The flavour saviours: more flavoursome fruit and vegetables could be on the way thanks to a better understanding of their genes.

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Flavourless tomatoes are all too common. They have likely been bred for colour, size and shelf life, at the expense of flavour. But major advances in molecular breeding in the past five years, combined with growing consumer demand for high quality fruit and vegetables, are paving the way for tastier produce.

Harry Klee, a horticultural scientist at the University of Florida in the US, says that about 25% of the genes that are critical to controlling the chemicals responsible for flavour in a tomato have so far been identified. Work on the tomato, an economically important crop, has attracted substantial research funding, he acknowledges. 'I expect in the next five years we'll identify another 25-50%.'

While work on the tomato is most advanced, scientists are slowly but surely identifying the genes that are critical to controlling the chemicals responsible for flavour for a variety of fruits and vegetables. With advances in gene identification, 'we can collect vast amounts of data about chemistry and vast amounts of data about gene expression and try to associate them in a way that helps us figure out which genes might be responsible for which changes in chemistry', says James Giovannoni, plant molecular biologist with the US Department of Agriculture and an adjunct professor of plant biology at Cornell University, US.

Seed companies will be able to use the technology to identify the one plant in the thousands of varieties in their germplasm collection that has the specific combination of genes that is linked to good flavour, says Klee. He notes that although gene technology is routinely used as a tool, the resulting plant can be bred traditionally and does not have to be genetically modified.

At Nunhems, Bayer CropScience's seed subsidiary, molecular markers are starting to replace the use of biochemical assays as part of the search for quality traits in several species. Using markers 'will save time and the breeder a lot of headache because you don't have to do the biochemical assay, which is more time consuming', says Roger Muren, Nunhems' global head of cell biology. 'The markers will allow us to test them a lot more efficiently.'

The firm has used the approach to develop its award-winning Intense tomatoes, designed to retain their firmness and flavour for use in sandwiches, precut salads and pizzas. A firm beef tomato variety, Intense tomatoes are bred to possess a very dense structure, ensuring that very little juice is lost when the fruit is cut.

Seed companies have used genetic markers for several years to breed for relatively simple inherited traits, such as disease resistance, Muren says. Recent advances in gene technology are expected to help companies breed for different aspects of flavour--a difficult task because of the hundreds of genes involved, encompassing taste, aroma, texture and appearance. 'When we take the jump to talk about flavour in fruits, there are many, many genes responsible, working in a complex, interactive way,' he continues.

Importantly, molecular tools give seed companies the ability to measure the presence or absence of a gene directly, eliminating confounding effects that can be caused by the environment. 'That gives you some real power when you are evaluating large populations,' says Muren. 'You can look at the DNA of an individual as a seedling and throw away 80% of the plants before growing them to maturity.'

Wild species also provide a key to improved flavour. 'There are huge opportunities to go into wild species and find the attributes and the traits that you're interested in,' says Giovannoni. For this reason, approaches using gene technology, while useful, are not required in certain cases because researchers may be able to find naturally occurring genes that give the required attributes, he says. Giovannoni estimates that the genetic basis of tomato varieties we eat today represent 1-2% of the actual possibilities in the wild germ plants.

Consumer demand for tomatoes and other fruit all year round has motivated the industry, over the last 50 years, to select fruits that are hard and slow to ripen, so they can get to market. 'The reason your tomatoes, apples and strawberries are so lousy today is because the vast majority of the public wants that fruit all year round,' says Klee. 'To achieve this, the produce either needs to be to grown in glasshouses, which is expensive, or in season in some place far from where you live.'

When fruit has to be shipped on a mass scale over large distances, it has to be harvested when it is only partly ripe or completely unripe. As fruit generally does not fully ripen when it is detached from the plant, this produces a lower quality fruit, says Giovannoni. 'That's the main reason why the vegetables and fruit you grow in your back yard are more flavourful than the ones you buy in the supermarket.'

The ripening process is critical to flavour. As fruits mature and expand, they generate starch, and as they ripen much of the starch is converted to sugar. Certain fruits, such as tomatoes, also accumulate organic acids, providing a flavour that combines sweet with acidic or sour. Volatile compounds, many of which are produced specifically at ripening, are also key to flavour.

Identification of the genes involved in fruit ripening will help the seed companies create better hybrids that will ripen more fully, and therefore produce a more flavoursome fruit. Scientists, including Giovannoni, are trying to identify the regulatory genes that are the molecular switches that initiate the ripening process.

In the past, research focused on how ethylene, which controls most of the events associated with fruit ripening, is controlled at the genetic level and on identifying the genes involved in the synthesis. 'We're asking what comes before that,' says Giovannoni. 'We know that a fruit like a tomato will at a certain stage begin producing a ton of ethylene and it will ripen very quickly. What's the signal that says "I'm a tomato fruit ready to ripen. Let's make a ton of ethylene now"?'

Inhibition of ripening is the method companies typically employ when produce has to be shipped large distances. Fruit is harvested unripe or a naturally occurring mutation or variant is used that gives slower or delayed ripening. Ripening can also be delayed by removing ethylene from the environment. Bananas, for example, are generally harvested unripe, shipped around the world and then stored in warehouses where they are gassed with ethylene to promote ripening.

Another method used to delay ripening involves treating produce with compounds, such as 1-methylcyclopropene (1-MCP), that blocks the response to ethylene.

The end result is that unripe fruit is typically more firm and so can be shipped and handled more easily. But whichever method is used, the fruit has to be harvested partially or fully unripe and never fully ripens. 'When you compound that with the fact that you have hybrids that are genetically not fully competent to ripen, you have a double whammy in terms of inhibition of ripening,' remarks Giovannoni.

During ripening, various volatile compounds are produced that combine to provide the aroma associated with that fruit. For tomatoes, cis-3-hexenal is the most important of the approximately 20 volatiles that we perceive to be part of the tomato aroma, says Klee. Derived from the breakdown of fatty acids, it has a concentration of 12000 parts per billion (ppb) and smells similar to freshly cut grass or hay.

The second most important volatile in tomatoes is beta-ionone, which has a floral, fruity aroma and is derived from the cleavage of beta-carotene. Beta-ionone is present at only 4ppb but is important because it can be easily detected. The impact of a chemical on flavour perception is determined by both its concentration and our ability to sense it, Klee explains.

Research conducted by Klee and Stephen Goff, from Syngenta Biotechnology, suggests that flavour perception is linked to the nutritional or health value of food (Science 2006, 311, 10). Virtually all the volatile chemicals that we perceive to be part of tomato aroma are derived by enzymes acting on a nutrient that is essential for the human diet, says Klee. They are mainly derived from essential fatty acids or essential amino acids. The other class is carotenoids--including lycopene, which gives the tomato its red colour--which have been found to have various health benefits, including anti-cancer properties.

When considering why we like these flavour and aromas, it is hard to separate nature from nurture. For example, says Klee, another important volatile in tomatoes, 2-phenylethanol, is the major contributor to the scent of roses. Do we like the smell of roses because it triggers a primordial response to indicate that there's something there that's good for us or because of the context in which roses have been presented to us, he asks.

The fact that many of these volatiles in a tomato are produced specifically at ripening suggests the fruit and the animals that are responsible for dispersing the seeds are co-evolving. 'The seeds are fully mature and need to be distributed. These chemicals are produced, which are perceived to be desirable for the animal. The animal comes and eats the fruit and disperses the seed,' adds Klee.

While this story can be applied to most fruits, the case is less clear for vegetables. We often pick them when they are immature and in many cases we are not even eating the seed part of the plant, says Klee, plus most vegetables need to be cooked to make them palatable.

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