Putting Down Your Roots: How Plants Know How to Do It

CAMBRIDGE, Mass. — The next time you pick up a bag of weed killer from The Home Depot, think about this: a chemical company probably spent years of testing and millions of dollars to develop an effective herbicide that is harmful to weeds but safe for you, your children, and your pets. Now a new study of root growth in a tiny weed called Arabidopsis thaliana suggests that genetics could help scientists save valuable time and money in developing better herbicides for the future.

Scientists at the Whitehead Institute for Biomedical Research report that they have cloned and characterized a plant gene called EIR1 (Ethylene Insensitive Root 1) that plays a critical role in the ability of roots to grow toward the earth in response to gravity. The roots of mutant weeds lacking EIR1 lose their ability to respond to gravity and are unable to grow downward into the soil. The findings are reported in the July 15 issue of Genes and Development by first author Dr. Christian Luschnig and his colleagues Ms. Paula Grisafi, Dr. Roberto Gaxiola, and Dr. Gerald R. Fink, director of the Whitehead Institute.

"These findings provide important new insights into age-old mysteries about root growth, and they also may have tremendous implications for the agricultural and pharmaceutical industries," says Dr. Fink. "Currently, most herbicides are developed by trial and error. Compounds first are tested for their ability to kill weeds, and then later tested, often for years, to ensure their safety in animals. Often the most effective ones turn out, in hindsight, to be the compounds that act against genes present only in plants but not in animals. Our findings suggest that one can design new classes of compounds targeted at plant-specific genes like EIR1 such that they would automatically be harmful to plants but safe for humans."

The Fink lab findings have additional implications for the agricultural industry. The genetic makeup of Arabidopsis is similar to that of food crops like rice and corn, so understanding genetic pathways that regulate the growth of this weed will lead to new approaches for the genetic improvement of agriculturally important crops. For example, since root development is one important way plants obtain nutrients, understanding the genetics of root growth could lead to new strategies for enhancing food production, particularly in arid climates. (Scientists use Arabidopsis as a model to study plant genetics because of its small size, short generation time, and abundant seed production.)

Plant Growth and Tropisms

In addition to its implications for the agricultural industry, the Fink lab's findings provide important information about plant physiology, and in particular about a phenomenon called tropism - the growth response by plants to external stimuli, such as light, temperature, water, and gravity.

Since Darwinian times, scientists have tried to get a handle on how plants are able to direct roots to always grow downward in search of the earth, and shoots to grow upwards in search of the sun. So great is the plant's directive that if a root is reoriented to lie horizontal to the surface of the earth, or in other words, turned 90 degrees with respect to gravity, it responds by altering its direction of growth, curving downward again until it finds its way into the earth.

Scientists have known that during root growth, the redistribution of a plant hormone called indole acetic acid (IAA) to the root tip is responsible for gravitropism. When the root tip is cut off, the plant no longer is able to grow downward. When roots are oriented horizontally, IAA accumulates along the lower side of the elongating zone. Cells on the top part of the root elongate, causing the downward curving of the root.

Researchers have speculated that the transport of IAA is facilitated by a gene that acts as a pump to redistribute the hormone up and down root cells as needed. The EIR1 gene isolated by the Fink lab may represent this pump. The case for EIR1 seems strong.

"When we studied the EIR1 gene, we found that it was very similar to bacterial genes that pump out toxins from bacterial cells," says Dr. Luschnig. And, when the scientists inserted the EIR1 gene into yeast cells, the yeast cells became resistant to fluorinated indolic compounds, suggesting that the EIR1 gene was helping yeast cells pump out the toxins. This suggests that EIR1 functions as an efflux pump in roots, and because EIR1 is expressed only in the roots and not other parts of the plant, it suggests that the gene is responsible for the root's response to gravity.

The study was supported in part by a Schroedinger Fellowship from the FWF, Austria, and by the PEW Latin American Program, the EC, and by a grant from the National Science Foundation.

Luschnig, C. et al. "EIR1, a Root Specific Protein Involved in Auxin Transport, is required for Gravitropism in Arabidopsis thaliana." Genes and Development. July 1998.