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Linked, if Not the Same, Mi-1 Homologues Confer Resistance to Tomato Powdery Mildew and Root-Knot Nematodes

¹Wageningen UR Plant Breeding, Wageningen University and Research Center, Droevendaalsesteeg 1, 6708PB Wageningen, The Netherlands; ²Department of Nematology, University of California, Riverside 92521, U.S.A.; ³Horticulture College of Northeast Agricultural University, Haerbin 150030, China; ⁴Laboratory of Nematology, Wageningen University, Droevendaalsesteeg 1, 6708PB Wageningen, The Netherlands; ⁵Laboratory of Entomology, Wageningen University, and EPG systems, Dillenburg 12, 6703CJ Wageningen, The Netherlands; ⁶Centre for BioSystems Genomics, The Netherlands Genomics Initiative/Netherlands Organization for Scientific Research, PO Box 98, 6700AA Wageningen, The Netherlands

On the short arm of tomato chromosome 6, a cluster of disease resistance (R) genes have evolved harboring the Mi-1 and Cf genes. The Mi-1 gene confers resistance to root-knot nematodes, aphids, and whiteflies. Previously, we mapped two genes, Ol-4 and Ol-6, for resistance to tomato powdery mildew in this cluster. The aim of this study was to investigate whether Ol-4 and Ol-6 are homologues of the R genes located in this cluster. We show that near-isogenic lines (NIL) harboring Ol-4 (NIL-Ol-4) and Ol-6 (NIL-Ol-6) are also resistant to nematodes and aphids. Genetically, the resistance to nematodes cosegregates with Ol-4 and Ol-6, which are further fine-mapped to the Mi-1 cluster. We provide evidence that the composition of Mi-1 homologues in NIL-Ol-4 and NIL-Ol-6 is different from other nematode-resistant tomato lines, Motelle and VFNT, harboring the Mi-1 gene. Furthermore, we demonstrate that the resistance to both nematodes and tomato powdery mildew in these two NIL is governed by linked (if not the same) Mi-1 homologues in the Mi-1 gene cluster. Finally, we discuss how Solanum crops exploit Mi-1 homologues to defend themselves against distinct pathogens.