A local scientist is sequencing the genomes of Burbank’s plums to verify their history and create a genetic roadmap for future plant breeders
Luther Burbank is still a well-known name around Sonoma County — it graces the region’s largest arts center and a couple of public gardens. Burbank was America’s most famous horticulturalist (that is, someone who specializes in growing fruit, vegetables and flowers). Since he lived in a time when most Americans still made their living from agriculture, his discoveries seemed as vital and important as those of his contemporaries Thomas Edison and Henry Ford, both of whom visited him here in Sonoma County.
Luther Burbank died in 1926, and over the ensuing 93 years, his reputation has faded considerably, but the plants he created live on. An essay in the Journal of Heredity in 2006 estimated that Burbank introduced between 800 and 1,000 new plants to the American horticultural universe, including the Russet-Burbank potato (still the most common potato in America), Shasta Daisies, the Elberta peach and the luscious Santa Rosa plum.
With only a high school education and no scientific training, Burbank was a self-taught genius and a relentless experimenter, who, it turns out, took extremely poor notes. Still, Burbank filled a collection of notebooks with his large, ungainly scrawl, describing in the briefest of terms his plant breeding experiments. Many of these descriptions are accompanied by hatchmarks — the meaning of which is still a mystery today — and fruit prints, which he made by cutting a fruit in half and pressing it onto the page.
Because Burbank was secretive about his plant breeding methods — and iffy with his notetaking — the origins of some of his most famous crosses are still mysterious.
Enter Rachel Spaeth, the garden curator at Luther Burbank Home & Gardens in Santa Rosa and a fourth-year Ph.D. candidate at UC Davis, working in the horticulture and agronomy graduate group.
Spaeth said she is “looking at all of the genetic diversity that we can find in Luther Burbank plums,” working to unravel their identity and ancestry.
“We kind of know what he said they (his hybrid crosses) were, and genetically, I should be able to tell that,” she said.
When talking to laymen, Spaeth likes to begin with this clarification: “When somebody says the word ‘plum,’ it’s a generic term and it’s kind of a misnomer, because ‘plum’ actually refers to between 17 and 40 different species of organisms,” Spaeth said, “depending on which kind of taxonomist you’re asking.”
Spaeth is looking at 60 living plums within 10 of those species. She is using plant material from living Burbank plum trees to sequence the genomes of Burbank’s plum varieties — and she ultimately hopes to make use of the roughly century-old DNA in Burbank’s fruit prints to confirm these identifications.
Spaeth said her goals are to verify Burbank’s claims about his crosses and to establish a genetic collection of characteristics, based on Burbank’s discoveries, for plant breeders in the future.
“One of the major goals is to just verify his claims,” she said, noting that Burbank’s breeding claims were often disbelieved. “Luther would say that these were the parents of something, but especially with his plum-apricot crosses, people didn’t really believe that he crossbred a plum with an apricot until 50 years after his death, when somebody else was able to reproduce a plum-apricot crop.”
A daring plant breeder
Long before genetic engineering was even dreamt of, Burbank took a swashbuckling approach toward plant breeding — blithely crossing species boundaries to create fertile hybrids, something you’re not supposed to be able to do.
“Luther was unique in that he would frequently cross different species, especially ones that people didn’t think were possible,” Spaeth said. “But if he saw that the flower morphology was similar or they had overlapping bloom periods, or if the fruit looked really similar, then he would deduce that, ‘OK, maybe I can cross these’ and he’d just try it — sometimes repetitively if he really wanted it to work, and sometimes he would get really good results.”
Spaeth said the Santa Rosa plum is probably the prime example of this.
“It’s a cross between a European plum and the Japanese plum. And in creating that hybridized cross, he created a plum that was partially self-fertile, but then also could be used to pollinize both European and Japanese plums. So the Santa Rosa plum became the universal pollinizer”— a role she said it still plays today. “Even if it’s not grown commercially for fresh market, they’ll always have a couple in the orchard just to make sure that they get a good fruit set on everything else.”
“The other thing is it’s really intermediate in bloom time,” she said. “So whenever you’re looking at plant patents for plums, almost all of them will tell you it flowers two weeks before Santa Rosa or two weeks after Santa Rosa. So really it’s kind of like the gold standard.”
A patient investigator
Before heading out to the orchards to gather plant material, Spaeth needed to get a comprehensive list of all the plums introduced by Burbank. She used two different source books, W.L. Howard’s “Luther Burbank’s Plant Contributions” (1945), a near comprehensive guide to everything Burbank introduced, and U.P. Hedrick’s “Plums of New York” (1911), a 750-page tome, filled with beautiful watercolor illustrations of plums.
Spaeth hit gold immediately.
“When I looked through the Hedrick book, I used an online PDF and searched for the keyword ‘Burbank,’ and I was able to discover six plums that Howard didn’t find, just because of the modern tools that we have available to us. So that was really exciting,” she said.
Then using that list as her starting point, she began gathering plant material — leaves and fruit — from plum trees at three different sites: Luther Burbank Home and Gardens in Santa Rosa (where there’s a small orchard with 34 varieties of plums grafted onto five trees); the Burbank Experiment Farm in Sebastopol (where some of the plum trees are so old they were planted by Burbank himself); and the USDA’s Wolfskill Experimental Orchards in Winters.
“If we find a cultivar that’s in all three places when we sequence the genome, it should all be identical,” Spaeth said. “And then we can say this is exactly that plum. Then that becomes the voucher.”
To make a definitive identification, “You have to corroborate material from three different sources,” she said. “Two different sources is OK; three is better. So if we have a sample from all three, and it comes back that one of them is different, then we have to do a little bit more legwork with phenotyping: so it would be like really looking at fruit quality, flowering time and all of that information to try to match it to the historic data that we have.”
Fruit prints then and now
Ultimately, she would like to do genetic testing on Burbank’s original fruit prints, which will require scraping some of the dried fruit matter off. Some of Burbank’s fruit prints are located at Luther Burbank Home & Gardens, but many more are located in the Library of Congress.
To figure out the smallest amount of materials she can take to get a good sample, Spaeth has been making her own fruit prints and practicing on those.
“The goal is to use my prints to see how little material possible you need to be able to get DNA. We’re using techniques that people would use in forensic archaeology, adding different compounds or increasing the amount of time that your reaction has to take place so that you can get good data out of it,” said Spaeth, who sometimes jokes that she feels like the Indiana Jones of the plant world.
When she’s confident in her sampling method, she’ll request permission to take samples from Burbank’s fruit prints in the Library of Congress.
Creating a genetic list of ingredients, a là Burbank
The ultimate goal of Spaeth’s genetic research is to pinpoint which sections of a plum’s genetic code create certain characteristics. This is done through quantitative trait locus (QTL) analysis, which is a statistical method that links phenotypic data (measurements of traits such as color, shape, disease resistance, etc.) with genotypic data in order to explain the genetic basis of a specific trait.
“The cool thing about Luther’s plums is that they have a huge range of not only genetic diversity, but phenotypic diversity. So you’re looking at every color under the rainbow of plums, every size, every shape, whether it’s free stone or cling stone,” Spaeth said.
“All of those characteristics get scored. And then we compare all of those characteristics to the genome,” she said. “You’re looking for QTL markers.”
“In other words, you’re looking for the recipe that makes the pointy bottom plum versus the recipe that makes a smooth bottom. And then once you know that recipe, people can use that information in future breeding experiments.”
According to Spaeth, the ability to create such a recipe “really emphasizes the importance of historic collections,” such as those at Luther Burbank Home & Gardens and the Luther Burbank Experiment Farm.
“Because we wouldn’t have this resource, if we didn’t have people that were curating and cataloging these sorts of things,” she said.
Bringing the past into the future
Knowing the recipe for a phenotypic trait allows researchers to target specific regions of the genome for genetic engineering as well as traditional breeding.
“From a genetic engineering perspective, knowing the recipe for the gene you want allows you to target and edit specific sequences,” Spaeth said. “This is highly useful if you want to do something like move a disease-resistance gene from cherry into plum without having to shuffle and sort the two genomes through traditional methods.”
What would Burbank think of modern genetic engineering?
"It is unfair to impart our 21st century mindset on someone who lived most of his life without electricity or cars,” Spaeth said. “However, if I had to venture a guess, I would say that Luther would be open to using all of the tools in the kit to further advance his breeding lines.”
By cracking the genetic code of Burbank’s hybrids and by creating a genetic database of their component parts, Spaeth is giving Luther Burbank a new place in the modern world so that future generations of scientists and plant lovers can benefit once more from his prodigious creativity.