Non-host resistance to the finger millet blast pathogen Magnaporthe oryzae is modulated by a resistance response to PWL1 and PWL2 in Eragrostis curvula but not in related Chloridoid species

Abstract

Magnaporthe oryzae, a devastating pathogen of finger millet (Eleusine coracana), secretes effector molecules during infection to manipulate host immunity. This study determined the presence of avirulence effector genes PWL1 and PWL2 in 221 Eleusine blast isolates from eastern Africa. Most Ethiopian isolates carried both PWL1 and PWL2. Kenyan and Ugandan isolates largely lacked both genes, and Tanzanian isolates carried either PWL1 or lacked both. The roles of PWL1 and PWL2 towards pathogenicity on alternative Chloridoid hosts, including weeping lovegrass (Eragrostis curvula), were also investigated. PWL1 and PWL2 were cloned from Ethiopian isolate E22 and transformed separately into Ugandan isolate U34, which lacked both genes. Resulting transformants harboring either gene gained varying degrees of avirulence on E. curvula but remained virulent on finger millet. Strains carrying PWL1 and/or PWL2 infected the Chloridoid species Sporobolus phyllotrichus and Eleusine tristachya, indicating the absence of cognate resistance (R) genes for PWL1 and PWL2 in these species. Other Chloridoid grasses, however, were fully resistant, regardless of the presence of PWL1 and/or PWL2, suggesting the presence of effective R genes against PWL and/or other effectors. Partial resistance in some E. curvula accessions to some blast isolates lacking PWL1 and PWL2 also indicated the presence of other AVR-R interactions. Related Chloridoid species thus harbour resistance genes that could be useful to improve finger millet for blast resistance. Conversely, loss of AVR genes in the fungus could expand its host range, as demonstrated by E. curvula’s susceptibility to finger millet blast isolates that had lost PWL1 and PWL2.