Introductory Information

Botany and History

    All cultivated blueberries belong in the section Cyanococcus of the genus Vaccinium of the heath family Ericaceae. Species within this section are often called the “true” or cluster-fruited blueberries. Wild representatives of Cyanococcus are found solely in North America . The most recent classification of the Cyanococcus species includes a total of 7 diploid species (V. boreale Hall & Aald., V. corymbosum L., V. darrowi Camp, V. elliottii Chapm., V. myrtilloides Michx., V. pallidum Ait., and V. tenellum Ait.), 6 tetraploid species (V. angustifolium Ait., V. corymbosum, V. hirsutum Buckley, V. myrsinites Lam., V. pallidum, and V. simulatum Small), and 2 hexaploid species (V. ashei Reade and V. constablaei Gray), with V. corymbosum  and V. pallidum occurring at diploid and tetraploid levels.

Blueberry species are also commonly grouped according to stature and referred to as the lowbush, highbush, and rabbiteye types. Lowbush plants are rhizomatous with stems from 0.30 to 0.60 m; highbush plants are crown forming and generally maintained between 1.8 and 2.5 m; rabbiteye plants are crown forming, but also are notable for suckering to varying degrees, and maintained between 2.0 and 4.0 m.

Of the major fruit crops, blueberry has been domesticated most recently, having been accomplished entirely within the twentieth century. Wild lowbush blueberry plants were probably the first blueberries to be cultivated, with native North Americans burning the heaths to increase yield. Commercial lowbush blueberry production, mainly V. angustifolium but also including V. myrtilloides, began in the 1860s in New England with the canning of fruit. Domestication of the highbush blueberry (mainly V. corymbosum) began in New Jersey in the early 1900s through the efforts of USDA botanist F.V. Coville and blueberry enthusiast E.C. White. The first commercial fruit from hybrid highbush plants was harvested in 1916. Efforts to domesticate rabbiteye blueberry began around 1893 with the transplantation of native seedlings by M.A. Sapp to his farm in northwestern Florida .

Importance

Economic significance

    Blueberry is a high value crop which can thrive on acidic, imperfectly drained sandy soils, once considered worthless for agricultural crop production. North America is the major producer of blueberries. Current North American total annual production of commercial blueberries approaches 140,000 t, with approximately 60% of that being cultivated (standard highbush, southern highbush, half-high, and rabbiteye) blueberries and about 40% being the managed wild, or semicultivated, lowbush blueberries. Total area devoted to growing commercial blueberries in North America is approx. 74,000 ha, consisting of 53,000 ha of managed wild stands of lowbush blueberries and 21,000 ha of cultivated blueberries. Most of the USA blueberry crop is grown in 12 states: Alabama, Arkansas, Florida, Georgia, Indiana, Maine, Michigan, New Jersey, New York, North Carolina, Oregon and Washington; Michigan, Maine, New Jersey and Oregon are the top producers. The Canadian blueberry crop is grown mainly in British Columbia, Nova Scotia, New Brunswick and Quebec. Worldwide, in addition to North America, commercial industries now exist in Europe, Australia, New Zealand, Chile and Argentina. 

Nutritional value

    Biomedical and epidemiological research suggest that the dietary antioxidants contained in fruits and vegetables may play an important role in preventing disease. Plant-derived antioxidants have been shown to function as singlet and triplet oxygen quenchers, free radical scavengers, peroxide decomposers, enzyme inhibitors, and synergists. Blueberries are one of the richest sources of antioxidants of all fresh fruits and vegetables. Compared with other fruits, canned blueberries are also a good source of iron, fair source of vitamin A, and about average in terms of protein, fat, carbohydrate, calories and calcium content. In addition, fresh blueberries are a fair source of vitamin C. 

Breeding and Genetics

    Because of a lack of fundamental sterility barriers between blueberry species of the same ploidy level (homoploid species), interspecific hybridizations have played, and continue to play, an important part in blueberry breeding programmes. Hybridizations between species of different ploidy levels (heteroploid crosses), although much more difficult to achieve, have also been utilized in blueberry breeding. 

    The initial blueberry breeding objectives were mainly focused on the fresh fruit characters of sweet and excellent flavor, large size, dry scar and plumpness at maturity, light blue color or bloom, and vegetative growth adequate to support a large crop of berries. With the expansion of the blueberry industry in North America and worldwide, modern blueberry breeding objectives now include generating plants with broader soil and climatic adaptation, disease and pest resistance, easy mechanical harvesting, extreme early and late harvesting, and good fruit quality including longer storage life. Broader climatic adaptation refers, on the one hand, to adaptation to warm, long-growing season areas through lower chilling requirements and increased heat and drought tolerance and, on the other hand, to adaptation to colder production areas through increased bud and wood cold tolerance and delayed flowering to avoid early spring frosts combined with earlier fruit maturation.

Ongoing Research of Rowland Laboratory

Research interest

Dr. Rowland’s research program focuses on identifying genes/molecular markers of horticultural significance in blueberry and making them available for marker-assisted breeding and transformation. Current projects use molecular and physiological approaches to better understand chilling requirement and cold tolerance in blueberry. Projects include: further saturating genetic maps of blueberry using Expressed Sequence Tag-Polymerase Chain Reaction (EST-PCR) markers and mapping QTLs associated with chilling requirement and cold hardiness in blueberry mapping populations; searching for new cold acclimation-responsive genes using ESTs and microarrays and subtracted/reverse subtracted libraries; and comparing the rate of deacclimation in different blueberry genotypes to identify slow deacclimating genotypes useful for breeding for spring frost tolerance. 

Specific objectives

Blueberry cDNA libraries and ESTs

    To gain a better understanding of changes in gene expression associated with cold acclimation in blueberry, the Rowland laboratory has undertaken a genomics approach based on the analysis of Expressed Sequence Tags (ESTs). Initially, two standard cDNA libraries were constructed using RNA from cold-acclimated and non-acclimated floral buds of the blueberry cultivar ‘Bluecrop’ (Vaccinium corymbosum L.) and about 1200 5’-end ESTs were generated from each of the libraries. About 100 3’-end ESTs were generated from the cold-acclimated library as well. The ESTs have been deposited into GenBank, making this the first publicly available EST database for Vaccinium. In addition, an aliquot of the non-acclimated library has been provided to scientists participating in the Floral Genome Project, who have generated additional ESTs from this library. 

    However, there are limitations to working with standard cDNA libraries. For example, random picking and sequencing of even several thousand clones from standard cDNA libraries will result in selection of clones representing more highly abundant transcripts because these clones will be present in the libraries at a higher frequency than those representing less abundant transcripts. Important regulatory genes, such as transcription factors, are often expressed at rather low levels and over a shorter timeframe. Thus, they can be missed using this approach. Therefore, we have also constructed subtracted and reverse subtracted libraries using procedures that help to increase the chances of finding rarer classes of transcripts by helping to normalize the distribution of clones. The subtracted library was prepared in such a way to enrich for transcripts that are expressed at higher levels in blueberry flower buds at 400 hours of cold exposure than at 0 hours of cold exposure and vice versa for the reverse subtracted library. 

    Putative functions have been assigned to cDNAs from all four libraries (cold acclimated standard, non-acclimated standard, subtracted, and reverse subtracted) that yielded high quality sequences based on homology to other genes/ESTs from GenBank, and these have been classified into functional categories. Contig analyses have been performed to cluster sequences derived from the same or very similar genes. The most highly abundant cDNAs that were picked many more times from one library than from the other have been identified as representing potentially differentially expressed transcripts. To date, northern analyses have been performed to examine expression of eight selected transcripts and seven of these have been confirmed to be preferentially expressed under either cold-acclimating or non-acclimating conditions. Only one of the seven transcripts, encoding a dehydrin, had been found previously to be up-regulated during cold acclimation of blueberry. Messages encoding a senescence-associated protein, early light-inducible protein, beta amylase, and an unknown protein were found to be up-regulated during cold acclimation. Messages encoding histone protein H3.2 and BURP-domain dehydration-responsive protein RD 22 were found to be down-regulated during cold acclimation. 

    This study has demonstrated that analysis of ESTs is an effective strategy to identify candidate cold acclimation-responsive transcripts in blueberry. The set of blueberry cDNAs is currently being used in microarray experiments.

Contact Information: 

Dr. L. Jeannine Rowland
USDA-ARS
Fruit Lab Building 010A
BARC West, 10300 Baltimore Avenue
Beltsville MD 20705
301-504-6654
rowlandj@ba.ars.usda.gov