significantly in AvocetYr27, but at 16 dpi, HR was often circumvented by large, intermingled fungal colonies in both lines. No resistance responses were observed in Avocet S (susceptible control). The very different histological patterns conferred by the six R genes suggests differences in the timing of the host–pathogen recognition and onset of host defence pathways.
Highlights
Yellow or stripe rust caused by the biotrophic fungus Puccinia striiformis f. sp. tritici (Pst) is one of the most destructive diseases of common wheat (Triticum aestivum) worldwide (Beddow et al, 2015)
A pilot study, comprising 36 wheat lines carrying different resistance genes (R genes) inoculated by six wheat-adapted Pst isolates, was executed prior to the main experiment
The isolate selected for the current study showed a high level of compatibility on susceptible control varieties, including the Avocet parental line, coupled with incompatibility for most of the resistant Avocet near-isogenic lines (NILs)
Summary
Yellow or stripe rust caused by the biotrophic fungus Puccinia striiformis f. sp. tritici (Pst) is one of the most destructive diseases of common wheat (Triticum aestivum) worldwide (Beddow et al, 2015). Tritici (Pst) is one of the most destructive diseases of common wheat (Triticum aestivum) worldwide (Beddow et al, 2015). The ability of this biotrophic pathogen to have multiple asexual cycles in a single crop season associated with abundant spore production, long-distance spore migration capacity and a high rate of mutation may contribute to the escalating threat of yellow rust worldwide (Hovmøller et al, 2010). ETI-associated HR is an effective defence strategy for the host plant, against biotrophic pathogens (Glazebrook, 2005), but effector recognition and the HR may vary significantly with respect to time and space (Wang et al, 2013). In a biotrophic host–pathogen interaction, the HR halts pathogen growth manifested by an incompatible phenotype, while in the absence of host–pathogen recognition, the pathogen may grow extensively and develop a compatible reaction
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