By Amy Maxmen
Bon appétit! A team of European researchers has decoded the genome of the delectable Périgord black truffle. Within its nucleotides reside secrets to the flavour and elusive lifestyle of this fungus, offering clues that could help a truffle industry that is fraught with unpredictable yields and a counterfeit market.
"There's a mystique around truffles. They're prized, rare and worth several thousand dollars per kilo," explains David Read, a mycologist at the University of Sheffield, UK, who was not involved in the genome study, which is published online in Nature. "People go out into the woods at night with dogs and pigs to locate them -- there's folklore behind it. And while most other mushrooms can be cultivated, the truffle has remained a serious challenge," he explains. "This report is a fundamental piece of work, yet it still leaves sufficient mystery in the area so that truffles can retain their influence on the human psyche."
The black truffle's scent has been variously described as earthy, dirty, musky or sexy. These 'black diamonds' sell for €1,000-€3,500 per kilogram ($1,345-$4,700) on today's market. Counterfeiters have been known to profit from the high price and demand for black truffles by replacing them with their cheaper yet similar-looking cousin, the Chinese truffle.
European truffle producers who sell black truffles specific to their region have been frustrated by sellers who make bogus claims about a truffle's origin. To sort out the scandals, mycologist Francis Martin, at the French National Institute for Agricultural Research in Nancy, and his colleagues are compiling a database of genetic markers to verify the geographic origins of black truffle populations. "The genome really changes everything," says Martin. "We now have a few thousand markers that will clearly be useful for professionals."
Scent from within
The genome of the black truffle (Tuber melanosporum) includes genes encoding flavour-related sulphur metabolites and enzymes that degrade amino acids. This reveals that its distinctive aroma and flavour is all produced in-house, and not by microbes native to the region in which the truffle grows, as many researchers had thought.
"When we started this work, many thought that truffles could be like cheese or wine, in that the microflora and yeast living on the truffles played a vital role in releasing volatile compounds," Martin says. "But we in fact demonstrated that the volatiles giving rise to the truffle's perfume are encoded in the truffle's genome."
The truffle's smell lures female pigs which, mistaking the aroma for an irresistibly scented boar, unearth the truffle and often consume it. When they excrete it, the truffle's spores are scattered around the forest floor. Other scents beckon the truffle fly, which visits the fungus in search of a mate but carries off fungal spores instead, spreading them between truffles.
Truffle cultivation is notoriously difficult, in part because of its clandestine life cycle as an underground symbiont, in which the fungus trades nutrients with oak-tree roots.
The T. melanosporum genome also reveals that the fungus reproduces sexually more often than researchers thought. Many growers rely on asexual truffle propagation, in which two haploid cells from a single fungus -- each with one copy of the genome -- fuse to form the diploid fruiting body (the truffle), which has two copies. Yet Martin and his team found two different sets of mating genes in the black truffle, suggesting that two strains of T. melanosporum with opposite mating types combined through sexual reproduction. Martin advises growers to use both mating types when inoculating oak trees, and to genetically fingerprint the truffles to be sure that they are not from the same family.
In 2008, Martin and his colleagues reported the genome ofLaccaria bicolor, another symbiotic fungus that is dependent on woody plants for nutrients. The 125-megabase black truffle genome is bigger than that of L. bicolor -- and four times larger than that of many fungi -- but it contains far fewer protein-coding genes. Besides sharing a handful of genes with L. bicolor that encode enzymes to degrade plant cell walls, the truffle lacks most of the genes that are involved in L. bicolor's symbiotic relationship with plants.
Mycologist David Hibbett of Clark University in Worcester, Massachusetts, says that he is amazed to learn that the two species of fungi share any genes involved in symbiosis at all. Because the truffle lineage separated from the lineage carrying L. bicolorbefore woody plants were around, they probably became symbionts independently, Hibbett says.
Although the sequenced genome opens up the possibility of genetically engineering other fungi to pack the same aromatic punch as the Périgord, gourmands who delight in truffled duck, white wine truffle sauce or truffled risotto are likely to wrinkle their noses at the thought of button mushrooms that have been engineered to smell like the real thing. "When you taste the black truffle on hot pasta, that is something you cannot forget," says Martin.