Red not purple

March 2014
Close up of a red tomato
Red vine tomato. CC image by Mrs Gemstone on Flickr
Although tomatoes have naturally high levels of many nutrients with health benefits, adding in a further class of anti-oxidants not naturally present has been a holy grail of genetic engineers since the 1990s. 

Such genetically-improved tomatoes would, it is hoped, conveniently piggy-back the already high consumption of tomatoes in the modern diet, and make up for the failure of the official '5-a-day for health' message. 

The anti-oxidants of interest are 'anthocyanins', which are the dark pigment in the skin of many fruits and vegetables, such as blueberries and aubergines.   

Science has indicated that:

“intake (of anthocyanins) in the human diet is associated with protection against coronary heart disease and an improvement in sight.  They might also prevent cholesterol-induced atherosclerosis, could have anti-inflammatory and anti-carcinogenic activities and could aid the the prevention of obesity and diabetes” (Gonzali).  

The health benefits described above have emerged from studies using black-currants, blueberries, tart cherries, elderberries, grape juice and seed, purple corn, purple sweet potato, red soyabeans, red beans and red wine. 

In the plant, anthocyanins comprise some 200 different substances arising from the same biochemical pathway, and are one class within a broad range important phytochemicals referred to as 'flavenoids'.  None of the anthocyanins exist in isolation, but is stabilised in (often large) complexes of other anthocyanins plus any number of some 4,000 species of flavenoids. 

Suggested natural functions of anthocyanins include protection of the plant from excess sunlight and a “crucial role in fertility and sexual reproductions” (Mazzucato).   “The anti-microbial activity of anthocyanins in general has been well-established” (Lila).  They are, therefore, highly influential at the most fundamental level of plant physiology and disease resistance. 

The announcement of the first truly successful GM high-anthocyanin tomato, created by scientists at the John Innes Centre (JIC) in England came in 2008.  Reported at the same time was that the purple tomatoes had been tested by feeding them to GM cancer-prone mice, where they increased life-span by about 30%.  

Five years later, the JIC announced that its unusually-hued GM tomatoes could not only ward off cancer, but had unexpected additional benefits to growers and all their customers.  Purple tomatoes, it seems, have a longer shelf-life because their novel anthocyanins confer resistance to 'grey mould', the tomato's most common fungal pest.  And not only this, but the GM fruit is better tasting because it exhibits delayed ripening, allowing the tomatoes to remain on the vine longer before harvesting.   

COMMENT  A win-win-win tomato (but how long this 'better taste' will last in tomatoes sitting longer on the supermarket shelf because they don't rot isn't mentioned). 

In January 2014 it was announced that 2000 litres of GM purple tomato juice specially grown in Canada for further JIC studies had arrived in the UK.  (If you're wondering about the nature of the further studies on this GM juice, see WHAT USE IS PURPLE GM JUICE? - March 2014) 


The purple tomatoes have two added artificial genes modelled on ones from an unrelated plant (snapdragon) which force the generation of an entire, complex, unnatural, biochemical pathway leading to multiple anthocyanin production.  Novel anthocyanins will form novel complexes with other phytochemicals, and both could be altered in unfortunate ways by environmental stresses.    

Note also the altered anti-microbial properties of high-anthocyanin tomatoes which will alter your gut microflora (with assured but unknown health effects), and that there is novel fungicidal activity (of questionable toxicity) associated with the novel anthocyanins.  Moreover, the physiology of the GM plants is clearly deranged enough to delay ripening.  Anthocyanins have been shown to have several powerful and varied pharmacological activities in humans (hence the health benefits described by Gonzali above).  There seems to be considerable scope here for the presence of unexpected factors harmful to our health in the purple tomatoes. 

Add to this that there can be no question (on either side of the Atlantic) of purple tomatoes being deemed 'substantially equivalent' to any conventional tomato.  Safety testing is going to have to be extensive. 

In the cold light of day, the excuse for going full-steam-ahead with GM tomatoes seems to hinge on a stack of data derived from other fruit and veg, and a single small-scale mouse feeding study in which 10 percent of the animals' diet was concentrated tomato.  That's an awful lot of tomatoes to eat.  In fact, it was enough to make the experimental mice eat  more than usual because the inclusion of so much tomato made their test-diets inadequate. 

Cancer UK was dismissive of the study because cancer and its causes are far too complex for such a simplistic solution. 

The recent World Cancer Research Report prepared by the UN World Health Organisation pointed out that fruit and vegetables do “not appear to be as strongly protective against cancer as initially believed”.  Rather, the organisation draws attention to the damaging roles of red meat, processed meat, junk-food-assisted obesity, sedentary life-style, alcohol and tobacco in the modern epidemic of cancers.  Adding vast quantities of GM tomatoes (of uncertain safety) into this mix isn't likely to have any marked effect. 

All the while, conventional tomato breeders have not been idle.  Research published in 2013 described inhibition of cultured human cancer cells by extracts of a conventionally-bred high-anthocyanin tomato, 'Sun Black'.  Scientists in Brazil have produced a conventionally bred purple tomato, and Oregon State University Extension Service has made its own conventionally bred 'Indigo Rose' high-anthocyanin tomato commercially available.  The high-risk GM route to such tomatoes isn't needed. 

It's very clear from the citations which establish the health-benefits of natural high-anthocyanin plants products, that there are lots of natural sources available.  Is there any reason to want GM purple tomatoes? 

As for 'better tasting' due to longer ripening on the vine, the British Tomato Growers Association points out that
"this may be an attractive  quality for overseas growers but not an issue for British growers.  British tomatoes ripen on the plant for optimum flavour and texture, and arrive in the shops in prime condition”.   

The health message seems to be buy British tomatoes, eat plenty of fresh and varied fruit and veg, and get your life-style under control.  No GM anywhere there!


Andrea Mazzucato et al., 2013, Novel phenotypes related to the breeding of purple-fruited tomatoes and effect of peel extracts on human cancer cll proliferation, Plant Physiology and Biochemistry 72

Silvia Gonzali et al., 2009, Purple as a tomato: towards high anthocyanin tomatoes, Trends in Plant Sciences 14:5

Yang Zhang et al., 2013, Anthocyanins Double the Shelf Life of Tomatoes by Delaying Overripening and Reducing Susceptibility to Gray Mold, Current Biology 23

Eugenio Butelli et al., 2008, Enrichment of tomato fruit with health-promoting anthocyanins by expression of select transcription factors, Nature Biotechnology 26:11

Mary Ann Lila, 2004, Anthocyanins and Human Health: An In Vitro Investigative Approach, Journal of Biomedicine and Biotechnology 2004:5

Purple tomatoes may keep cancer at bay, John Innes Centre Press release, October 2008

Sarah Boseley, Alcohol and obesity fuel cancer surge, Guardian 4.01.14

Genetic Modification,, accessed February 2014

GM purple tomato hype rears its head again, GM Watch 11.01.14

Nick Collins, Genetically modified purple tomato 'tastier than normal varieties', Telegraph 23.05.13

Tomatoes, said to be the world's most popular fruit, can be made both better-tasting and longer-lasting thanks to UK research with purple GM varieties, John Innes Centre Press Release 23.05.13

 “Purple tomato can beat cancer”, GM Watch 2012

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