Abstract
Highly specialized obligate plant-parasites exist within several groups of arthropods (insects and mites). Many of these are important pests, but the molecular basis of their parasitism and its evolution are poorly understood. One hypothesis is that plant parasitic arthropods use effector proteins to defeat basal plant immunity and modulate plant growth. Because avirulence (Avr) gene discovery is a reliable method of effector identification, we tested this hypothesis using high-resolution molecular genetic mapping of an Avr gene (vH13) in the Hessian fly (HF, Mayetiola destructor), an important gall midge pest of wheat (Triticum spp.). Chromosome walking resolved the position of vH13, and revealed alleles that determine whether HF larvae are virulent (survive) or avirulent (die) on wheat seedlings carrying the wheat H13 resistance gene. Association mapping found three independent insertions in vH13 that appear to be responsible for H13-virulence in field populations. We observed vH13 transcription in H13-avirulent larvae and the salivary glands of H13-avirulent larvae, but not in H13-virulent larvae. RNA-interference-knockdown of vH13 transcripts allowed some H13-avirulent larvae to escape H13-directed resistance. vH13 is the first Avr gene identified in an arthropod. It encodes a small modular protein with no sequence similarities to other proteins in GenBank. These data clearly support the hypothesis that an effector-based strategy has evolved in multiple lineages of plant parasites, including arthropods.
Highlights
Many gene-for-gene interactions are manifestations of the biological interplay that occurs between plant resistance proteins and plant pathogen effector proteins [1,2,3,4,5]
Fluorescence in situ hybridization (FISH) of bacterial artificial chromosomes (BACs) to the polytene chromosomes of the Hessian fly (HF) was used to test the fidelity of the walk (Figure 1, Table S1)
Genetic analysis performed during the chromosome walk indicated that the likely position of vH13 was between the ends of a single HF BAC (Hf5p7; Figure 1)
Summary
Many gene-for-gene interactions are manifestations of the biological interplay that occurs between plant resistance proteins and plant pathogen effector proteins [1,2,3,4,5]. Plant pathogens use their effector proteins to defeat basal plant immunity and modify plant cell biochemistry and development [6]. The resulting R-protein/effector interaction elicits a plant resistance response called effector-triggered immunity (ETI) [2], which restricts the proliferation of the pathogen. It remains a reliable approach to effector discovery [10]
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