Endangered oak is first cryopreserved plant at Frozen Zoo’s biological bank

SAN DIEGO

Joseph Ree, a former post-doctoral associate with San Diego Zoo Wildlife Alliance, works with specimens of endangered Nuttall’s scrub oak trees. The oaks were the first plants cryopreserved and deposited into the association’s Frozen Zoo.

Samples from 18 endangered Nuttall’s scrub oak trees were prepared to be quickly frozen with liquid nitrogen. photos courtesy San Diego Zoo Wildlife Alliance

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By Karen Kucher

Union-Tribune

For the first time, a plant specimen has been cryopreserved and placed into the San Diego Zoo Wildlife Alliance’s Frozen Zoo, a step scientists believe could help the endangered scrub oak species survive — and save other plants.

Samples from 18 Nuttall’s scrub oak trees were deposited in the biodiversity bank last August, joining living cell lines from more than 11,500 individuals representing more than 1,330 species. The Frozen Zoo, which turned 50 this year, stores genetic material for future conservation use and holds cells, embryos and gametes from mammals, birds, fish, reptiles — and now plants.

Each sample is stored in liquid nitrogen at -320 degrees Fahrenheit. The collection of specimens is kept at the Beckman Center for Conservation Research near the San Diego Zoo Safari Park.

Nuttall’s scrub oak, which grows in San Diego County, was picked for the project because it is listed as endangered by the International Union for Conservation of Nature. Zoo officials say conserving the species is important not only to preserve botanical diversity but because the trees sustain wildlife and ecosystems.

Zoo officials said nearly one-third of the world’s 450 oak species are at risk of disappearing because of habitat loss, invasive pests and fire.

“Cryopreserving Nuttall’s scrub oak is an enormous accomplishment for plant conservation,” Christy Powell, horticulture manager at the zoo, said in a statement. “Unlike many species, oaks cannot be stored in traditional seed banks. Cryopreservation offers us an incredible tool to ensure their survival — alongside habitat protections and living collections.”

More than 500 native plants in San Diego County are held in a Native Seed Bank at the zoo. Those seeds are dried down to a certain relative humidity and then frozen at -20 degrees Celsius, which is close to the temperature of a typical home freezer.

But some species, like oaks, cannot be preserved in that manner. That’s because acorns are a fleshy fruit and contain too much moisture for traditional freezing techniques. When those methods were tried, ice crystals formed and killed the plant embryo, said Katie Heineman, program officer for biodiversity banking at the alliance.

With cryopreservation, the plant material is treated and frozen very quickly at a very cold temperature so ice crystals can’t form.

Heineman said researchers will remove a shoot tip from an oak and grow it in a small jar so it becomes “like a mini tree” with a stem and other parts. Researchers then will take the oak’s somatic embryo, which she said essentially is a clone of the plant, and that’s what is cryopreserved.

Other institutions are pursuing similar work, including the Cincinnati Zoo, which is also working on cryopreservation of oaks and other native U.S. plants, as well as the U.S. Department of Agriculture and The Huntington Library, Art Museum and Botanical Gardens in San Marino. Zoo officials said those partners were critical to the project.

“We have a robust horticulture program where we do a great deal of micropropagation of species, so this really just combines two strengths of the organization: our know-how in growing plants, and our ability to freeze things, in a really exciting new avenue for conservation,” Heineman said.

There’s a lot of trial and error in determining the right protocols, such as figuring out the correct rate of freezing and other steps. “It is kind of like a recipe with lots of different pieces,” she said. When the wrong steps are taken, the plant won’t regrow after being frozen.

Heineman said zoo officials hope to train others so protocols can be developed for other high-priority species, so as much genetic diversity can be saved as possible.

The Frozen Zoo has notched some reproductive breakthroughs over the years, including in 2023 when researchers were able to successfully freeze sunflower sea star sperm, thaw it and fertilize eggs that resulted in larvae developing with a high rate of success.

Heineman said the alliance was awarded a three-year grant from the Institute of Museum and Library Services to expand into other oak species, including the Channel Island Oak. Scientists also plan to look at orchid species.

“We have a large orchid propagation program at the San Diego Zoo, and orchid seeds are almost like dust, almost the size of pollen grains … We want to evaluate how we can cryopreserve those plants,” she said. “Just in general, we really hope to eventually be a cryo-repository for California native plants that need this type of biobanking.”