Tomato fends off plant parasite

A parasitic plant capable of devastating food crops has met its match with the domestic tomato, which has evolved a way to kill off the pest. Now scientists understand how, and are hoping that the tomato strategy may be transferred to existing vulnerable crops...

Cuscuta reflexa, also known as "dodder" in some places, grows from a seed but completely lacks leaves and roots. But it does have the ability to "sniff out" nearby plant victims towards which it extends tendrils that wind up the plant stems.

The tendrils produce structures called haustoria, which are like a drilling apparatus. These penetrate and plug the parasite directly into the vascular system of the host plant, allowing the parasite to tap off nutrients and water, to the detriment of its host.

Many important food crops, like soy beans and coffee, are affected by dodder species, which are widespread across the globe.

Infestations by the parasite can significantly dent yields and it is impossible to remove chemically, because poisoning it also poisons the host plant. The pest also times its flowering to coincide with the host plant, and the seeds it sets can remain dormant in soil for years at a time, making it very hard to eradicate from a crop.

Now University of Tubingen researcher Markus Albert and his colleagues have stumbled on an unlikely answer, which they found lurking in a domestic tomato plant.

Albert noticed that a species of tomato was resistant to the parasite. "Places on the stem where the parasite tried to penetrate turned brown, and the a couple of weeks later the parasite was dead," he explains. "When we cut across the tomato stem, we saw that the parasite haustorium could not grow inside."

To unravel the means by which the humble tomato, Solanum lycopersicum, uniquely appears to be able to fend off dodder, the team set about crossing their resistant tomato plants with different species that were vulnerable to the parasite to create a range of hybrid plants that either were or were not susceptible to attack.

This meant that they were able to compare, genetically, the make-up of susceptible plants with tomatoes still capable of defending themselves to unpick the genetic basis of the defence mechanism.

"We found a genetic region," explains Albert, "that codes for a receptor that detects a chemical made by the parasite. When the receptor sees this chemical it triggers a defence system in the plant that repels the parasite."

Having identified the receptor the team were then able to transfer the relevant DNA to other - formerly susceptible - plant species and demonstrate that they too became resistant to the pest.

"There's still more to work out though," says Albert. "Because some of our crossed tomato plants lacking the receptor were still resistant to parasite attack. So there is more to find out yet!"

The ultimate goal of the work will be to transfer the genetic machinery underlying the chemical "tripwire" set by the tomato plants into important crop species to protect them. But on the basis of the experiments the team have done so far, things look rosy, or possibly tomato-y...