Effects of the Lr34 and Lr46 rust-resistance genes on other diseases of wheat
Lr34 and Lr46 are adult plant resistance genes providing durable resistance to biotrophic diseases of wheat like rusts and powdery mildew. In seedlings, these genes increased susceptibility to Septoria tritici blotch (STB), caused by Zymoseptoria tritici, in near-isogenic lines (NILs) of spring wheats Lal Bahadur (LB) and Avocet and mutant lines developed from LB, but not in an Lr34 NIL of Jupateco. A similar effect was observed in adult plants with artificial inoculation in polytunnels. The role of leaf age in Lr34 and Lr46 resistance to mildew and susceptibility to STB was tested. It was hypothesised that enhanced senescence can make leaves more resistant to biotrophs but more susceptible to necrotrophs. In young leaves, LB was less susceptible to STB than Lr34 or Lr46 NILs. The opposite pattern was observed in older leaves. There was higher expression of genes associated with senescence and cell death in LB-Lr34 than in LB, indicating that Lr34 may enhance senescence. Metabolites associated with senescing leaves accumulated to a higher level in Z.tritici-infected LB-Lr34 NILs than in LB. In seedlings, Lr34 and Lr46 favoured the non-biotrophic fungi Magnoporthe grisea, the wheat blast pathogen, and Ramularia collo-cygni, the Ramularia leaf spot pathogen of barley. Lr34 reduced spot blotch (Cochliobolus sativus) in field trials but no conclusive results were obtained for tan spot (Pyrenophora tritici-repentis) or Fusarium head blight (FHB; Fusarium graminearum). These results presented indicate that there may be significant consequences for the use of Lr34 and Lr46 to control rust and mildew in areas where necrotrophic diseases are prevalent, including some which have not previously been economically significant on wheat, and that plant breeding strategies to control multiple diseases simultaneously are required. They also indicate that the potential to breed varieties in which the adverse effects of these genes are mitigated.
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