Two quantitative trait loci, Dw1 and Dw2, are primarily responsible for rootstock-induced dwarfing in apple

The apple dwarfing rootstock‘Malling9’(‘M9’)has been used worldwide both to reduce scion vigour and as a genetic source for breeding new rootstocks.Progeny of‘M9’segregate for rootstock-induced dwarfing of the scion,indicating that this trait is controlled by one or more genetic factors.A quantitative trait locus(QTL)analysis of a rootstock population derived from the cross between‘M9’בRobusta5’(non-dwarfing)and grafted with‘Braeburn’scions identified a major QTL(Dw1)on linkage group(LG)5,which exhibits a significant influence on dwarfing of the scion.A smaller-effect QTL affecting dwarfing(Dw2)was identified on LG11,and four minor-effect QTLs were found on LG6,LG9,LG10 and LG12.Phenotypic analysis indicates that the combination of Dw1 and Dw2 has the strongest influence on rootstock-induced dwarfing,and that Dw1 has a stronger effect than Dw2.Genetic markers linked to Dw1 and Dw2 were screened over 41 rootstock accessions that confer a range of effects on scion growth.The majority of the dwarfing and semi-dwarfing rootstock accessions screened carried marker alleles linked to Dw1 and Dw2.This suggests that most apple dwarfing rootstocks have been derived from the same genetic source.

Genome-wide association studies in apple reveal loci of large effect controlling apple polyphenols

Apples are a nutritious food source with significant amounts of polyphenols that contribute to human health and wellbeing,primarily as dietary antioxidants.Although numerous pre-and post-harvest factors can affect the composition of polyphenols in apples,genetics is presumed to play a major role because polyphenol concentration varies dramatically among apple cultivars.Here we investigated the genetic architecture of apple polyphenols by combining high performance liquid chromatography(HPLC)data with~100,000 single nucleotide polymorphisms(SNPs)from two diverse apple populations.We found that polyphenols can vary in concentration by up to two orders of magnitude across cultivars,and that this dramatic variation was often predictable using genetic markers and frequently controlled by a small number of large effect genetic loci.Using GWAS,we identified candidate genes for the production of quercitrin,epicatechin,catechin,chlorogenic acid,4-O-caffeoylquinic acid and procyanidins B1,B2,and C1.Our observation that a relatively simple genetic architecture underlies the dramatic variation of key polyphenols in apples suggests that breeders may be able to improve the nutritional value of apples through markerassisted breeding or gene editing.