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

Authors: FONG, STEPHANIE - Rutgers University; KAWASH, JOSEPH - Orise Fellow; WANG, YIFEI - Ohio University; JOHNSON-CICALESE, JENNIFER - Rutgers University; Polashock, James; VORSA, NICHOLI - Rutgers University

Submitted to: Tree Genetics and Genomes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/17/2020
Publication Date: 1/6/2021
Citation: Fong, S.K., Kawash, J., Wang, Y., Johnson-Cicalese, J., Polashock, J.J., Vorsa, N. 2021. A low malic acid trait in cranberry fruit: genetics, molecular mapping, and interaction with a citric acid locus. Tree Genetics and Genomes. 17(1):1-14. https://doi.org/10.1007/s11295-020-01482-8.
DOI: https://doi.org/10.1007/s11295-020-01482-8

Interpretive Summary: American cranberry fruit has a tart, acidic taste. The fruit acidity of cranberry is significantly higher than other freshly consumed fruits, e.g., tangerines and apples. This requires the addition of sugar to cranberry products to make them more palatable. The primary contributors to acidity in cranberry fruit are citric and malic acids. Levels of both citric acid and malic acid each reach approximately 8 mg/g fresh weight. The present study characterizes a unique cranberry with naturally low malic acid (˜ 4 mg/g fresh fruit). Our results show that the low malic acid trait is controlled by a single recessive gene. Using genomic techniques, we localized the causal gene to chromosome 4 in the cranberry genome. Markers that we identified which are linked to this gene will be used by plant breeders to speed breeding for low malic acid in commercial cranberry varieties. New low malic acid cranberry cultivars will allow the food industry to manufacture more healthy reduced added sugar products for consumer benefit.

Technical Abstract: Commercial cranberry (Vaccinium macrocarpon Ait.) varieties have relatively high fruit 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 (CAE) in commercial cultivars. Thus, considerable amounts of sugar need to be added (‘added-sugar’) into products such as sweetened-dried cranberries and juices. Within our cranberry germplasm collection, a unique accession was identified with fruit having a TA ˜1.50%, where MA concentration was reduced to ~4 mg/g fresh fruit, compared to ~8 mg/g in current cultivars. A series of populations derived from this accession exhibited single locus Mendelian inheritance with co-dominant alleles for the low MA and lower TA traits. The germplasm accession harbored a low MA allele (mala) in the heterozygous state (Mala/mala) and yielded progeny with an even lower MA (~2 mg/g FW) phenotype when homozygous (mala/mala). MA was reduced approximately 75 percent in these populations compared to standard cultivars. The homozygous mala/mala locus also depressed fruit CA and quinic acid concentrations. Genotyping-by-sequencing (GBS) identified a region on Chromosome 4 associated with low MA. The combined segregation of three half-sib populations derived from the low MA accession allowed generation of effective (within <1cM) KASP markers for marker assisted selection (MAS). Dihybrid populations were developed, having a previously described low CITA locus allele, cita, with the mala alleles of this study to explore the interaction of alleles at both loci.