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

The high acidity of commercial cranberry (Vaccinium macrocarpon) fruit is principally the result of relatively high concentrations of malic acid (MA) and citric acid (CA), and partially quinic acid. These acids contribute to the high titratable acidity (TA), a measure of tartness, of cranberry fruit. Thus, considerable amounts of sugar ('added-sugar') is required in products such as 'sweetened-dried cranberries.' Initial phenotyping of the Rutgers cranberry germplasm collection identified a unique accession with TA of 1.50% (compared to 2.13-2.50% in commercial cultivars) and reduced MA concentration (4 mg/g fresh fruit compared to 8 mg/g). High performance liquid chromatography (HPLC) for MA concentrations of a series of populations derived from this wild germplasm exhibited inheritance of a low MA (lower TA) trait consistent with single locus Mendelian inheritance with co-dominant alleles. The germplasm accession harbored a low MA allele (mala) in the heterozygous state (Mala/mala) and yielded progeny with 2 mg/g MA phenotype when homozygous (mala/mala). These progenies had 75% less MA than standard cultivars. The mala/mala locus also depresses quinic acid concentrations. Genotyping by sequencing (GBS) identified a region on Chromosome 4 associated with low MA. Three small, half-sib populations derived from the low MA germplasm accession were used to generate effective (within <1cM) KASP markers for marker assisted selection (MAS). Additionally, populations that introgressed a previously described low CA locus, cita, with the mala alleles described herein were developed to explore the interaction of alleles at both loci. These markers facilitate MAS for TA for less acidic cranberry fruit and reduced added-sugars in cranberry products.