Fruit Breeding - Peach Breeding to Improve Fruit Quality

Recently, fossil peach pits from the late Pliocene strata were discovered in southwestern China (Su et al. 2015). The pits are more than 2.5 million years old and are identical to those of modern cultivars and similar in size to smaller modern cultivars. This suggests that peaches evolved their modern morphology through natural selection under natural conditions before being domesticated by humans.

Peach size was improved much later through domestication and breeding. In 1562, the Spanish introduced peaches to North America in St. Augustine, Florida. When John Smith described the fauna in Jamestown, Virginia, in 1629, he said: "Here [Virginia] are likewise great Peach-orchards, which bear such an infinite quantity of Peaches, that at some Plantations they beat down to the Hoggs fourty bushels a year."

In the early 1700s, the English Surveyor-General of North Carolina, John Lawson, said peaches grew as luxuriantly as weeds: "We are forced to take a great deal of care to weed them out. Otherwise, they make our Land a Wilderness of Peach trees." It is interesting that peach was considered invasive during the early colonial period, but today, wild stands of peaches don't exist. However, an account by John Bartram of Philadelphia in the late 1700s indicated that peach trees were short-lived due to borers, and new trees had to be planted almost annually.

Until peach breeding programs were established, peach cultivars were selected from chance seedlings and were named by growers. Some examples of such cultivars included 'Early Crawford,' 'Late Crawford,' 'Reeves,' and 'Iron Mountain.' Some of these cultivars were parents of important cultivars. For example, 'Elberta' was a seedling from a 'Chinese Cling' tree growing next to an 'Early Crawford' tree in an orchard owned by Samuel H. Rumph in Marshall, Georgia in 1870. 'Elberta' was the most heavily planted peach cultivar in the U.S. for several decades. Professor M.A. Blake, at Rutgers University, used 'Elberta' as a parent to produce 'Golden Jubilee, 'Goldeneast,' and 'Primrose.' Although never verified, 'Elberta' may be a parent of 'J.H. Hale' introduced by Michigan State University. J.H. Weinberger at the USDA in Fort Valley, Georgia, used progeny of 'Elberta' to produce 'Valient,' 'Vedette,' 'Halberta,' and 'Redelberta.' Developing a new variety requires many seedlings. When I was at Rutgers, the peach breeder Fred Hough told me that about one in every 20,000 seedlings is as good as its parents. Apples are even more challenging - he said about 1 in every 50,000 apple seedlings is as good as the parents.

In 1806, the English horticulturist/botanist, Thomas Knight, was the first to produce a peach cultivar by controlled cross-pollination. Today, the most common method for producing new cultivars is by crossing two parents with desirable characteristics and selecting offspring with a combination of desirable characteristics for further crosses or to introduce to the industry. Initially, breeders selected for red skin color, large fruit, firmness, cold hardiness, and flavor. By the 1940s, breeders also selected for season extension and low chilling for southern regions. During the last 20 years, as information concerning consumer acceptance has been generated, breeders have also been selecting for fruit quality.

There are probably many definitions of "quality," but the definition I like is "the chemical and physical characteristics that appeal to and are acceptable to consumers." These factors include fruit shape, skin color, flesh texture, sugar and acid concentrations, flavor, and aroma. Some of these characteristics, such as fruit shape (round vs. flat), are controlled primarily by one gene. Other traits are controlled by more than one gene and may not be independent. Because many genes may be involved, it is difficult to combine many favorable traits by traditional breeding methods, and some genes controlling favorable traits may be closely linked with genes of undesirable traits. Therefore, desirable and undesirable traits may be inherited together. Over the past 25 years, new methods of plant regeneration, molecular biology, and gene transfer have provided strategies for manipulating specific genes to develop cultivars with several desirable traits.

Peach has only eight pairs of chromosomes compared to 17 for apple, and the genome size of peach is less than half that of apple, so peach is easier to study. Over the past couple of decades, peach breeders have been trying to identify genes that control certain characteristics. The first step involved sequencing the peach genome. Sequencing involves determining the order of the DNA nucleotides or bases in a genome. This information can be used to map the genome by determining the order and distance between genes on a chromosome. The DNA of the peach was first sequenced from a 'Lovell' peach tree growing at the Musser Fruit Research Farm at Clemson University. Peach breeders around the world used and added to this information to identify the genes responsible for important traits.

Basic Genetic Information

For those who forgot their high school biology, I will try to briefly review genetic terminology. A chromosome is a long strand of DNA. DNA is composed of a string of building blocks, called nucleotides. A gene is a segment of a DNA molecule that is the basic unit of heredity, which is transferred from a parent to an offspring and controls the characteristics displayed in an offspring. Each gene is located at a specific location on a chromosome in two copies. One copy is inherited from each parent. The copies may not be the same, and when the copies of a gene differ from each other, they are known as alleles. Some alleles are dominant. In peach, white flesh color is dominant over yellow (Connors, 1920). The difference between peach and nectarine is that skin fuzz is controlled by a single dominant allele. Sometimes a characteristic is controlled by a single allele, and other times multiple alleles act together. Nectarine flavor is controlled by other genes, so it is possible to develop peaches with nectarine flavor. Since the allele for fuzz is dominant, peaches can be homozygous for fuzz (2 alleles for fuzz), or they can be heterozygous for fuzz (one allele for fuzz and one for no fuzz). All nectarines are homozygous for nectarine skin (no fuzz).

The RosBREED Project

The RosBREED project is a USDA-SCRI-funded project focused on developing and applying modern DNA tests and related breeding methods to develop new cultivars of rosaceous crops. Thirty-five scientists from 22 breeding programs at 14 U.S. institutions, plus some international cooperators, are focusing on eight crops, including peach and apple.

The group developed the indel G DNA test to determine if a young seedling will be a nectarine or a peach. They are beginning to understand the inheritance of peach flavor. A region on peach chromosome 4 is associated with sugar and acid levels. A single DNA test was developed to test seedlings for these genes. Three alleles control peach fruit texture (freestone vs. clingstone), and freestone is the dominant allele. There are two regions on chromosome 4 that influence peach fruit texture, and a DNA test was developed for these regions. Different genes are involved in bacterial spot resistance for leaves and the fruit separately and together. Researchers at Clemson University identified alleles associated with different levels of peach fruit resistance to bacterial spot, and DNA tests with 80% accuracy in predicting fruit resistance are available to screen seedling populations. These screening methods allow breeders to cull out young seedlings lacking desirable characteristics before they are planted in the field.

Other Peach Genetic Research

Spanish peach breeders (Cantiin et al. 2010) studied the inheritance of important characteristics and found that among 15 progenies, bloom date varied little, but days from bloom to harvest varied from 80 to 150 days. Their data suggest that traits such as yield, fruit weight, and soluble solids concentration are controlled by more than one gene. Work at NC State University showed that the red flesh trait found in 'Rutgers Redleaf' and 'Harrow Blood' rootstocks is controlled by a single gene, designated bf (Werner et al., 1998). Red flesh color in peach has historically been associated with poor eating quality. However, red flesh peaches with high concentrations of anthocyanin may have health benefits, so some breeders are trying to develop red flesh peaches with good eating quality. A group from China recently characterized genes controlling 12 peach quality traits (Cao et al. 2016). They identified genes for flesh adhesion and texture, flesh color, and color around the pit, kernel taste, fruit pubescence, fruit shape, low-acid, and flower shape. Researchers at Clemson University evaluated 132 peach and nectarine cultivars for bioactive compounds and antioxidant capacity. Antioxidant capacity and phytochemicals tended to be highest in late-season cultivars and yellow-flesh heirloom cultivars, such as 'Elberta,' 'Jerseyqueen,' and white-fleshed 'Belle of Georgia' (Abdelghafar et al. 2018).

California researchers (Dhanapal and Crisosto, 2013) are unraveling the genetic control of chilling injury in peach and nectarine, which is important for maintaining the good eating quality during and after storage. They identified the regions of chromosome associated with flesh mealiness, flesh browning, and flesh bleeding.

Modern Breeding Methods

For more than 20 years, plant breeders have been using marker-assisted selection (MAS) to help identify young seedlings with desirable traits. Molecular or genetic markers are used to identify specific genes. The markers are a string sequence of nucleic acid that make up a segment of DNA. The markers are located near the DNA sequence of the desired gene. The markers and the genes are close together on the same chromosome (sometimes called genetic linkage) and tend to stay together as each generation of plants is produced. Genetic linkage maps can be developed that show the location of genes on a chromosome. New DNA sequencing techniques have also been used to identify candidate genes related to plum pox virus, graft incompatibility, and flowering time in stone fruit. Over time, these maps will become more detailed. With this information, a piece of leaf from a newly germinated seedling can be analyzed to determine if that individual has the characteristic of interest. Instead of planting hundreds of seeds from a cross and waiting several years for a tree to produce fruit, breeders can eliminate seedlings lacking the desired traits, so only the desirable seedlings will be grown in the field. Linkage maps for peach have been used for fruit size, firmness, acidity, individual sugars (fructose, glucose, sucrose, and sorbitol), aroma, peel-related phenolic compounds, and disease resistance.

Geneticists are still in the early stages of mapping the peach genome, but the map is being improved. As the map becomes more detailed and techniques for identifying and mapping genes become faster and less expensive, breeders will be able to select promising seedlings earlier in the program and shorten the time from making a cross to releasing a new cultivar. New cultivars will also likely possess combinations of desirable traits not previously available.

Literature Cited

• Abdelghafar, A., R. Burrell, G. Reighard, and K. Gasic. 2018. Antioxidant capacity and bioactive compounds accumulation in peach breeding germplasm. J. Amer. Pomol. Soc. 72:40-69.
• Cao, K. et al. 2016. Genome-wide association study of 12 agronomic traits in peach. Nature Communications 7:13246. doi: 10.1038/ncomms13246
• Cantin, C. M., Y. Gogorcena, and M. A. Moreno. 2010. Phenotypic diversity and relationships of fruit quality traits in peach and nectarine [Prunus persica (L.) Batsch] breeding progenies. Euphytica 171:211-226. DOI 10.1007/s10681-009-0023-4.
• Connors, C.H. 1920. Some notes on the inheritance of unit characters in the peach. Proc. Amer. Soc. Hort. Sci. 16:24–36.
• Dhanapal, A.P. and C.H. Crisosto. 2013. Association genetics of chilling injury susceptibility in peach (Prunus persica (L.) batsch) across multiple years. 3 Biotech. 3:481-490. doi: 10.1007/s13205-012-0109-x
• Su, T., P. Wilf, Y. Huang, S. Shang, and Z. Shou. 2015. Peaches preceded humans: fossil evidence from SW China. Scientific Reports 5:16794. DOI: 10.1038/srep16794
• Werner, D.J., M.A. Dreller, and J.X. Chaparro. 1998. Inheritance of the blood-flesh trait in peach. HortScience 33:1243-1246. DOI:10.21273/HORTSCI.33.7.1243