A low malic acid trait in cranberry fruit: genetics, molecular mapping, and interaction with a citric acid locus

The fruit of commercial cranberry (Vaccinium macrocarpon Ait.) cultivars have relatively high concentrations of malic acid (MA) and citric acid (CA), and, to a lesser extent, quinic acid. These acids contribute to the high titratable acidity (TA), a measure of tartness, of cranberry fruit, which typically ranges from 2.3 to 2.5% citric acid equivalents in commercial cultivars. Thus, considerable amounts of sugar are added (“added sugar”) in products such as sweetened-dried cranberries and juices. Within our cranberry germplasm collection, a unique accession was identified with fruit having a TA ≈ 1.5%, where MA concentration was reduced to ~ 4 mg/g fresh weight (FW), compared to ~ 8 mg/g FW in current cultivars. Inbred crosses derived from this accession yielded progeny with a lower MA phenotype (~ 2 mg/g FW). Observed segregation indicated the accession was heterozygous (Mala/mala) for a low MA allele (mala) and very low MA progeny were homozygous (mala/mala). MA was reduced approximately 75% in these populations relative to standard cultivars. The homozygous mala/mala locus also depressed fruit CA and quinic acid concentrations. Quantitative trait loci mapping identified a region on chromosome 4 associated with low MA. The combined segregation of three half-sib populations derived from the low MA accession generated effective (within < 1 cM) Kompetitive allele-specific PCR (KASP) markers for use in breeding of cranberry cultivars with lower MA. Dihybrid populations were developed having a previously described low CA allele, cita, with the mala alleles of this study to explore the interaction of alleles at both loci.