Truffles and Chinese Gooseberries

October 22, 2014

Bacteria are sometimes treacherous, as when they cause infection, but they are sometimes useful. Baker's yeast gives us bread, and vintner's yeast gives us wine. Science facilitates winemaking, as it does for every industrial process. Oenology, a word derived from the Greek, οινος (wine) and λογια (study of), is the science of winemaking.

The bacteria in wine are just there for their ability to produce ethanol from sugar, but some bacteria impart flavor to food. Cheese is made from milk by several methods, but one of these is the use of fermentation bacteria to convert the milk sugar, lactose, into lactic acid. The lactic acid curdles the milk, and the bacteria impart flavor. One cheesemaking bacteria is Lactococcus.

The truffle, the fruiting body of the Ascomycota fungus, is a prized and expensive food additive whose price is nearly that of gold. Truffles are a good example of the economic principle that scarcity will force an increase in price until an equilibrium of supply and demand is reached. Cultivation of truffles is difficult, since they are found only in symbiotic association with the roots of certain trees, such as oak, hazelnut, beech and birch.

Black truffles from Istria, Croatia. (Wikimedia Commons image by Sl-Ziga.)

Truffles are prized for their flavor, and a recent study by scientists from the Georg-August University of Goettingen (Goettingen, Germany), the Institute for Molecular Biosciences (Frankfurt am Main, Germany), and the Université de Lorraine (Champenoux, France) has shown that the flavor of white truffles comes mostly from soil bacteria trapped inside the truffle fruiting bodies.[1-2] A report on this research is contained in a recent issue of the journal, Environmental Microbiology.[1]

White truffles from the Piedmont region of Italy are the most expensive class of truffle, costing about 5,000 Euros/kilogram. The less expensive (2,000 Euros/kilogram) black truffles are found in the Périgord region of Southern France. The research team studied the white truffle, Tuber borchii, which is native to Europe but is now cultivated in New Zealand and Argentina.[2]

It's been suspected for the last decade that bacteria might contribute to truffle flavor. The study identified thiophenes, sulfur-containing volatile chemicals produced by bacteria.[1] These volatile sulfur compounds make up a portion of the distinctive truffle smell. Pigs and dogs are able to find truffles, which grow underground, because of the sulfur smell.[2]

One test of this hypothesis was to treat the truffles with antibacterial agents. The antibacterial agents fully suppressed the production of thiophene volatiles, while application of fungicide had no affect.[1] Says Richard Splivallo, a study author from the Georg-August-University,

"However, our results cannot be transferred to other types of truffles because the compounds we investigated are only found in the white truffle Tuber borchii... We don't just want to know which part of the truffle flavor is produced by bacteria. We are also interested in how the symbiosis between fungi and microorganisms has evolved and how this benefits both symbiotic partners."[2]

This research was funded by the German Research Foundation (Deutsche Forschungsgemeinschaft).

Have you ever eaten a Chinese gooseberry? It's likely that you have, but didn't realize it, since these are now known as kiwifruit. Kiwifruit, the fruit of the the Actinidia chinensis, is native to China, but it was first produced in New Zealand in 1906. By a clever rebranding in 1974 to identify it with the cute, flightless kiwi bird, this fruit has gained considerable market share. Nearly 1.5 million tons of kiwifruit are produced annually.

Whole and cut kiwifruit. (Wikimedia Commons photo by André Karwath, via Aka.)

The kiwifruit has a high vitamin C content, and Actinidia chinensis has become an important model plant for studies in the production of vitamin C, carotenoids and flavoloids. That's why an International Kiwifruit Genome Consortium (IKGC) was initiated in 2011 to sequence the whole genome of Actinidia chinensis. A draft of this genome has just been published by a large international team of scientists from twelve institutions, including Hefei University of Technology (Hefei, China), Sichuan University (Chengdu, China), and Cornell University (Ithaca, New York).[3-6]

A Chinese variety of the kiwifruit plant, “Hongyang,” was used to produce the draft sequence. The genome was found to have a total length of 616.1 mega-base-pairs (Mb) and it contains 39,040 genes.[3] For comparison, the human genome is about 3,200 Mb long, and it contains just 20,000–25,000 protein-coding genes. Kiwifruit has many genetic similarities to other plant species, including potatoes and tomatoes.[5]

Kiwifruit diverged from the tomato and potato in a way that modified its vitamin C, flavoloids and carotenoids.[3] About 8,000 genes are found to be in common with tomato, rice, grape, and the mustard weed, Arabidopsis.[5]

The kiwifruit plant (Actinidia chinensis) has many familiar cousins, as shown in this taxonomic tree. Shown here are Solanum lycopersicum (tomato), Solanum tuberosum (potato),Vitis vinifera (common grape vine), Citrus Sinensis (orange), Theobroma cacao (cocoa tree), Carica papaya (papaya), Brassica rapa (turnip family),Arabidopsis thaliana, Populus trichocarpa (western balsam poplar),

Ricinus communis (castor oil plant), Cajanus cajan (pigeon pea), Glycine max (soybean), Citrullus lanatus (watermellon), Cucumis melo (muskmelon), Cucumis sativus (cucumber), Fragaria vesca (strawberry), Malus domestica (apple), and Prunus mume (Chinese plum). (Adapted from fig. S1 of the supplementary information for ref. 3, in turn based on published sources.)[4]

The data showed that about 80 million years ago there was an extensive expansion of genes when an entire extra copy of the genome was added. This event was followed about 27 million years ago by an extensive gene loss.[5] When genes are duplicated like this, the extra genes can mutate to perform new functions. This neofunctionalization was beneficial to kiwifruit.[5]

Says Zhangjun Fei, a study author and an associate professor at Cornell University, “The duplication contributed to adding additional members of gene families that are involved in regulating important kiwifruit characteristics, such as fruit vitamin C, flavoloid and carotenoid metabolism.[5]

References:

1. Richard Splivallo, Aurélie Deveau, Nayuf Valdez, Nina Kirchhoff, Pascale Frey-Klett, and Petr Karlovsky, "Bacteria associated with truffle-fruiting bodies contribute to truffle aroma," Environmental Microbiology Early View (2014), doi: 10.1111/1462-2920.12521).
2. On the trail of the truffle flavor - Soil bacteria contribute to the taste and smell, Goethe University Frankfurt Press Release, September 30, 2014.Press release in German can be found here.
3. Shengxiong Huang, Jian Ding, Dejing Deng, Wei Tang, Honghe Sun, Dongyuan Liu, Lei Zhang, Xiangli Niu, Xia Zhang, Meng Meng, Jinde Yu, Jia Liu, Yi Han, Wei Shi, Danfeng Zhang, Shuqing Cao, Zhaojun Wei, Yongliang Cui, Yanhua Xia, Huaping Zeng, et al., "Draft genome of the kiwifruit Actinidia chinensis," Nature Communications, vol. 4, article no. 2640 (October 18, 2013), doi:10.1038/ncomms3640. This is an open access article with a PDF file available here.
4. Supplementary Information for ref. 3 (PDF File).
5. Melissa Osgood, "Surprises discovered in decoded kiwifruit genome," Cornell University Press Release, October 25, 2013.
6. Kiwifruit Genome Database at Cornell University.