By David N. Leff
A generation ago, there was a fad in which lonely people talked to their house plants. They were convinced the occupants of their flowerpots could understand the spoken word and appreciate the music their owners also played for them.
Despite this transient anecdotal testimony, we know botanical specimens do not rap with human speech or song.
What we don't know much about is how plants move, grow and differentiate.
"In comparison to animal growth regulators," observed plant molecular biologist Christian Luschnig, at the Whitehead Institute for Biomedical Research, in Cambridge, Mass., "our knowledge is approximately zero.
"We have known for more than half a decade already," he continued, "that the plant hormone auxin — also known as indole acetic acid — has some major influence on plant growth and differentiation. And one thing that has been suggested for quite some time is that auxin needs to be distributed in the plant in order to reach its target cells."
Plants grow in two directions — up and down. The shoot cells that form leaves, stems, twigs and branches reach toward the sun for the photosynthetic energy that fuels their growth.
At the other extreme from this phototropism are the bottom-feeders, the root cells, which extend downward toward the center of the earth, pulled by gravity. The burrowing cells form root hairs that take up water and nutrients from the soil.
"When you grow a plant," Luschnig pointed out, "and its root hits some kind of underground obstacle, it finds a way to move around the impediment in a kind of detour. Then the root resumes its gravitropic descent."
Anyone with a green thumb can demonstrate this gravitational spell. Dig up a small seedling and lay it flat, horizontal to the earth. Without benefit of spoken instructions, the plantlet responds by changing its direction of growth. In a matter of hours, its roots curve downward again, aimed at the earth's center of gravity.
What happens is that the plant hormone auxin collects along the lower side of the plant's elongation zone. Cells on top of the root elongate, causing it to curve downward.
Charles Darwin reported this effect as far back as 1880.
What he didn't know was that the redistribution of auxin to the root's tip energizes and directs this gravi-tropism. The proof is simple: Snip off the tip of the root and gravity loosens its tug.
Luschnig and his fellow scientists at the Whitehead have discovered a mutant plant gene that seems to play a starring role in the drama of root gravitropism. Its name, EIR1, stands for 'ethylene-insensitive root 1.'
Ethylene is a gaseous chemical produced by plants to trigger their ripening, presumably as a regulator of auxin transport. The mutant EIR1 protein, as its name implies, is resistant to this growth-regulating gas.
Gene Manages Auxin Transport
Moreover, Luschnig observed, "mutant EIR1 is root-specific, so that root growth no longer responds to gravity or obstacles. Nor do the roots form nutrient-absorbing hairs. Furthermore, they are resistant to a number of experimental added compounds that inhibit auxin transport. That gave us the first hint that this gene might be effective in such transport."
Luschnig is first author of a paper in the July 15, 1998, issue of Genes & Development, a twice-monthly journal published by the Cold Spring Harbor Laboratory, of N.Y. Its title is "EIR1, a root-specific protein involved in auxin transport, is required for gravitropism in Arabidopsis thaliana."
A. thaliana, a small, easily-cultivated weed, is to plant geneticists what Escherichia coli is to molecular biologists — a lab model of choice.
In the greenhouse atop the Whitehead laboratories, Luschnig told BioWorld Today, "we grew 10,000 A. thaliana seedlings on vertically oriented plates. Under normal conditions, their roots grow straight down. But some of the plants did not follow this gravitropic route; their roots took off in random directions.
"We screened these to find the mutant genes, which led to the defective proteins. And that's how we found the normal EIR1 gene itself."
Molecular biologist Gerald Fink, the Whitehead's director, is senior author of the paper. "Currently," he told BioWorld Today, "most herbicides are developed by trial and error. Compounds first are tested for their ability to kill weeds, then later tested — often for years — to ensure their safety in animals. Our findings suggest that one can design new classes of compounds targeted at plant-specific genes like EIR1, such that they would be harmful to plants but safe for humans."
Auxin Transporter Is Herbicide Target
In agricultural economic terms, Luschnig pointed out, "our results point to using the auxin transporter as a target for engineering herbicides and improving crop yields in arid zones. We report evidence that it's a membrane protein from its localization in yeast cells. There it seems to be targeted to the plasma membrane and has restricted similarity to a number of bacterial membrane transporters.
"Some of these transporters," he went on, "appear to be involved in pumping toxic compounds out of the cells.
"When we overexpressed the mutant EIR1 gene in yeast cells, we got more resistance to toxic indolic compounds, which are fluorinated analogues of auxin. Such auxin-related compounds," Luschnig explained, "are already used as herbicides. If you can engineer the protein in a way that has a higher efficiency at pumping out some of these analogues, plants expressing this engineered protein would of course have an advantage in nature, when used as a herbicide, as they are more resistant to this class of compound."
To describe the weed-killing mechanism, Luschnig said, "Just imagine that you had pumped up some kind of growth factor, somewhat similar to when you treat plants with high doses of auxin. So the entire regulation of growth and development will be screwed up and soon after, the plant is going to die."
He allowed that "there are efforts on the way to patent this Whitehead herbicide approach," and added that in recent months "it has turned out that in this highly competitive field, there are at least three other groups out there, which are very close to us and also have submitted some results.
"Right now," Luschnig concluded, "I'm using this EIR1 mutant to try some genetic approaches to finding other proteins that might also be involved in the transport of auxin." *