Department of Horticulture, New York State Agricultural Experiment Station, Cornell University, Geneva, NY 14456, USA.
Mol Genet Genomics. 2012 Aug;287(8):663-78. doi: 10.1007/s00438-012-0707-7. Epub 2012 Jul 18.
Acidity levels greatly affect the taste and flavor of fruit, and consequently its market value. In mature apple fruit, malic acid is the predominant organic acid. Several studies have confirmed that the major quantitative trait locus Ma largely controls the variation of fruit acidity levels. The Ma locus has recently been defined in a region of 150 kb that contains 44 predicted genes on chromosome 16 in the Golden Delicious genome. In this study, we identified two aluminum-activated malate transporter-like genes, designated Ma1 and Ma2, as strong candidates of Ma by narrowing down the Ma locus to 65-82 kb containing 12-19 predicted genes depending on the haplotypes. The Ma haplotypes were determined by sequencing two bacterial artificial chromosome clones from G.41 (an apple rootstock of genotype Mama) that cover the two distinct haplotypes at the Ma locus. Gene expression profiling in 18 apple germplasm accessions suggested that Ma1 is the major determinant at the Ma locus controlling fruit acidity as Ma1 is expressed at a much higher level than Ma2 and the Ma1 expression is significantly correlated with fruit titratable acidity (R (2) = 0.4543, P = 0.0021). In the coding sequences of low acidity alleles of Ma1 and Ma2, sequence variations at the amino acid level between Golden Delicious and G.41 were not detected. But the alleles for high acidity vary considerably between the two genotypes. The low acidity allele of Ma1, Ma1-1455A, is mainly characterized by a mutation at base 1455 in the open reading frame. The mutation leads to a premature stop codon that truncates the carboxyl terminus of Ma1-1455A by 84 amino acids compared with Ma1-1455G. A survey of 29 apple germplasm accessions using marker CAPS(1455) that targets the SNP(1455) in Ma1 showed that the CAPS(1455A) allele was associated completely with high pH and highly with low titratable acidity, suggesting that the natural mutation-led truncation is most likely responsible for the abolished function of Ma for low pH or high acidity in apple.
酸度水平极大地影响水果的口感和风味，从而影响其市场价值。在成熟的苹果果实中，苹果酸是主要的有机酸。多项研究证实，主要数量性状位点 Ma 主要控制果实酸度水平的变化。Ma 基因座最近在 Golden Delicious 基因组第 16 号染色体上的一个包含 44 个预测基因的 150kb 区域内被定义。在这项研究中，我们通过将 Ma 基因座缩小到包含 12-19 个预测基因的 65-82kb 区域内，确定了两个铝激活的苹果酸转运蛋白样基因 Ma1 和 Ma2，作为 Ma 的强候选基因，具体取决于单倍型。Ma 单倍型通过对来自 G.41（Mama 的苹果砧木基因型）的两个细菌人工染色体克隆进行测序来确定，该克隆覆盖了 Ma 基因座上的两个不同的单倍型。在 18 个苹果种质资源中的基因表达谱分析表明，Ma1 是控制果实酸度的 Ma 基因座的主要决定因素，因为 Ma1 的表达水平远高于 Ma2，并且 Ma1 的表达与果实可滴定酸度显著相关（R (2) = 0.4543，P = 0.0021）。在 Ma1 和 Ma2 的低酸度等位基因的编码序列中，在 Golden Delicious 和 G.41 之间未检测到氨基酸水平的序列变异。但是，两种基因型之间的高酸度等位基因差异很大。Ma1 的低酸度等位基因 Ma1-1455A 主要以开放阅读框中第 1455 位碱基的突变为特征。该突变导致提前出现终止密码子，使 Ma1-1455A 的羧基末端比 Ma1-1455G 截断 84 个氨基酸。使用针对 Ma1 中的 SNP(1455)的标记 CAPS(1455)对 29 个苹果种质资源进行调查显示，CAPS(1455A)等位基因完全与高 pH 值和低可滴定酸度相关，表明自然突变导致的截断很可能是导致 Ma 对苹果低 pH 值或高酸度失去功能的原因。
