At the root of the apple

Despite what Pennsylvania folklore would lead you to believe, the apples you know and love don't come directly from seeds.

fire blight apple infection
Tim McNellis

Fire blight, an insidious bacterial infection, can wipe out entire apple orchards. The bacteria attack and kill from the inside out, leaving brown and brittle trees that look like they've been set on fire.

"Plant a Golden Delicious seed and you won't get a Golden Delicious tree," says Tim McNellis, assistant professor of plant pathology at Penn State. Too much genetic variability lurks inside the seed. Instead of a prized lineage, you're likely to get a version of the infamous crab apple, the "mutt" of the apple world.

To reproduce cultivars like the sweet, blushing Gala and the crisp, tart Macintosh generation after generation, apple breeders must make clones. The 2,000-year-old technique involves grafting the shoots and branches of an existing apple tree, called a scion, onto a new trunk and root system, called a rootstock, and planting the hybrid in the ground.

But what if you want to introduce new traits into your favorite old apple, make it bigger, better, heartier? "You can do it by using the rootstock to transfer certain qualities to the scion," McNellis explains.

Breeders have been doing this for years, he adds, but with little understanding of how the process works. It's not a case of jumping genes, he says; it's a case of influence. Somehow, rootstocks can turn on genes already present in the scions. McNellis is trying to elucidate the link between those turned on genes and traits like size, number of fruit, and disease resistance. Linking genes and traits, he says, might make for better breeding programs.

McNellis is focused on one apple particularly important to Pennsylvania growers—the Gala—and one trait: disease resistance. Entire orchards of Gala and other varieties have been lost to insidious enemies like the fire blight bacteria, Erwinia amylovora, which rots trees from the inside out, leaving them brown and brittle and looking like they've been set on fire.

flowers with blue background
Tim McNellis

This montage shows a portion of a flowering apple tree and an apple shoot displaying symptoms of fire blight disease against the backdrop of a cDNA-AFLP differential display gel. cDNA-AFLP is a genetic tool that allows researchers to compare the expression of certain genes with observable traits.

In earlier work, McNellis and a team of researchers compared Gala scions that were grafted onto two different kinds of rootstocks. Using powerful new genetic tools, the team "scanned the entire genetic makeup of the plant to determine which genes were active in Gala scions on each rootstock," McNellis explains. One kind of rootstock turned on twice as many stress tolerance genes in the scion. Those scions, they observed, were more resistant to fire blight.

This year, McNellis, working with Rob Crassweller at University Park and Jim Travis at Penn State's Fruit and Research Extension Center in Biglerville, Pennsylvania, will expand the research to include over 20 different rootstock varieties, each able to influence the Gala in a different way. This spring, the grafted trees will be planted in orchards in Biglerville and Pennsylvania Furnace. This summer, the researchers will again compare observable traits with gene expression.

"If we can identify a gene that is associated with greater resistance to bacterial pathogens, then we can use that information in a tree-breeding program," says McNellis. That could mean helping Pennsylvania growers choose the right rootstock to protect generations of Gala apple trees from the ravages of fire blight.

Tim McNellis, Ph.D., is assistant professor of plant pathology in the College of Agricultural Sciences, mcnellis@psu.edu. His research is funded by the National Science Foundation's Plant Genome Project.

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Last Updated February 01, 2005