Cryo Validation: Conserving potato biodiversity for the long term

Changes in the climate, landscapes, and pest and disease pressure are driving a necessity to transform agriculture, which requires preserving the agrobiodiversity breeders and farmers need to achieve adaptation. Yet some of the same factors that threaten the future of farming endanger agrobiodiversity in the field today, making the conservation of plant genetic resources in genebanks, or ex situ, more important than ever.

Biodiversity safeguarded in genebanks is frequently shared with crop breeders or farmers, and it could be key to overcoming threats to food production that scientists have yet to even image. It is thus vital to conserve as much agrobiodiversity as possible, and for as long as possible; preferably forever.

For scientists at the International Potato Center (CIP) genebank, which conserves the world’s most diverse collections of potato and sweetpotato biodiversity, forever increasingly looks feasible. That’s because a growing portion of the more than 4,900 cultivated potato accessions held in the genebank are now conserved using cryopreservation, in which plant tissue samples are stored in liquid nitrogen at a temperature of -196 °C.

Those frozen accessions can theoretically remain viable for centuries, and as scientists explain in an article recently published in Frontiers in Plant Science, the genebank has made great progress in implementing cryopreserving for potato over the past decade. By undertaking experiments and monitoring results on a wide genetic diversity of accessions, a team coordinated by CIP scientist Rainer Vollmer – lead author of the above article – obtained results that could be extrapolated over much of the potato collection, developing a protocol that reduced the time required and cost of cryopreserving potato, while increasing the average recovery rate.

Conservation challenges

Genebanks primarily safeguard seeds, but potato and sweetpotato are “clonal” crops, for which farmers plant tubers or vines that are genetically identical to mother plants. CIP thus conserves those crops – under the mandate of the International Treaty on Plant Genetic Resources for Food and Agriculture – as tiny plants in test tubes. Those in vitro plantlets need to be removed from their test tubes and regenerated every two or three years, when tissue cuttings are taken and grown into new plantlets to replace them; a process that is labor-intensive and expensive.

In vitro conservation is not only used for potato and sweetpotato but also clonal crops such as cassava, banana or yam, and crops with recalcitrant seeds that can’t be dried and stored at a low temperature. There is a risk of accessions becoming mixed up or contaminated with pathogens during the regeneration process, so cryopreservation is both safer and more cost and space efficient than in vitro, and offers the option of preserving genetic materials for anywhere from decades to centuries.

“We already know that cryopreservation is the future for clonal crops and for crops with recalcitrant seeds, but there is still a lot of research that needs to be done,” said Vania Azevedo, Head of the CIP genebank and Biodiversity for the Future Program.

To ensure that cryopreserved samples grow into healthy plants when thawed, a team led by Vollmer assessed the post-thaw viability of an array of accessions after two, four and eight years in liquid nitrogen. Future viability experiments are planned for every 16, 32 and 64 years from the start of cryopreservation at CIP.

The average recovery rate of potato accessions has been improved, and though cryopreservation of sweetpotato is more challenging than potato, recovery rates for both crops are improving. According to Vollmer, lessons learned from potato have been applied to sweetpotato and can also be applied to other clonal crops.

Cryopreservation achievements

More than 83% of the cultivated potato in vitro accessions in the CIP genebank have been backed up with cryopreserved copies, and Vollmer expects the entire collection to be cryopreserved within four years. He explained that the size of the in vitro collection will be reduced in the future as accessions nobody has requested for a decade will be progressively stored only in the cryobank. However, if needed for research, breeding or by farmers, they can be thawed, recovered, and ready to be shipped as in vitro plantlets in 50-55 days.

While improving the cryopreservation of potato and sweetpotato agrobiodiversity conserved by the CIP genebank, scientists have shared the knowledge and skils they’ve developed with colleagues at other genebanks in Latin America and the Caribbean. They’ve also begun expanding the CIP cryobank’s capacity in order to preserve safetly copies of the clonal crop and recalcitrant-seed crop collections of other genebanks.

Azevedo explained that the COVID-19 pandemic highlighted one of many advantages of cryopreservation. “During lockdown, the routine maintenance of the in vitro collection was disrupted by restrictions in the number people allowed to work in the genebank, but the cryobank continued its normal functioning because just one person was needed to fill the cryotanks with liquid nitrogen every two weeks” she said.

“The pandemic showed us how fragile in vitro collections of clonal crops can be,” she observed.

According to Azevedo, by reducing the cost and space needed for germplasm conservation, cryopreservation can enable the genebank to safeguard more biodiversity than before.

“Last year, CIP analyzed more than a thousand native potato landraces in collaboration with Andean communities in Peru’s Pasco and Lima regions, and we identified 90 that weren’t represented in the potato collection,” explained Azevedo.

“The farmers donated these lines to CIP and a copy of each was saved in the cryobank. In return, their communities will always be able to have those landraces repatriated for free, after they’ve been cleaned of yield-reducing diseases, which is good for their future harvests,” she said.

“By facilitating such interventions, cryopreservation is helping ensure that the CIP genebank benefits local farmers and conserves more genetic diversity, which could end up benefiting people anywhere in the world, in this or future generations,” Azevedo said.

Blog by Gaia Luziatelli and David Dudenhoefer

This work is supported by the Global Crop Diversity Trust and contributors to the CGIAR Trust Fund.