Shining, Unhappy Plants

It is the dead of night, one summer just after the turn of the next century. Despite the darkness, a Midwestern farmer is surveying his acres of crops. From several clumps of plants scattered randomly throughout his fields there emanates an eerie blue glow. The farmer worries: The plants are obviously under stress.

If scientists in the United Kingdom are right, this scene might be played out all over the world. Glowing blue plants may someday provide an early-warning system that will alert farmers to infection and herbivore attack in time for defensive action.

At the Institute of Cell and Molecular Biology at the University of Edinburgh, a team led by plant biochemist Anthony Trewavas has been developing a genetic-engineering program to meet this goal. They are working with a protein that causes certain marine creatures, such as the jellyfish Aequorea victoria to give off light when they are attacked by predators. In response to touch, jellyfish cells fill rapidly with calcium ions, which act as a cellular alarm signal during the organism’s response to stress. The calcium ions bind to various molecules, including the protein aequorin. In binding to calcium aequorin gains an influx of energy, which it dissipates by giving off photons. In other words, it glows.

Plant cells also have an electrical response to stresses such as infection, touch and cold shock. Calcium ions pour in, again playing a signaling role in mobilizing the organism’s defenses. Trewavas and his team wanted to effectively amplify the calcium signal so that the farmer could lend a helping hand to a stressed plant. He reported the team’s latest results at the annual Science Festival of the British Association for the Advancement of Science in Newcastle-upon-Tyne in September.

A motivation for the research is the widespread use of blanket spraying of pesticides. Farmers practice blanket spraying in anticipation of infection or infestation because they would lose crops if they waited for visible signs of attack on leaf surfaces–if you wait, it is often too late to rescue the harvest. Farmers equipped with an early-warning system might be able to spray in time to prevent losses, and to spray only areas affected.

In the early stages of their work, the Edinburgh team transferred the genes that code for the fluorescent calcium-binding protein aequorin from the jellyfish into tobacco plants and mosses. They succeeded in their first goal: When wounded or infected or otherwise stressed, test plants responded quickly by giving off a very faint blue glow, detectable by ultrasensitive camera equipment.

“At the moment,” says Trewavas, “the light is not visible to the naked eye, but that is because this is a jellyfish gene, not a plant gene.” The jellyfish gene includes a number of DNA sections (codons) that plants use rarely, if ever, and this difference in how the genetic information is arranged limits plants’ ability to “read” the gene. “That means we need to resynthesize the gene to optimize it for plants,” he said.

The team hopes to increase expression of the protein, using appropriate promoters, so that the glow is visible in darkness. The choice of promoters could also make the signal more specific, so that, for instance, it would indicate a response to infection rather than to cold shock. Even if one seed in a thousand produced a plant capable of glowing, the warning would be more effective than that achieved in experiments using microinjected fluorescent dyes. Dyes that respond to accelerated calcium flow have been used to monitor plant stress, but these techniques are limited to single or small groups of cells.

Trewavas is optimistic that his technology will be available to farmers by 2000. “If the jellyfish can do it,” he says, “then so can we.” Neal Stewart, Jr., assistant professor of biology at the University of North Carolina at Chapel Hill, shares Trewavas’s bullish outlook and is beginning his own research. “I think that perhaps the year of commercialization may be optimistic–maybe not–but new and improved fluorescent proteins should be on line soon.”

The reference for my original article on this topic is American Scientist, Volume 84, Issue 1, p.25-26