By Sean Henahan, Access Excellence

First reports of the tomatoes natural defense system came more than 20 years ago. However, until recently,  most plant physiologists believed the intercellular warning signals were chemical in nature.

"Proving that a genuine electrical signal can turn on genes is important  because it raises the possibility that we may be able to use electrical stimulation as an environmentally sound means for increasing crop resistance to pests," says Dr. Eric Davies, professor and head of NC State's Department of Botany.

Davies and colleagues coauthor found that pin levels rose three- to five-fold throughout the plant within 15 minutes of an electrical stimulation, and up to 15-fold within a hour.  Levels began to revert within two hours.

The researchers found that wounding a leaf with a low flame also could spur large, rapid  increases in temporary pin production. However, this seemed to occur via a separate signaling mechanism.  The sudden loss of hydraulic tension in the dead tissue of the burned cells causes a plant-wide hydraulic pressure surge which triggers an electrical reaction -- called a variation potential -- in adjacent living cells.  Unlike action potentials, which spur uniform pin production plant-wide, variation potentials were found to promote varying levels of pin production, with the highest levels occurring in cells nearest the wound.

The researchers also found that variation potentials may spur systemic expression of calmodulin, a gene that plays a key role in initiating many information processes in plants, including their responses to cold and heat, gravity and touch.

To rule out the possibility the signals were chemical in nature, researchers attached a cooling ring to leaf petioles prior to wounding them.  This prevented the leaves from transmitting chemicals out through their phloem, or vascular system.  The researchers also analyzed the electrical signals transmitted from the wounded leaf.

"We found that if you cut the leaf off before the electrical signal was transmitted, there was no change in gene expression outside the wounded leaf itself.  But if we cut it off after the transmission of the signal it made no difference.  Gene expression throughout the plant was neither stopped nor reduced," Davies says.

In the short term, the findings are likely to increase understanding of the fundamental intercellular communication processes used by plants. In the longer term plant biologists will evaluate the feasibility and effectiveness of electrically stimulated gene expression as a means of strengthening crop resistance to pests.

"By proving that both electrical and hydraulic signals can trigger gene-specific expression, we show that plants are a bit more complex than we once thought. They can distinguish between various environmental stimuli and respond with the most appropriate type of intercellular signaling mechanism -- be it chemical, electrical, hydraulic or a combination thereof," he said, adding:

"We need to broaden the scope of our investigations and give these so-called 'lower life forms' a bit more respect. It's only when we recognize and understand the full range of sophisticated intercellular communication processes employed by plants that we will best understand how to manipulate and modify them."

The study appears in the June 1997 issue of the the European science journal Planta.