Graduate student Yurah Kim evaluating organic acid HPLC profiles in a progeny (family) segregating for the low malic acid gene.
Commercial cranberry cultivars have high concentrations of acids, about five times that of fruit eaten fresh, that contribute to high titratable acidity (TA)—or tartness within the fruit.
To offset the tartness for consumers, higher amounts of sugar need to be added in the cranberry products, such as sweetened-dried-cranberries and juices. The added sugar has deterred consumers from increasing cranberry product consumption despite cranberry health benefits.
Within the cranberry germplasm collection at Rutgers University, a unique cranberry plant was identified with fruit having a lower percent TA than is normally found in commercial cultivars and may offer cranberry products with less added sugar.
Crosses using this plant identified a naturally occurring genetic trait that produced fruit with a lower malic acid, which impacts TA.
Utilizing these populations—segregating for the low malic acid traits—it was possible to generate effective genetic markers for use in breeding of cranberry cultivars with TA.
Key Words
Locus: fixed position on a chromosome where a genetic marker is located
Allele: one of two or more versions of a gene
Heterozygous: two different versions of a gene present in an individual.
Homozygous: two copies of the same version of a gene in an individual
Introduction
Cranberries are known for their health benefits, but due to the fruit’s relative tartness, most cranberry products typically contain up to 40% added sugars (Ocean Spray Cranberries Inc. 2019).
Cranberry fruit has three organic acids that contribute to tartness or TA:
malic (MA)
citric (CA)
quinic acid (QA)—though to a lesser extent
Modeling MA in peach showed that both MA and CA are significant contributors to TA (Lobit et al. 2002).
A germplasm accession was collected from a native cranberry population in Suffolk County, NY which exhibited lower MA in a subsequent germplasm screen.
If TA is affected by the concentration of MA, then reducing the latter will reduce overall acidity/tartness in cranberry fruit, thus reducing the amount of added sugar needed in cranberry products.
Objectives of this Study
As the first characterization of the genetics of a qualitative low MA trait in cranberry fruit, the objectives of this study were to:
describe the inheritance of the mala allele (a low MA allele), its effect on TA and MA, and its relationship with CA and QA.
identify, develop, and validate molecular markers that could be used to select for the low MA trait.
determine the effect of the mala among other cranberry genotypes that have the cita allele (a low CA allele)
Experiments
1993
A germplasm accession, NJ93-57, was collected and exhibited lower MA in a subsequent germplasm screen.
2004-2012
An initial cross, of the germplasm accession with cv. Mullica Queen (MQ), was made in 2004 to generate the CNJ04-52 population (Fig. 1). Between 2004 and 2012, various crosses were made to produce varying populations.
Crosses were made manually during April and May in the greenhouse during 2004–2012.
Seedlings were maintained in the greenhouse and in the field for at least three years.
2017 and 2018
During the spring and summer, flowering plants were taken outside for bee open pollination.
Fruits were collected from each individual in each population once a year during late August and September in 2014–2018 for analysis of TA and organic acids with HPLC .
Leaf tissue was collected in the spring of 2017 for DNA extractions.
Results
The homozygous mala plants have a dwarf-like growth habit which is likely not commercially viable .
This study characterized a low MA trait in cranberry fruit derived from a native germplasm accession (NJ93-57) and determined its interaction with genotypes at the CITA locus.
NJ93-57 was determined to be heterozygous (Mala/mala). In an F2 population, derived from an F1 Mullica Queen × NJ93-57, progeny with a MA concentration as low as ~ 2 mg/g were recovered (compared to the typical 6–8 mg/g range).
This progeny has the lowest level MA phenotype reported in cranberry and is a result of a mala/mala homozygous genotype.
The high correlation of MA levels across years indicates a strong qualitative genetic effect of the mala/mala genotype.
However, significant differences in MA and TA between populations with the low MA trait, indicate general genetic background also influences acidity to some extent.
To characterize and map the low MA trait, mala, three populations (119 unique individuals) were phenotyped and genotyped. The three populations segregated for the low MA trait (~ 2 mg/g FW) consistent with a single, co-dominant gene in a Mendelian pattern and we named the locus MALA.
The mala allele had a significant effect on QA and CA as well as TA.
Although the mala/mala genotype yields TA < 1%, there is a caveat, as the homozygous mala plants have a dwarf-like growth habit which is likely not commercially viable.
Conclusion
A low MA trait was characterized, and a genetic locus was identified that presents a decreased TA to below 1%—within the range of fruit that is consumed fresh (Kallio et al. 2000).
Other than variation in acidity, there were no other apparent fruit quality traits affected.
Markers have been identified and are being developed for genetic screening, including screening the germplasm collection for low MA.
Identifying the genes controlling CA and MA accumulation would allow greater ability for marker-assisted selection and gene editing applications in the future. This will allow seedlings of no value to be culled—saving space, time, and money.
Continued work will contribute to breeding efforts to develop commercially viable cranberry cultivars that require less added sugar.
