QTL Analysis for Adventitious Rooting and Assessing the Phloem-to-xylem Ratio: Comprehensive Characterization of Adventitious Rooting in Apple Rootstocks

Dwarfing rootstocks are widely used in apple cultivation. We focused on adventitious rooting, crucial for vegetative propagation, and the phloem-to-xylem ratio, an indicator of dwarfing. Malus prunifolia and M. domestica ‘M.9’ were used as vigorous rootstocks with adventitious rooting ability and dwarf rootstocks without it, respectively. QTL analysis showed that QTLs for adventitious rooting (Cp) and the phloem-to-xylem ratio (Rb4) were located in the same genomic region, suggesting a potential relationship between the traits. The exon-intron structure, cDNA, and predicted amino acid sequences of the genes in the regions narrowed down by fine mapping for Cp were analyzed along with their expressions during cutting. Long transposable element-like insertions were found in MD17G1113200 and MD17G1113300 on the M. prunifolia genome unlike M. domestica. The gene product of MD17G1113300 in M. prunifolia was predicted to be 384 amino acids shorter than that of M. domestica in the C-terminal region, with different expression levels between the species during cutting. In MD17G1113900, functional amino acid substitutions and different expression levels during cutting were found between the species. When comparing other candidate genes for Cp between the species, different expression levels during cutting were found in MD17G1113400, MD17G1113500, and MD17G1113600. The physiological roles of these candidate genes and their relationships with adventitious rooting ability are discussed. Stereomicroscopic observation of the bases of cuttings revealed distinct morphological characteristics of M. prunifolia compared with M. domestica in terms of callus formation and rooting from lenticels and calli. Fully understanding the mechanisms determining adventitious root formation and developing highly accurate DNA markers are anticipated in the near future, and this paper will serve as a foundation for these achievements.