Plants under pressure: the touch that stunts.

Plants under pressure: The touch that stunts

Plants feel -- a fact recognized in the 1970s when scientists showed that many forms of touch affect plant growth. Now, researchers have isolated specific plant genes that "turn on" in response to pressure cues such as wind, rain and human touch.

Plants respond to changes in the environment, but unlike people, they can't runfor shelter. Instead, they must adapt by changing their own development. For example, coastal trees buffeted by powerful winds and heavy rains assume a bent-over posture as they grow, which leaves them shorter and sturdier than more protected inland trees. In the laboratory, touching a plant's leaves cna elicit the same growth changes as wind and rain.

In indoor experiments measuring the effects of environmental stimuli on the growth of Arabidopsis, a member of the mustard family, researchers at Stanford University found that pressure stimuli activated five genes. Moreover, plants touched twice daily by the researchers did not grow as tall as untouched plants, says Ronald Davis, who reports the work with Janet Braam in the Feb. 9 CELL.

Plants seems to sense touch and translate that information into increased gene expression, which ultimately alters their development, Davis says.

The researchers detected gene expression by measuring the amounts of messenger RNA (mRNA) in plant cells. Thirty minutes after spraying plans with water, they found that mRNA levels had increased 10 to 100 percent over those in unsprayed plants.

But the gene activation did not stem from any special property of water, the team found. The same genes switched on just as readily, cut them with a scissors touched leaves, cut them with a scissors or simulated blowing wind with a hair dryer. Plants receiving no direct stimulation showed no increase in mRNA, Davis says. Nor did genes turn on in plants exposed only to music, increased humidity or a change of location.

To determine whether three weeks of human touch would affect growth patterns, the researchers measured the lengths of the stems and stalks of untouched plants and compared them with plants touched twice daily. They calibrated their measurements from photographs of the plants to avoid inadvertently turning on the touch-activated genes. They found that the stalks of untouched plants reached an average length of 41 centimeters, compared with 18 cm in touch plants.

Although scientists have yet to demonstrate a mechanism by which touch alters plant growth, the Standard group suggests one, based on its discovery that three of the touch-activated genes code for calmodulin or related proteins. Scientists know that calmodulin, a receptor within cells, responds to environmental signals by binding calcium -- a key cellular communicator. When a plant reponds to direct stimulation, calcium levels increase inside cells. And when enzymes act on calcium-saturated calmodulin receptors, a cascade of cellular events ensues. One such event might be the activation and regulation of the touch-activated genes, the Stanford researchers propose. In effect, Calmodulin may turn on the very genes that encode it, Davis hypothesizes.

Calmodulin receptors also exist in human brains, where they are important in responding to sensory stimuli. Davis speculates that plants and people may detect environmental changes through similar sensory pathways. "It may be a very ancient conserved mechanism that evolved before animals and plants diverged durign evolutoin," he suggests.

He also speculates that calmodulin plays a role in rearranging plant-cell structure. The rearrangement may be required for expanding cell diameter and inhibiting cell elongation resulting in shorter plants, Davis says.