Co-located quantitative trait loci for resistance to Agrobacterium tumefaciens, Phytophthora cinnamomi, and P. pini in Juglans microcarpa × J. regia hybrid rootstock

Authors: RAMASAMY, RAMESH - University Of California, Davis; LUO, MING-CHENG - University Of California, Davis; LESLIE, CHARLES - University Of California, Davis; Velasco, Dianne; Ott, Natalia; McClean, Ali; DANDEKAR, ABHAYA - University Of California, Davis; Aradhya, Mallikarjuna; BROWN, PATRICK - University Of California, Davis; Browne, Greg; Kluepfel, Daniel; WESTPHAL, ANDREAS - University Of California; DVORAK, JAN - University Of California, Davis

Submitted to: Horticulture Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/26/2021
Publication Date: 5/1/2021
Citation: Ramasamy, R.K., Luo, M., Leslie, C.A., Velasco, D.M., Ott, N.J., McClean, A.E., Dandekar, A.M., Aradhya, M.K., Brown, P.J., Browne, G.T., Kluepfel, D.A., Westphal, A., Dvorak, J. 2021. Co-located quantitative trait loci for resistance to Agrobacterium tumefaciens, Phytophthora cinnamomi, and P. pini in Juglans microcarpa × J. regia hybrid rootstock. Horticulture Research. 8. Article 111. https://doi.org/10.1038/s41438-021-00546-7.
DOI: https://doi.org/10.1038/s41438-021-00546-7

Interpretive Summary:

Technical Abstract: Soil-borne plant pathogens represent a serious threat to commercial walnut production. Grafting a scion variety onto a resistant rootstock is a proven strategy for dealing with this hazard. Here, we describe genetic mapping of resistance to Agrobacterium tumefaciens and two species of Phytophthora in Juglans microcarpa, a North American wild relative of cultivated Persian walnut (J. regia). Two half-sib J. microcarpa mother trees were crossed with J. regia cv. Serr, producing 353 and 400 hybrids. The hybrids were used in SNP marker development by genotyping-by-sequencing and mapping. Two genetic maps were constructed for J. microcarpa and two were constructed for Serr. The maps based on male meiosis (J. regia) were about 20% longer than the maps based on female meiosis (J. microcarpa). Similar sex-related differences in genetic map lengths were observed within J. regia. The hybrids were clonally propagated, and clones were used in the assessment of resistance to A. tumefaciens, P. cinnamomi, and P. pini. A single quantitative trait locus (QTL) was mapped for each pathogen in the same short region in the middle of the long arm of J. microcarpa chromosome 4D. Resistance to all three pathogens was associated with the same haplotype. These features of resistance to a bacterial pathogen and two Oomycetes pathogens and markers for selection of resistant progeny will greatly accelerate breeding of resistant rootstocks to sustain commercial walnut production.