| Apple [Malus domestica Borkh.) is one of the most important fruit crops in the world. Soluble sugars and organic acids are the major components of fruit tastes, and have a strong impact on the overall organoleptic quality of fruits. In this study, soluble sugars and malic acid contents were quantified using high-performance liquid chromatography (HPLC) in mature apple fruits. Consequently, association mapping was conducted to investigate the association between acidity and candidate genes in apple germplasm. The main results were as follows:1. The phenotype database consists of soluble sugars and malic acid contents in mature fruits of 321 cultivated apple and 43 wild relatives was constructed. Soluble sugars are mainly composed of sucrose, fructose and glucose, fructose and sucrose represented the major components of soluble sugars in cultivated apple fruits, whilst fructose and glucose were the major items of sugars in wild apple fruits. Malic acid is the predominant organic acid in appe fruits, and wild apple fruits were significantly more acidic than cultivated apple fruits, whilst the average concentration of total sugars and sweetness index were quite similar between cultivated and wild apple fruits. Thus, acidty played an important role during apple flavor quality domestication.2. Malic acid content was positively correlated with glucose content and negatively correlated with sucrose content in apple. This suggests that selection of fruit acidity could have an effect on the proportion of soluble sugar components in apple fruits.3. A major QTL (designated Ma) for acidity has been mapped on the linkage group 16 in apple, a gene encoding aluminum-activated malate transporter (ALMT) was identified as the candidate gene Ma1 in the Ma locus using region based association mapping. The Ma locus consists of two alleles, Ma1 and mal. Ma1 resides in the tonoplast and its ectopic expression in yeast was found to significantly increase the influx of malic acid into yeast cells, suggesting it may function as a vacuolar malate channel and can increase the malate concenteation in fruit. In contrast, mal encodes a truncated protein (loss 84 amino acid) because of a single nucleotide substitution of G with A in the last exon. Since this truncated protein resides within the cell membrane, it is deemed to be nonfunctional as a vacuolar malate channel result in low acidity.4. In order to analysis whether the homologous genes of Mal had an effect on the acidity of apple, the apple ALMT genes can be divided into three families and the Mal gene belongs to the ALMTâ…¡ family. The ALMT â…¡ genes taged molecular marker were developed, and candiadate gene association analysis was conducted. The result shown that only the Mal gene is significantly associated with malic acid content.5. The genotype of Ma locus was analyzed in apple germplasm, and the frequency of the Ma1Ma1 genotype is very low in apple cultivars but is high in wild relatives, which suggests that apple domestication may be accompanied by selection for the Mal gene. In addition, variations in the malic acid content of mature fruits were also observed between accessions with the same genotype in the Ma locus. This therefore suggests that the Ma gene is not the only genetic determinant of fruit acidity in apple.All results above not only provide theoretical basis and material security for the genetic improvement of apple fruit quality, but also offer a new strategy for identifying the genes involved in complex quantitative traits in other fruit species. |
PDF Full Text Request