In their quest for the perfect tomato plant – one that produces an abundance of firm, blemish-free fruit – breeders have forgotten the most important thing: taste. But researchers are studying tomato DNA to find a way to restore their old-fashioned flavor, while keeping all the benefits of modern varieties.
Have tomatoes really lost their flavor? That’s what a lot of foodies say – and science has proven them right. The crux of the problem lies in modern varieties’ DNA, as decades of artificial selection have turned the fruit into the bland commodity now sold in supermarkets.
Researchers have measured this loss of flavor and can now even offer solutions. Denise Tieman, a biologist at the University of Florida, has pinpointed the chemical compounds that give tomatoes their unique taste and identified the genes involved. Her findings are unequivocal: most tomato varieties used in intensive farming today are genetically incapable of making enough flavor compounds to be detected.
A delicate, subtle taste
Tomatoes suffer more from artificial selection than any other fruit or vegetable. That’s because of their highly complex flavor mechanisms. Whereas a banana’s flavor essentially comes from a single chemical compound, Tieman found that a tomato’s depends on dozens. Some are present in only miniscule amounts, with concentrations of just a few parts per billion. But the right balance of all these compounds is what makes tomatoes tasty.
Because this balance is so tenuous, and the concentrations of some compounds so small, even an ostensibly harmless genetic mutation could alter the fruit’s taste, according to Tieman. But such mutations are exactly what breeders seek to exploit, whether to create giant tomatoes, miniature dogs, hairless cats, or violet tulips. Artificial selection has been around for centuries, but its consequences are not always well understood.
Tieman showed that 13 of a tomato’s flavor compounds are especially vulnerable to selection processes. “Modern varieties were engineered for maximum yields and disease resistance, and to withstand shocks during transportation. The effect on flavor has been a gradual, unintended consequence,” she said.
Taste testing and genetic sequencing
Tieman’s research, recently published in the prestigious Science magazine,1 started with a simple taste test. Volunteers were asked to rate the tastes of various types of wild, heirloom and modern tomatoes. By combining their subjective evaluations with her chemical analyses, Tieman pinpointed 13 flavor compounds that were missing in contemporary varieties.
Meanwhile, researchers at the Agricultural Genomics Institute at Shenzhen finished sequencing the full genomes of over 400 tomato varieties. They identified the genetic mutations in modern tomatoes that had caused those 13 compounds to more or less disappear. Their painstaking work took over 15 years but resulted in a complete genetic map of the fruit’s taste. “Our goal wasn’t to make genetically-modified tomatoes, but to develop a tool that would let growers select varieties knowing full well what to expect,” said ???.
Sugar: another culprit
When it comes to the insipidness of modern tomatoes, a 2012 study2 laid the blame on another factor. Researchers at the University of California, Davis found that today’s varieties are sorely lacking in sugar, another taste-enhancing compound. And they identified the gene responsible: SIGLK2.
This gene had been mutated to produce attractive, uniform tomatoes – the holy grail for tomato farmers, since they make harvesting easier. These modified tomatoes can be picked while still a smooth, vibrant green, and become red by the time they are displayed in stores a few days later. The motley appearance of heirloom tomatoes is less effective and can put off consumers. Who wants blemished tomatoes in their salad?
But that beauty has a price: sugar. “Beautiful fruit that tastes like carboard,” said the researchers, whose work was also published in Science. The starch in the modified tomatoes is not properly transformed as they ripen.
Without realizing it, tomato breeders across the globe – all striving for the same goal – engineered the same genetic mutation. The SIGLK2 gene mutates easily; it has a section with six of the same elements in a row, making it prone to DNA copying errors. The upshot is that unrelated tomatoes from around the world all have the same flaw. By placing a functional copy of the SIGLK2 gene back into mutated varieties, the researchers were able to increase sugar production by 40%. But, alas, the dreaded blemishes returned.
But it goes beyond genetics
The problem of tasteless tomatoes is not due solely to genetics. Other factors, such as a premature harvest or being shipped in refrigerated trucks, also play a role. “These processing issues are undoubtedly just as important as genetics. But if the tomatoes are bland to start with, then there’s no way they’re going to get any better,” said Tieman.
Tieman is now working on developing varieties whose taste can withstand refrigeration. Thanks to her prior research, she knows exactly which important taste-related genes are active in the species she is studying. So all that’s left is the tedious work of cross-breeding generation after generation of plants, the old-fashioned way.
She admits that supermarkets are nowhere near being able to sell tomatoes that are as tasty as our grandmothers’. “Heirloom varieties with an irregular appearance simply taste better. But I’m optimistic. A genuine awareness does seem to be building among consumers, which could turn things around. We have started handing out seeds of our varieties on campus, and they are much more popular than we expected. It’s gratifying to see people so directly appreciate the results of our work,” Tieman said.
- Denise Tieman, Guangtao Zhu, Marcio F. R. Resende Jr., Tao Lin, Cuong Nguyen, Dawn Bies, Jose Luis Rambla, Kristty Stephanie Ortiz Beltran, Mark Taylor, Bo Zhang, Hiroki Ikeda, Zhongyuan Liu, Josef Fisher, Itay Zemach, Antonio Monforte, Dani Zamir, Antonio Granell, Matias Kirst, Sanwen Huang, Harry Klee. A chemical genetic roadmap to improved tomato flavor, Science (2017) DOI: 10.1126/science.aal1556.
- Ann L. T. Powell, Cuong V. Nguyen, Theresa Hill, KaLai Lam Cheng, Rosa Figueroa-Balderas, Hakan Aktas, Hamid Ashrafi, Clara Pons, Rafael Fernández-Muñoz, Ariel Vicente, Javier Lopez-Baltazar, Cornelius S. Barry, Yongsheng Liu, Roger Chetelat, Antonio Granell, Allen Van Deynze, James J. Giovannoni, Alan B. Bennett. Uniform ripening Encodes a Golden 2-like Transcription Factor Regulating Tomato Fruit Chloroplast Development, Science (2012) DOI: 10.1126/science.1222218.