Farmers may one day be able to target pesticides to only those parts of their fields that are at risk of disease simply by noting which ones are glowing, according to researchers.

Scientists have genetically engineered the natural immune system of the tobacco plant to make it change colour or glow in the presence of viruses, bacteria and other pathogens known to reduce crop yields, which normally force farmers to apply costly pesticides. In laboratory tests, these 'photosensory' plants turn red or produce a fluorescent glow when infected.

Although the system has so far been tested only in tobacco plants, lead researcher Neal Stewart, a plant scientist at the US-based University of Tennessee, told SciDev.Net it could easily be adapted to other crops.

Instead of planting entire fields with these plants, he suggested they could be strategically placed throughout the field. This would introduce only a small number of GM plants into the environment, so farmers could choose not to harvest them while still benefiting from their pathogen-detecting ability. "If we know something bad is happening, the farmer can treat certain areas rather than the whole field," he said.

Stewart's sensory plants could also be adapted to detect landmines by changing their leaf colour. The next step, said Stewart, is to conduct field tests to discover exactly how the new plants respond to different pathogens in a field setting. "It would be very exciting to partner with research groups in developing countries that might not have access to a lot of pesticides," he said. "Knowing early on that a plant disease could sweep through might make a big difference to [those] farmers."

But Andrija Finka, a biologist at the University of Lausanne, Switzerland, expressed concerns that farmers in developing countries would not be able to grow the high-tech plants. "It will be really hard to maintain them," he told SciDev.Net.

And Janice Lake, a plant physiologist at the University of Sheffield, United Kingdom, said that plant responses to different stresses may be too similar to distinguish, because the same types of molecules can signal a range of problems. "I'm not sure how you would identify exact stresses," she said.

The researchers presented their work at the 12th World Congress of the International Association for Plant Biotechnology, in the United States, last month (6-11 June).