Toward Association Mapping of Candidate Genes With Drought Tolerance in Perennial Ryegrass.

Association mapping identifies quantitative trait loci (QTLs) by examining marker-trait associations at a natural population level. It has been developed as a novel, powerful, and high resolution mapping technique for fine mapping of traits, identifying functional markers, and validating genes and alleles of interest. The overall goal of this project is to use the candidate gene-association mapping approach to identify genes/markers linking to drought tolerance in perennial ryegrass (Lolium perenne L.). A natural population of 200 perennial ryegrass accessions from 43 countries was obtained from the USDA National Plant Germplasm System. The ploidy level was tested using the flow cytometry technique, and 192 diploid accessions were used as the mapping population. The initial screening for drought tolerance in a greenhouse showed that approximately 10 % and 12 % of these accessions exhibited good and poor drought tolerance, respectively. The diploid accessions were propagated by tillers and transferred to the field for further evaluating their drought tolerance. A total of 105 simple sequence repeat (SSR) markers were used for amplifying DNA in 192 accessions to assess the genetic diversity and population structure. The results from the cluster and principal component analysis showed that the perennial ryegrass germplasm panel was diverse without forming major population groups, suggesting that this collection would be an ideal population for association mapping between candidate genes and phenotypic traits. A list of 80 key genes that play an important role in drought resistance were collected and used for association analyses of candidate genes with drought tolerance in perennial ryegrass using the unified mixed model. Association mapping possesses great potential for complex trait dissection.