Sep 25, 2012
Study reveals identity of the apple acidity gene

Cherries are simply classified into sweet and tart, based on their taste. Although apples are not usually categorized in the same way, apple fruit tartness, or acidity, has a wide range in both fruit pH (2.8-4.5) and titratable acidity (TA, 0.07-1.87 percent)

The acceptable range of fruit acidity for dessert apples is pH 3.1-3.8 and TA 0.3-1 percent. Fruits with pH lower than 3.1 and TA higher than 1 percent are considered too sour, while those with pH higher than 3.9 and TA lower than 0.3 percent will taste flat or flavorless, i.e. too low in acidity.

Common apples are known for their sweetness and flavor, such as Delicious, Fuji, Honeycrisp, Gala, Golden Delicious and McIntosh. But these apples all have an acidity level well within the acceptable range.

Sweeter apple varieties exist, such as Sweet McIntosh, Sweet Delicious, Sweet Caroline and others. These apples contain very little acid and always offer the flat taste. Most consumers do not like such flat taste due to low acid. This makes the low acid/sweeter apples of limited market value, if any.

Malic acid is the most predominant organic acid in mature apple fruit, and almost solely determines fruit acidity levels.

In apple genetics, the gene that controls malic acid levels is called Ma. Gene Ma has two different versions or alleles, Ma and ma. Allele Ma is responsible for high/medium acid, allele ma for low acid. Generally, apple plants receive one allele from their seed parent and another allele from their pollen parent. This gives rise to three possible genotypes in apple: MaMa, Mama and mama. Obviously, genotype MaMa produces fruit of high/medium acid, whereas genotype mama bears fruit of low acid with flat taste. In genotype Mama, the high acid allele Ma is dominant over the low acid allele ma, thereby producing high/medium acid fruit as well.

Among named apple varieties, approximately 13 percent are of genotype MaMa, 80 percent of Mama and 7 percent of mama.

In apple breeding, most crosses are made between two apple varieties of the Mama genotype. The progeny of these crosses segregate in an expected ratio as the following: one quarter of genotype MaMa, one half of Mama and another quarter of mama.

Although growing the quarter of seedlings of genotype mama in breeding populations is a waste of time and money, apple breeders have not been able to discard these seedlings before tasting their fruit, as the identity of the Ma gene has been unknown.

Recently, one of our studies successfully revealed the identity of the Ma gene. Based on its DNA sequence information, the Ma gene encodes a member of a plant-specific protein family, called the ALMT (aluminum-activated malate transporter) family. As a malate (salt of malic acid) transporter, gene Ma is a master regulator controlling malic acid levels in apple fruit cells. A single DNA base mutation causes allele ma to be abnormal in functionality, leading to low acid.

To enable marker-assisted breeding, we have developed a DNA marker, called CAPS1455, to specifically detect the DNA base mutated in allele ma. Testing of marker CAPS1455 in the large apple collection of more than 1,000 accessions in the USDA apple germplasm repository in Geneva, N.Y., proves that the marker perfectly predicts the three genotypes – MaMa, Mama and mama – and their corresponding acidity levels.

It is expected that applying the Ma gene-based functional marker CAPS1455 in apple breeding programs will significantly increase the breeding efficiency by allowing selection at the early seedling stages. Consequently, the number of seedlings in breeding populations that need to be planted in orchards can be cut by at least 25 percent, saving time and cost and hopefully speeding up the release of superior apple varieties.

Kenong Xu, Cornell University





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