The RAP1 Gene Confers Effective, Race-Specific Resistance to the Pea Aphid in Medicago truncatula Independent of the Hypersensitive Reaction

Abstract

Plant resistance to pathogens is commonly associated with a hypersensitive response (HR), but the degree to which the HR is responsible for incompatibility is subject to debate. Resistance to aphids is likely to share features with resistance to pathogens but is less well understood. Here, we report effective resistance to the pea aphid Acyrthosiphon pisum in Medicago truncatula. Aphids lost weight and died rapidly (within two days) on the resistant genotype Jemalong, which developed necrotic lesions following infestation. Lesions were induced by nonvascular intracellular stylet punctures by aphids, remained localized to the site of stylet entry, stained for the presence of reactive oxygen species, and were similar to the HR induced by the bacterial pathogen Pseudomonas syringae pv. phaseolicola. The implication that aphid-induced lesions confer resistance was tested by quantitative trait loci analysis using recombinant inbred lines derived from a cross between Jemalong and the susceptible genotype DZA315.16. One major locus, RAP1, was identified that was sufficient to confer race-specific resistance against the pea aphid and was mapped to the middle of chromosome 3. Surprisingly, a separate locus, mapping to the top of chromosome 3, governed aphid-induced HR, indicating that the HR-like lesions are not required for RAP1-mediated aphid resistance.

Cryptic Virulence and Avirulence Alleles Revealed by Controlled Sexual Recombination in Pea Aphids

“…The F 1 progeny populations from PS01 selfing and both PS01 3 N116 crosses all induced necrosis on A17 host plants but almost never on DZA315.16 (Table S1). Because the PS01 3 N116 clones segregated 1:1 for virulence on A17, it is clear that necrosis is not a necessary element of host resistance, confirming previous data of Stewart et al (2009). Further evidence for the disconnect between necrosis and resistance came from the NILs, which lack the functional AIL allele: here the full spectrum of resistance was found despite the complete absence of induced necrotic symptoms from any aphid clones on either rNIL or sNIL hosts.…”

“…However, because all cross and self progeny were virulent on DZA315.16, it is clear that V is not an essential prerequisite for virulence on all M. truncatula hosts. Previous QTL mapping indicated that a large proportion of the PS01 resistance resides at the RAP1 locus carried by A17 (Stewart et al 2009), and we therefore concluded that the dominant virulence allele V is required for suppression of RAP1-dependent resistance. On this basis, V and RAP1 likely represent a gene-for-gene interaction.…”

“…Seeds of NILs derived from a mapping population from a cross between A17 and DZA315.16 were generated by Stewart et al (2009). Seeds were scarified with sandpaper and soaked in distilled water for 3-4 hr at room temperature and then incubated overnight on damp filter paper in petri dishes in darkness at 4°.…”

“…In this study, we report the inheritance of aphid virulence and avirulence functions using the pea aphid-M. truncatula model. The RAP1 gene in M. truncatula confers race-specific resistance to pea aphid clone PS01 but is ineffective against clone LL01 (Stewart et al 2009). Similar to LL01, another clone, N116, was virulent on RAP1 genotypes and on a wide range of other natural and cultivated M. truncatula genotypes (Kanvil et al 2014).…”

“…Jemalong A17 and DZA315.16 are highly polymorphic and have been used as parents to generate recombinant inbred lines (RILs) developed from an F 2 mapping population (Thoquet et al 2002;Torregrosa et al 2004). Mapping of QTL from this population identified two loci (RAP1 and RAP2) conferring pea aphid resistance and a third independent locus controlling a hypersensitive-like (HR-like) response that results in aphid-induced lesions (AIL) (Stewart et al 2009). From the RILs, near-isogenic lines (NILs) were developed that differed at the RAP1 genomic region, enabling further testing of virulence-resistance relationships.…”