Most soldiers in the biotech revolution think the public will eventually accept genetically modified (GM) foods, thereby ending hostilities. However, science must first offer something of value, such as improved nutrition. Just making life easier for farmers with pest-resistant crops won't outweigh real or imagined risks to people or butterflies. That's the message of a new consumer poll done by Roper Starch Worldwide for the American Farm Bureau Federation.1
Metabolic or nutritional genomics--using genes to improve the nutritional value of plants--excites Dean DellaPenna, professor of biochemistry at the University of Nevada, Reno.2 He's even testified before Congress about it. Says DellaPenna, "I'm enormously hopeful. This has the potential like very few other things to benefit mankind." DellaPenna has reason to be enthusiastic: Recent reports, including one from his own laboratory, point to improved plant output traits in the form of extra vitamins.
Easy as A and E
Vitamins A and E are linked by common biosynthetic pathways and functions. Animals make the vitamin A, or retinol, they need for vision by cleaving provitamin A carotenoids--mostly orange-colored b-carotene--they get from plants. But millions of children go blind each year or die because of diets low in fresh fruit and vegetables, and many more are at risk. Plants make carotenoids and tocopherols (vitamin E) in plastids, including chloroplasts, where they function as pigments and antioxidants in photosynthesis. Those same antioxidant properties are important in human nutrition beyond recommended minimum daily allowances, or RDAs. Extra carotenoids and tocopherols reportedly enhance cardiovascular health, prevent cancer, and slow aging processes.
Agricultural enhancement of carotenoids and tocopherols would be a real boon to public health, especially in developing countries, since dispensing vitamins isn't cheap or easy. DellaPenna thinks "this is a way to get compounds to people who need them. Let them grow their own." But which plants to choose and how to engineer the traits?
Since rice is an important food staple worldwide, it's an obvious target for nutritional improvement. A rice grain is mostly endosperm tissue whose specialized plastids store starch. Unfortunately, endosperm doesn't make chlorophyll or b-carotene--that's why it's mostly white--so agronomists lack the raw material for traditional breeding. Enter biotechnology. In a gene-jockeying tour de force, a team led by Peter Beyer of the University of Freiburg, Germany, and Ingo Potrykus of Zurich's Swiss Federal Institute of Technology coaxed rice endosperm into making carotenoids by introducing foreign genes for three enzymes of the biosynthetic pathway.3
The first gene, from daffodil, encodes phytoene synthase, which combines two 20-carbon compounds called geranylgeranyl diphosphate (GGPP) into colorless 40-carbon phytoene. A second desaturase enzyme encoded by a bacterial gene introduces four double bonds to make red-colored lycopene (plants normally use two enzymes to do the job). The third enzyme, a daffodil cyclase, completes the pathway to ß-carotene. The researchers also had to make sure that the genes turned on in endosperm and the resulting enzymes moved into the plastids, so they added special promoters and chloroplast transit sequences.
Beyer and Potrykus didn't need fancy instruments to see their success: Transgenic rice grains were golden yellow. Still, analysis turned up ß-carotene and other nutritionally valuable carotenoids. Just 300 grams of rice each day should prevent vitamin A deficiency.
Surprisingly, transgenic lines without the daffodil cyclase gene still made ß-carotene, prompting speculation that endosperm's own cyclase was active or lycopene made by the desaturase induced it. That's important because the vector carrying the daffodil gene also had an antibiotic resistance marker to aid selection. If the extra cyclase isn't necessary, breeders can delete the resistance marker to head off Frankenfood brickbats. "There is no scientific reason to be afraid of ... marker genes in transgenic foodstuffs," insists Potrykus, adding, "We realize that psychological reasons also have to be taken [seriously]."
Rice breeders in Asia, Africa, and Latin America are busy moving the trait into common varieties. Beyer thinks that despite "many unforeseen problems ... rather related to organization and paperwork than to science, it should take two to three years." The hard part may be getting people to accept the yellow color.
Just a Spoonful of Oil
While rice grabbed headlines, scientists from California-based Calgene and parent company Monsanto added provitamin A to canola oil. Led by senior scientist Christine Shewmaker, the team transformed canola with a phytoene synthase gene from the same Erwinia bacterium tapped by the Europeans for its desaturase. The industry effort took advantage of the fact that canola's embryo plastids are green and therefore primed to make carotenoids. The embryo's cotyledons also assume the storage function of endosperm and accumulate lots of oil. In fact, 35 to 50 percent of the canola seed is oil. By goosing the embryo with another phytoene synthase gene, Shewmaker hoped to overexpress the enzyme, thereby boosting the seed's low carotenoid content. The lipid-loving carotenoids should end up in extracted oil, which is exactly what happened.4
Besides small amounts of lycopene, transgenic seed had lots of a- and b-carotene. In fact, total carotenoid increased 50-fold compared to controls--more than in tomato or carrot. Oil extracted from the seeds was dark orange, and Shewmaker estimates it would take "just a half teaspoon to get the RDA." But extra carotenoid consumed from provitamin-A-rich oil beyond that is still beneficial.
Shewmaker isn't sure why one extra gene adds up to so much carotenoid. For one thing, a whole lot more carbon flows through the pathway--mature transgenic seeds have 400 percent more total GGPP units. Despite the uncertainty, she sees canola and rice combining to improve health: "What's really nice is there's a variety of different methods to deliver provitamin A. We can reach different people, in different areas, using different foodstuffs."
E Too, Dean?
Dean DellaPenna did for vitamin E what Shewmaker's team accomplished for A, but a year earlier. The 20-carbon tocopherols, like carotenoids, are lipid soluble. In fact, 60 percent of dietary vitamin E in the United States comes from vegetable oil. Unfortunately, while a-tocopherol is the most nutritious form of the vitamin, vegetable oils are rich in its immediate precursor, g-tocopherol. Apparently, the final step leading to a-tocopherol, governed by the enzyme g-tocopherol methyl transferase (g-TMT), is limiting, so postdoctoral fellow David Shintani, together with DellaPenna, revved it up.5
In a complex series of molecular steps that included mining a cyanobacterial genome, Shintani and DellaPenna identified the g-TMT gene in canola's cousin Arabidopsis. They then transformed Arabidopsis with a seed-targeted extra copy of the gene. Transgenic seeds with the most g-TMT flipped their tocopherol levels from nearly all g to more than 95 percent a. Total tocopherol didn't change.
Given the potencies of various tocopherols, vitamin E levels jumped ninefold in transgenic Arabidopis oil, to nearly 100 international units. Arabidopsis oil isn't a hot commodity, but lots of people use canola. DellaPenna thinks fortifying vegetable oils to the same extent as Arabidopsis would "reduce incidence of heart attack by 50 percent, so benefits for even developed countries would be substantial," adding, "I'm not doing commercial crops, but companies I'm working with are."
One Plus One Equals ... One?
If vitamins A and E are so important, why not do both in the same oil-rich seed? It sounds reasonable, but it may not be easy. Plastids make carotenoids and tocopherols from GGPP along different branches of the same basic pathway. Since they compete for the same carbon skeletons, upping one may offset the other. Shewmaker's canola had 50 times more carotenoid but 50 percent less total tocopherol, mainly at the expense of the g form (but with considerable variability in a). Chlorophyll also fell in transgenic seed--not surprising since it too forms from plastid GGPP. With less g-tocopherol to work with, adding extra g-TMT would probably be pointless and perhaps harmful.
DellaPenna thinks Shewmaker's study "is just fantastic, but it points out how little we know about multiple pathway manipulations in plants." Adds Shewmaker, "if you want to stack traits, we have to be cognizant of how metabolic pathways interact." Still, if the latest studies say anything, it is to never underestimate the benefit of a little genetic tinkering.
Barry Palevitz (firstname.lastname@example.org) is a contributing editor for The Scientist.
1. American Farm Bureau Federation Web site: www.fb.com/2000annual/amnews/gapover4.html
2. D. DellaPenna, "Nutritional genomics: manipulating plant micronutrients to improve human health," Science, 285:375-9, July 16, 1999.
3. X. Ye et al., "Engineering the provitamin A (b-carotene) biosynthetic pathway into (carotenoid-free) rice endosperm," Science, 287:303-5, Jan. 14, 2000.
4. C.K. Shewmaker et al., "Seed-specific overexpression of phytoene synthase: increase in carotenoids and other metabolic effects," Plant Journal, 20:401-12, November 1999.
5. D. Shintani and D. DellaPenna, "Elevating the vitamin E content of plants through metabolic engineering," Science, 282:2098-100, Dec. 11, 1998.
|Biotech Breeds Strange Crop|
Vitamin-enriched plants may not qualify as conjured Frankenfoods, but biotech wizards still make things go bump in the night. The world first learned of Ingo Potrykus' golden rice at last summer's International Botanical Congress, courtesy of a press briefing and news release from Potrykus' sponsor, the Rockefeller Foundation.1 In preparing this story on vitamin-enriched plants, and with the August announcement in mind, The Scientist asked Rockefeller if the rice work was published. "I am not aware of any peer-reviewed journals publishing Dr. Potrykus' research yet," responded spokesman George Soule. The study appeared in Science just 11 days later. Why hadn't Rockefeller mentioned it? Explains Soule, "The intention was to let Science, not Rockefeller Foundation, promote this news. As a peer-reviewed journal, Science has a degree of credibility and objectivity that we as the project's primary funder might not be viewed as having."
Science indeed hyped the work, with a separate perspective piece. But was it really news? The Associated Press and Reuters thought so, covering the article as if last summer never happened.
Even more surprising, Science played second banana on the rice article--Potrykus and colleague Peter Beyer first sent it to Nature, which rejected it without peer review. According to Potrykus, "Nature was not even willing to show the paper to referees. Nature was on an antitransgene track at that time. They preferred to publish about crazy experiments such as feeding Bt pollen to monarch butterflies."
Like seed in spring, strangeness keeps sprouting. Rice lead author Xudong Ye moved from Potrykus' lab in Zurich to Agrecetus in Madison, Wis. Agrecetus is a division of Monsanto, which did the canola work.
The strangest harvest may yet materialize in the Illinois boardroom of grain processor Archer Daniels Midland (ADM). By asking farmers to segregate GM from traditional crops, ADM generated shock waves last September that reverberated from commodity markets to Monsanto's St. Louis headquarters. Not long afterward, ADM formed a new nutraceutical division geared toward "functional food products and supplements such as ... vitamin E." ADM claims "an all-natural product line that contains countless items with real nutritional benefits, from soy protein to vegetable oil to vitamins and more." But what will ADM do when farmers show up with transgenic canola, corn, or soybean oil containing gobs of vitamin A and/or E?
1. R. Lewis, "Rice delivery," The Scientist, 13:31, Aug. 30, 1999.