Snapshot Issue 31 February 2007

Polygalacturonase-2

It is not an uncommon sight to step over streams of tomato juice pouring down the gutters of small Italian countryside villages. This occurs in the late summer when tomato sauce is prepared in huge amounts, to be bottled and stored over the winter months. Thanks to their climate, the Italians produce tons of tomatoes every year. Some countries cannot however, and their tomatoes have to be imported. Most of the tomatoes the Italian peasants use would not look pretty on a shop shelf, nor would they survive days of transport or produce acceptable tomato ketchup upon arrival.

Indeed, one great problem with tomatoes is that not only do they ripen fast but they also become very soft: unlike an overripe grape that shrivels to dryness, a tomato becomes liquid and is generally messy. Consequently, for years producers have been seeking ways of slowing down the process of ripening, in order to be able to send their tomatoes to the other end of the world without them arriving spoiled and hence commercially undesirable.

Why does fruit ripen in the first place? A tomato’s hope – like any fruit’s – is to disperse its seeds. It can do this in two ways: either by perishing in such a way that it ends up by releasing its pips, or by seducing – by way of its flavour, colour and softness – a consumer that will digest and ultimately disseminate the pips. Tomatoes naturally prefer the first option, while tomato producers prefer the second. The ripening of a tomato demands a battery of enzymes, one of which is known as polygalacturonase-2, which depolymerizes polyurenide molecules – such as pectin for example. Polyurenides are essential to the making and maintenance of the texture and strength of the cell wall. When a tomato is ready to ripen, the production of polygalacturonase-2 increases and polyurenide breakdown is triggered off, resulting in the softening of the tomato.

Understanding how a tomato matures on the molecular level led to the production of the first transgenic tomato in the 1990s, in which the activity of polygalacturonase-2 was suppressed. In doing so, not only was the ripening of tomatoes delayed but it was discovered that the viscosity properties of tomato paste – used in all kinds of sauces – were also improved. Adding genetically modified tomatoes to a plate of salad is something that is not yet unanimously accepted , in the future, transgenic fruit may prove to be one of the best solutions for distributing fruit worldwide and selling it while it’s still pleasing to the eye.

Polygalacturonase-2, Solanum lycopersicum (Tomato): P05117

L'édition française de cette chronique est disponible dans l'Instantanés du mois de Prolune.

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