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"Long before it's in the papers"
June 29, 2012

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How tomatoes lost their flavor

June 29, 2012
Courtesy of Science, UC Davis
and World Science  Staff

Breed­ers have un­know­ingly bred the fla­vor out of toma­toes by fa­vor­ing those with a nice un­iform col­or, sci­en­tists are re­port­ing.

It’s hoped the find­ing could help grow­ers re­cap­ture the old, sweet fla­vor of toma­toes—which, as they sit on su­per­mar­ket shelves to­day, of­ten seem not to taste much dif­fer­ent from the pack­ag­ing they sit in.

Varieties of heirloom tomatoes. (Image courtesy Ann Powell, UC Davis)
The find­ing, re­ported in the June 29 is­sue of the jour­nal Sci­ence, could have im­plica­t­ions for the U.S. to­ma­to in­dus­try, which har­vests over 15 mil­lion tons of the fruit yearly for pro­cess­ing and fresh-market sales.

“This in­forma­t­ion… pro­vides a strat­e­gy to re­cap­ture qual­ity char­ac­ter­is­tics that had been un­know­ingly bred out of mod­ern cul­ti­vat­ed toma­toes,” said Ann Pow­ell, a bio­chem­ist at the Uni­vers­ity of Cal­i­for­nia Da­vis and one of the lead au­thors of the stu­dy.

For about 70 years, breed­ers have se­lected to­ma­to va­ri­eties with un­iformly light green fruit be­fore rip­en­ing. These toma­toes then turn red evenly as they rip­en, and they look nice in a su­per­mar­ket dis­play. Pow­ell and col­leagues say the gene at the heart of un­iform rip­en­ing codes for the pro­duc­tion of a mol­e­cule called GLK2, which is a tran­scrip­tion fac­tor, mean­ing it go­verns ge­net­ic ac­ti­vity. 

GLK2 boosts the fruit’s ca­pa­city for pho­to­syn­the­sis, the pro­cess of con­vert­ing sun­light to sug­ars, Pow­ell and col­leagues found. The mol­e­cule al­so aids the pro­duc­tion of  ly­copene, a health pro­mot­ing com­pound. But the un­iform-rip­en­ing muta­t­ion dis­ables GLK2, the re­search­ers found. This leads to in­fe­ri­or de­vel­op­ment of pho­to­syn­the­sis-enabling cel­lu­lar struc­ture called choloro­plasts, and in turn, low­er pro­duc­tion of key in­gre­di­ents that give toma­toes their sweet­ness.

Re­search­ers at the uni­vers­ity be­gan stu­dying the genes in­flu­enc­ing to­ma­to de­vel­op­ment and rip­en­ing af­ter screen­ing to­ma­to plants for cer­tain tran­scrip­tion fac­tors that might play a role in both col­or and qual­ity. They were par­tic­u­larly in­ter­est­ed in toma­toes they saw that were un­usu­ally dark green be­fore rip­en­ing. Part­ner­ing with re­search­ers at Cor­nell Uni­vers­ity in New York and in Spain, who were map­ping re­gions of the to­ma­to ge­nome, the sci­en­tists disco­vered two tran­scrip­tion fac­tors, GLK1 and GLK2, that con­trol the de­vel­op­ment of chloro­plasts.

The re­search­ers scoured a col­lec­tion of mu­tant and wild spe­cies of toma­toes es­tab­lished at UC Da­vis by the late Pro­fes­sor Charles Rick be­gin­ning in the 1950s. They disco­vered that dark green toma­toes that nat­u­rally pro­duce GLK2 pro­duced ripe fruit with more sug­ars or sol­u­ble solids, im­por­tant for pro­cess­ing toma­toes, as well as more ly­copene.

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Breeders have unknowingly bred the flavor out of tomatoes by favoring those with a nice uniform color, scientists are reporting. The finding could help growers recapture the old, sweet flavor of tomatoes—which, as they sit on supermarket shelves today, often seem as though they don’t taste much different from the packaging they sit in. The finding, reported in the June 29 issue of the journal Science, could have implications for the U.S. tomato industry, which harvests over 15 million tons of the fruit yearly for processing and fresh-market sales. “This information… provides a strategy to recapture quality characteristics that had been unknowingly bred out of modern cultivated tomatoes,” said Ann Powell, a biochemist at the University of California Davis and one of the lead authors of the study. For about 70 years, breeders have selected tomato varieties with uniformly light green fruit before ripening. These tomatoes then turn red evenly as they ripen, and they look nice in a supermarket display. Powell and colleagues say the gene at the heart of uniform ripening codes for the production of a molecule called GLK2, which is a transcription factor, meaning it governs genetic activity. GLK2 boosts the fruit’s capacity for photosynthesis, the process of converting sunlight to sugars, Powell and colleagues found. The molecule also aids the production of sugars and lycopene, a health promoting compound. But the uniform-ripening mutation disables GLK2, the researchers found. This leads to inferior development of photosynthesis-enabling cellular structure called choloroplasts, and in turn, lower production of key ingredients that give tomatoes their sweetness. Researchers at the university began studying the genes influencing tomato development and ripening after screening tomato plants for certain transcription factors that might play a role in both color and quality. They were particularly interested in tomatoes they saw that were unusually dark green before ripening. Partnering with researchers at Cornell University in New York and in Spain, who were mapping regions of the tomato genome, the scientists discovered two transcription factors, called GLK1 and GLK2, that control the development of chloroplasts. The researchers scoured a collection of mutant and wild species of tomatoes at the university established at UC Davis by the late Professor Charles Rick beginning in the 1950s. They discovered that dark green tomatoes that naturally produce GLK2 produced ripe fruit with more sugars or soluble solids, important for processing tomatoes, as well as more lycopene.