A highly diversified NLR cluster in melon contains homologs that confer powdery mildew and aphid resistance

Abstract

Podosphaera xanthii is the main causal agent of powdery mildew (PM) on Cucurbitaceae. In Cucumis melo, the Pm-w resistance gene, which confers resistance to P. xanthii, is located on chromosome 5 in a cluster of nucleotide-binding leucine-rich repeat receptors (NLRs). We used positional cloning and transgenesis, to isolate the Pm-w^{WMR 29} gene encoding a coiled-coil NLR (CC-NLR). Pm-w^{WMR 29} conferred high level of resistance to race 1 of PM and intermediate level of resistance to race 3 of PM. Pm-w^{WMR 29} turned out to be a homolog of the Aphis gossypii resistance gene Vat-1^{PI 161375}. We confirmed that Pm-w^{WMR 29} did not confer resistance to aphids, while Vat-1^{PI 161375} did not confer resistance to PM. We showed that both homologs were included in a highly diversified cluster of NLRs, the Vat cluster. Specific Vat-1^{PI 161375} and Pm-w^{WMR 29} markers were present in 10% to 13% of 678 accessions representative of wild and cultivated melon types worldwide. Phylogenic reconstruction of 34 protein homologs of Vat-1^{PI 161375} and Pm-w^WMR 29 identified in 24 melon accessions revealed an ancestor with four R65aa—a specific motif in the LRR domain, evolved towards aphid and virus resistance, while an ancestor with five R65aa evolved towards PM resistance. The complexity of the cluster comprising the Vat/Pm-w genes and its diversity in melon suggest that Vat homologs may contribute to the recognition of a broad range of yet to be identified pests and pathogens.

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