Science - August 28, 2014

The tomato that never sleeps

Roelof Kleis

Our common or garden tomato needs its sleep. Six hours a night, minimum. But this can change. Mexican PhD student Aaron Velez-Ramirez developed a variety that can do without sleep. What is the secret of this party tomato?

The tomato plant’s sleep requirements are a nuisance. To tomato growers, that is. After all, a plant which grows both day and night is more profitable. So, five years ago, seed giant Monsanto came up with a nice project: a light-tolerant tomato. Aaron Velez-Ramirez hit the global headlines recently with his discovery of the gene that makes the tomato resistant to light. He will graduate with a PhD for his study next month. Resource got a sneak preview. Eight questions and answers about the tomato that never sleeps. 

Why does a tomato need sleep anyway?   

Sleep is not the right word, perhaps. It is a question of rest, or, to be more precise, periods without light. Night-time rest. Tomatoes need sunlight in order to grow. Horticulturalists stimulate their growth with extra light, but there is a limit to how much you can do that. Our tomato needs at least six hours of rest per day. That was discovered at the end of the nineteen twenties. Exposing the plant to more light than that causes damage. The leaves discolour and eventually the plant even dies. How that is possible has been a riddle until recently. 

How do you create a tomato that can cope with 24 hours of light? 

The answer is simple: imitation. Because wild tomatoes can handle continuous light. That was clear from (almost forgotten) research in the nineteen sixties. Velez-Ramirez successfully cross-bred the light-tolerance of the wild tomato into two of the currently popular commercial varieities, Idooll and Westland. Then he went in search of the secret behind the tolerance. According to co-supervisor Wim van Ieperen (Horticultural Chains chair group), it is pure coincidence that our standard tomato cannot cope with light. ‘At some point in the seventeenth century we domesticated precisely that one wild tomato species that could not cope with light.’ 

What makes a tomato light-resistant? 

The key is in a single gene, CAB-13. CAB stands for chlorophyll a/b binding. The gene is one of a whole family of genes whose characteristics are a product of proteins which play a role in photosynthesis. ‘CAB-13 protein is a component of the antennae which capture light and take it to the photosynthetic reaction centre,’ explains Velez-Ramirez. ‘There are many proteins which do that. CAB-13 is one of the rarer ones. Exactly what it does, we don’t know.’ In the light-sensitive tomato, too much light inhibits the expression of this gene. Quite how that works is not known. In the light-tolerant tomato, that brake is absent and in fact more of the protein is manufactured.’ 

So the mystery of light tolerance has not been solved completely? 

By no means. Velez-Ramirez discovered that phytochrome A plays an important role too. Phytochromes are proteins which play a role in registering light from the surroundings. They perform an important function in the regulation of plants’ night-and-day rhythm. If you change a tomato plant genetically so that it makes extra phytochrome A, the plant will also become tolerant of continuous light. Velez-Ramirez puts this effect of phytochrome A down to the development of chloroplasts in the cell. These are organs in the cell in which photosynthesis takes place. Continuous light disturbs these ‘photosynthesis factories’, causing the leaf to become pale. ‘There is probably some kind of miscommunication in the cell,’ explains the other co-supervisor Dick Vreugdenhil (Laboratory for Plant Physiology). But no one knows which signal is responsible for that. It is clear that phytochrome A destroys this signal and limits or even prevents the damage. Whether and how these processes – the expression of CAB-13 and the role of phytochrome A – are connected is still not understood. 

And are there no other changes to the tomato? 

Cross-breeding is a tricky process. You never know beforehand exactly what the result will be. You gain one characteristic but you might lose others. Genetic processes are still a bit of a black box. But the introduction of light tolerance does not appear to have changed anything about the tomato itself, says Velez-Ramirez. The new line does, however, have slightly smaller, thicker leaves. ‘In principle that is not good because that plant has to invest more energy in thicker leaves. On the other hand, smaller leaves mean the leaves at the top do not block all the light. So on balance, it might be better like this. That still needs to be tested.’’ 

How does the sleepless tomato taste? 

To be honest, Velez-Ramirez does not know: he has never tasted the tomato. The same goes for his co-supervisors. ‘As far as we know, the tomato is exactly the same and there are no other changes apart from light tolerance,’ says co-supervisor Vreugdenhil. His colleague Van Ieperen sees not reason to assume that the new tomatoes would have undergone any other changes. ‘We know that CAB-13 in plants has a function in photosynthesis. No other functions are known.’ 

The new plant yields 10 percent more tomatoes with continuous light. But does that make it more efficient, given that the lamps have to stay on longer?  

To be honest, we don’t know yet. ‘We expect that it will be, but it is hard to say right now,’ says Van Ieperen. ‘Six hours more light is six hours more electricity. But it is not as simple as that, because you can then turn down the heating. A lot of the light is converted into heat. On the other hand, extra light means more evaporation and therefore more moisture in the air in the greenhouse, which has to be extracted. That costs energy too.’ Quite how the balance of costs and benefits will work out is hard to say before production-scale trials.  

Are tomato growers keen to have a light-tolerant tomato? 

PhD researcher Velez-Ramirez thinks they are. ‘The light-tolerant tomato offers the grower more flexibility. A normal tomato plant can cope with a maximum of 18 hours of light per day. These plants have no limits. You can give them 16 hours or 20 hours in a row. It is up to the grower to decide that. In the winter, when there is not much sunlight, longer periods of artificial light can be a good idea. It is an additional management option. You can even give plants light individually in order to end up with a harvest that is as homogeneous as possible. This could radically change the way tomatoes are grown in the winter.’