On 10 October 2019, a dog began pawing excitedly at the ground beneath a young oak tree in western France. Its owner eased it out of the way and pulled an Italian white truffle (Tuber magnatum Pico) from the earth. Knobbly, covered in soil and about the size of a hen’s egg, it was not much to look at, but the fungal discovery nonetheless generated ripples of excitement among researchers, chefs and truffle growers worldwide.
That’s not just because T. magnatum is the most expensive truffle species, for which wealthy gastronomes are willing to pay up to US$11,000 per kilogram. Although more than 90% of the also highly sought-after black Périgord truffles (Tuber melanosporum) served in restaurants today are farmed, previous attempts to cultivate their more elusive white counterparts had failed.
That changed three years ago, when the Lagotto Romagnolo, the Italian dog breed commonly used as a truffle hunter, unearthed the first Italian white truffle confirmed to have been cultivated outside its natural range. The dog made the find at its owner’s plantation in the Nouvelle Aquitaine region of France, but the precise location is being kept secret to deter thieves.
Scientists at a laboratory run jointly by France’s National Research Institute for Agriculture, Food and the Environment (INRAE) and the University of Lorraine in Nancy reported1 that since that first T. magnatum truffle was unearthed, two more were found at the site in 2019 and four in 2020. In an article published last month in Le Trufficulteur, the magazine of the French Federation of Truffle Growers, the researchers report the cultivation of 26 truffles last year2.
“I was very happy to hear these results,” says Alessandra Zambonelli, a mycologist at the University of Bologna, Italy, who has studied Italian white truffles for more than 40 years, and whose own attempts to cultivate them in the 1980s failed. “I was sure it was possible to cultivate T. magnatum, but only now do we have the scientific proof.”
The INRAE project is helping growers to better understand the optimal conditions for cultivating Italian white truffles. Some scientists think the breakthrough could help to reverse falls in harvests of wild truffles that have been linked to climate change. Researchers also hope the work will help them to answer outstanding questions about the life cycle of the species and understand why it is so much harder to farm than are other truffles.
Tuber magnatum’s natural range is more limited than those of other sought-after truffles, growing as it does in parts of Italy, southeastern France, the Balkans and Switzerland. It is highly prized for its intense, some say intoxicating, aroma and flavour, variously described as reminiscent of garlic, fermented cheese and methanethiol — the additive that gives domestic gas its smell. Prices fluctuate in line with supply, which varies according to climatic conditions. These hit an all-time high in 2021, when US prices were more than triple what they were in 2019.
Most land plants form symbiotic relationships with fungi to access extra water and mineral nutrients. In return, the plants provide their fungal partners, which grow around and into their root tips, with carbon-rich nutrients. These associations are known as mycorrhizae. What most people call truffles are, in fact, just the spore-containing fruiting bodies of the fungus.
In the 1970s, French scientists successfully induced Périgord truffles to form mycorrhizal associations with tree seedlings by inoculating the seedlings with their spores. The same technique was used at the time to produce trees with T. magnatum mycorrhizae. More than 500,000 of these were planted in Italy. But when researchers later began using the polymerase chain reaction (PCR) technique to accurately identify truffle mycorrhizae, fruiting bodies and the root-like mycelia, it became clear that this species’ physical characteristics had been poorly described, and that, as a result, many of the trees had in fact partnered with less sought-after truffle species.
Some sites in Italy did produce T. magnatum truffles 15–20 years after planting, but only in areas where the species occurs naturally. “It is likely that those found so long after being planted came from chance colonization of host plants by native T. magnatum strains in the environment,” says Claudia Riccioni, a plant and fungal biologist at Italy’s Institute of Biosciences and BioResources in Perugia.
After the Italian white and Périgord truffles, the next most sought-after species is the summer truffle (Tuber aestivum), which grows in many European countries and sells for much less than its more highly regarded cousins. Plantations of T. aestivum have been established in France, Italy, Scandinavia, Germany and elsewhere.
In 1999, INRAE researchers joined forces with Robin Pépinières, a nursery based in Saint-Laurent-du-Cros, southern France. Genetic analysis confirmed that the nursery had produced trees that partnered with T. magnatum, leading, from 2008, to the establishment of plantations in France1. In 2018, the INRAE group selected five of these, all outside the part of southeastern France where T. magnatum grows naturally, to see whether it had become established and to record the conditions under which any truffle fruiting bodies were produced.
PCR tests confirmed the fungus’s mycelia were present in soil samples taken from near the trees at four of the locations. The first three truffles, found in Nouvelle Aquitaine, were discovered four-and-a-half years after the inoculated trees had been planted. Further PCR tests confirmed they were T. magnatum. The 26 truffles found in 2021 were unearthed beneath 11 different trees, with 5 under one of them. The largest weighed 150g.
Mycologists Claude Murat and Cyrille Bach, both members of the INRAE–University of Lorraine lab, were present when one of the four fruiting bodies produced in 2020 was discovered. Asked how sure he was that the truffle grew in the plantation and hadn’t originated elsewhere, Murat said: “I’m 100% sure. We could see the soil had not been disturbed and that grasses were growing there.”
Previous attempts to cultivate Italian white truffles failed in part because their life cycle remains poorly understood. Twenty years ago, it was widely assumed that truffles, including T. magnatum, were self-fertile. However, research then showed they have one of two ‘mating type’ genes, and that the mycelia of individuals of different mating types must meet for reproduction to occur3.
A remaining unresolved puzzle is why researchers have found T. magnatum mycorrhizae much harder to locate than those of other truffles. Mycologist Paul Thomas works to establish joint ventures with truffle growers through Mycorrhizal Systems, his UK-based company. He inoculated host trees with T. magnatum, and generated mycorrhizae at the company’s greenhouses in Preston, but these did not last long, so the trials were abandoned.
“When you find fruiting bodies, you quite often can’t find mycorrhizae,” says Thomas, “and sometimes you get mycorrhizae but no fruiting bodies. Perhaps, in the case of T. magnatum we’ve become too focused on linking truffle production to mycorrhizae.”
When Zambonelli’s group analysed soil from four Italian white-truffle sites over three years, they found a correlation between production of fruiting bodies and a location’s concentration of DNA from T. magnatum mycelia4. Some researchers began to suspect that the host–fungus relationship might not be as important as previously thought, and that T. magnatum might be saprotrophic, meaning that it digests dead or decaying organic matter.
However, a 2018 comparison5 of the genomes of truffle species with those of several saprotrophic fungi showed this to be unlikely. “T. magnatum has very few plant-wall-degrading enzymes, which does not support the saprotrophic hypothesis,” says Riccioni, one of the study’s authors. Other researchers have tried to explain the elusiveness of T. magnatum mycorrhizae by pointing out that other truffles can form endophytic relationships with plants in which they which live throughout them, not just at their roots.
Murat wonders whether he and others have just been looking in the wrong place. “We look on the roots down to 20 centimetres, never looked at 50 centimetres, even though we know other mycorrhizae can be found at those depths,” he says. “Or perhaps they produce mycorrhizae just for a very short time; we just don’t know.”
A growing body of research shows that microorganisms have important roles in truffle life cycles. A 2015 review found that bacteria in T. magnatum fruiting bodies help to create the truffles’ odours6. Zambonelli and her colleagues found that bacteria in T. magnatum fruiting bodies can fix nitrogen for nutritional purposes7. Another Italian team found that microbes commonly associated with white truffles are involved in fruiting-body maturation8. “Some bacteria could also help T. magnatum become established at tree roots and fruiting-body formation,” says Zambonelli.
A changing climate
Gathering accurate statistics on truffle yields before cultivation is difficult, although it is widely accepted that these fell significantly during the twentieth century. One study reports that Périgord truffle harvests in France collapsed from 500–1,000 tonnes annually in the 1900s to 10–50 tonnes by the 2000s. Yields in Italy declined, too, but not by as much, and mostly in the first half of the twentieth century9.
The reasons for falls in truffle harvests are complex and vary by location, but researchers have blamed depopulation, loss of knowledge about truffle hunting and deforestation. Some of the older men who featured in the highly rated 2020 documentary The Truffle Hunters, set in Piedmont, northern Italy, say they will take what they know about truffles to the grave rather than pass it on to younger generations because of the greed they see in the industry.
More recently, some researchers have highlighted climate change as another cause of declining yields. Truffle gastronomy and tourism are economically and culturally important in places where truffles occur naturally. That’s certainly true in parts of Croatia, where, from 2003 to 2013, reported annual harvests were 1–3 tonnes for Italian white truffles and 1–6 tonnes for Périgords, except for the years 2009, 2010 and 2013, when they fell to 0.1–0.5 tonnes.
Field mycologist Željko Žgrablić at the Ruđer Bošković Institute in Zagreb says truffles have become harder to find on the Istria peninsula, where he grew up, in part because of increasingly frequent and severe droughts. Yields have also been affected by big increases in wild-boar populations as a result of warmer winters. The animals forage for the truffles and reduce human harvests, and, according to Žgrablić, also damage the fungus’s mycelia. “The climate has become unpredictable, with more extremes,” says Žgrablić. “It’s hard to prove it, but I think we have fewer white truffles as a result.”
In a 2019 study, Thomas analysed annual Périgord truffle yields in the Mediterranean region over a 36-year period10. He concluded that decreased summer rain and increased summer temperatures significantly reduced subsequent winter harvests. He forecast declines of 78–100% in harvests between 2071 and 2100 as a result of further predicted warming. “White truffles need relatively moist soil, so in its natural range it might be okay in mountainous areas but particularly vulnerable in areas where falls in rainfall are predicted,” says Thomas.
Beyond producing the first confirmed cultivated white truffles, the INRAE project is also generating data on the optimal conditions for production. The soil temperature at the site that yielded the truffles was around 20 °C in the summer, and Murat says that the team’s tests suggest white truffles need more water than do Périgords.
So could the increasing knowledge of how best to get Italian white truffles to grow be adopted more widely to help reverse declining yields? Fruiting bodies have been confirmed at only one site, so other growers are waiting to see whether this success will be repeated elsewhere. Murat is in the process of trying to confirm recent claims from two other owners that they, too, have cultivated T. magnatum truffles.
Thomas is downbeat about the future of Italian white-truffle cultivation. “In parts of Spain, more and more orchards can no longer irrigate because of water shortages. Already, in France, it is hard to get permission to extract water from rivers for irrigation, and that’s only going to get worse.”
Oak trees inoculated with Périgord- and summer-truffle spores are due to be planted later this year in Croatia as part of a collaboration run by the state-owned Croatian Forests. If successful, the group could try white truffles. Žgrablić, who is part of the project, is also advising an enthusiast who planted 650 seedlings inoculated with T. magnatum, also in Croatia, earlier this year. “We’re seeing increasing interest from private investors in cultivating Italian white truffles,” he says. “There is certainly a lot of potential, but what the results will be, I can’t tell.”
Alongside his research work, Murat acts as a scientific consultant for WeTruf, a company he co-founded in Nancy that provides advice and monitoring services for truffle farmers. He is cautious about the potential for white-truffle cultivation, if optimistically so. “We are careful when people tell us they want to start big white-truffle plantations,” says Murat. “I tell them ‘we are only at the beginning, we don’t know if it will succeed or not’. But I think there will be more and more plantations, and, if they apply good management practices, I hope, more and more truffles.”
Source: Ecology - nature.com