Cryo success at Queensland delivers 1st ‘frozen’ avocado tree

It’s taken more than 40 years, but cryopreservation research at the University of Queensland has finally paid off – at least for those seeking to freeze avocado plants in order to protect them in an uncertain climate future.

“The aim is to preserve important avocado cultivars and key genetic traits from possible destruction by threats like bushfires, pests and disease,” says Chris O’Brien, a doctoral candidate at the university. He cites the example of laurel wilt, a fungus with the potential to wipe out the germplasm (the critical genetic material) of an avocado that grows in Florida in the United States.

Led by O’Brien, the Australian and U.S. researchers who worked on the cryopreservation technique say it’s a world-first success with the avocado plant. They published their findings this week in the Plant Cell, Tissue and Organ Culture (PCTOC) arm of the Journal of Plant Biotechnology.

O’Brien’s quest is not new when it comes to cryopreservation, which has been used to freeze other plants such as potatoes, bananas, grapevines and apple, and relies on the same technology that humans use when freezing eggs and sperm to later retrieve. Saving the plant life has long been a research vision at The Huntington Library, Art Museum, and Botanical Gardens in California, which collaborated on the work.

“Preserving plant genetic material, or germplasm, is a critical part of safeguarding the world’s food supply, creating new varieties of plants, and being able to bring back plants that have been destroyed,” the Huntington center explains.

Many people are familiar with the Svalbard Global Seed Vault, where millions of seeds shipped from global seed banks are preserved underground in a remote corner of the Arctic. “But seed banking doesn’t work for plants that produce few or no seeds, plants that don’t grow true from seed, or plants with seeds that can’t be frozen,” the Huntington adds. “In these cases, cryopreservation of specialized plant tissue is necessary.”

Dr. Neena Mitter of Queensland’s Centre for Horticultural Science says what they’re doing is “taking a small, tiny, tiny cutting from an avocado tree,” and then plunging it into liquid nitrogen. At minus 196 degrees Celsius, the liquid nitrogen maintains its temperature without electricity so the cutting can remain frozen until it’s time to regrow the avocado.

The process can allow for a single shoot-tip to grow up to 500 plants. First the avocado tip is placed on aluminum foil to allow for rapid cooling and rewarming, and then it is placed in a cryotube for storage in the liquid nitrogen. The frozen avocado shoot tips take about 20 minutes to recover once they’re retrieved and rehydrated in the lab.

“In about two months they have new leaves and are ready for rooting before beginning a life in the orchard,” says O’Brien, who now has 80 such avocado plants growing in a greenhouse. The recovered avocado trees will be monitored for flowering times and fruit quality, with field trials to follow.

“I suppose you could say they are space-age avocados – ready to be cryo-frozen and shipped to Mars when human flight becomes possible,” said Mitter. “But it is really about protecting the world’s avocado supplies here on earth and ensuring we meet the demand of current and future generations.”