Research Progress on Genetic Basis of Fruit Quality Traits in Apple (Malus × domestica)

Abstract

Identifying the genetic variation characteristics of phenotypic traits is important for fruit tree breeding. During the long-term evolution of fruit trees, gene recombination and natural mutation have resulted in a high degree of heterozygosity. Apple (Malus × domestica Borkh.) shows strong ecological adaptability and is widely cultivated, and is among the most economically important fruit crops worldwide. However, the high level of heterozygosity and large genome of apple, in combination with its perennial life history and long juvenile phase, complicate investigation of the genetic basis of fruit quality traits. With continuing augmentation in the apple genomic resources available, in recent years important progress has been achieved in research on the genetic variation of fruit quality traits. This review focuses on summarizing recent genetic studies on apple fruit quality traits, including appearance, flavor, nutritional, ripening, and storage qualities. In addition, we discuss the mapping of quantitative trait loci, screening of molecular markers, and mining of major genes associated with fruit quality traits. The overall aim of this review is to provide valuable insights into the mechanisms of genetic variation and molecular breeding of important fruit quality traits in apple.

Keywords: QTLs; apple; genes; genetic characteristics; quality traits.

FIGURE 1

Quantitative trait loci (QTLs), molecular markers, major-effect genes, and miRNAs associated with apple fruit weight. The main QTLs associated with fruit weight include fw1, fw2, and fszg08.8 (Devoghalaere et al., 2012; Chang et al., 2014). The major-effect genes include the auxin response factors MdARF6 and MdARF106, and APETALA 2 family TF MdAP2 (Devoghalaere et al., 2012; Yao et al., 2015). The main miRNAs include miR172g, miR172h, and miR172p (Yao et al., 2015; Duan et al., 2017). The function of miR172p has been elucidated in transgenic “Royal Gala” apple.

FIGURE 2

Red pigmentation of apple fruit skin and flesh. (A) A gypsy-like long terminal repeat retrotransposon (designated redTE) was inserted 3297 bp upstream of MdMYB1, thereby activating the expression of MdMYB1 and controlling the redness of the fruit skin. (B) MdAGO4s, MdDRM2s, and MdRDM1 interact with each other and form an effector complex. MdAGO4s recruit MdDRM2s, which catalyze CHH methylation of the MdMYB1 promoter. MdMYB1 then regulates anthocyanin accumulation to determine the coloration. M, a -CH₃ (methyl); ABS, AGO4 binding sequence. (C) Model showing autoregulation of the R6 and R1 promoters by MdMYB10. The MdMYB10 promoter in red-fleshed apple contains six 23 bp repeating microsatellite sequences (R6), which confer MdMYB10 with self-activation. The MdMYB10 promoter in white-fleshed apple contains only one 23 bp repeating microsatellite sequence. (D) MdMYB10 and its homolog MdMYB110a are involved in the red pigmentation of type I and type II red-fleshed apples, respectively.