AUTHOR=Liu Jun-Jun , Sniezko Richard A. , Sissons Robert , Krakowski Jodie , Alger Genoa , Schoettle Anna W. , Williams Holly , Zamany Arezoo , Zitomer Rachel A. , Kegley Angelia 

TITLE=Association Mapping and Development of Marker-Assisted Selection Tools for the Resistance to White Pine Blister Rust in the Alberta Limber Pine Populations

JOURNAL=Frontiers in Plant Science

VOLUME=11

YEAR=2020

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2020.557672

DOI=10.3389/fpls.2020.557672

ISSN=1664-462X

ABSTRACT=<p>Since its introduction to North America in the early 1900s, white pine blister rust (WPBR) caused by the fungal pathogen <italic>Cronartium ribicola</italic> has resulted in substantial economic losses and ecological damage to native North American five-needle pine species. The high susceptibility and mortality of these species, including limber pine (<italic>Pinus flexilis</italic>), creates an urgent need for the development and deployment of resistant germplasm to support recovery of impacted populations. Extensive screening for genetic resistance to WPBR has been underway for decades in some species but has only started recently in limber pine using seed families collected from wild parental trees in the USA and Canada. This study was conducted to characterize Alberta limber pine seed families for WPBR resistance and to develop reliable molecular tools for marker-assisted selection (MAS). Open-pollinated seed families were evaluated for host reaction following controlled infection using <italic>C. ribicola</italic> basidiospores. Phenotypic segregation for presence/absence of stem symptoms was observed in four seed families. The segregation ratios of these families were consistent with expression of major gene resistance (MGR) controlled by a dominant R locus. Based on linkage disequilibrium (LD)-based association mapping used to detect single nucleotide polymorphism (SNP) markers associated with MGR against <italic>C. ribicola</italic>, MGR in these seed families appears to be controlled by <italic>Cr4</italic> or other R genes in very close proximity to <italic>Cr4</italic>. These associated SNPs were located in genes involved in multiple molecular mechanisms potentially underlying limber pine MGR to <italic>C. ribicola</italic>, including NBS-LRR genes for recognition of <italic>C. ribicola</italic> effectors, signaling components, and a large set of defense-responsive genes with potential functions in plant effector-triggered immunity (ETI). Interactions of associated loci were identified for MGR selection in trees with complex genetic backgrounds. SNPs with tight Cr4-linkage were further converted to TaqMan assays to confirm their effectiveness as MAS tools. This work demonstrates the successful translation and deployment of molecular genetic knowledge into specific MAS tools that can be easily applied in a selection or breeding program to efficiently screen MGR against WPBR in Alberta limber pine populations.</p>