Putting Down Your Roots: How Plants Know To Do It

Scientists at the Whitehead Institute for Biomedical Research reportthat they have cloned and characterized a plant gene called EIR1 (EthyleneInsensitive Root 1) that plays a critical role in the ability of roots to growtoward the earth in response to gravity. The roots of mutant weeds lacking EIR1lose their ability to respond to gravity and are unable to grow downward intothe soil. The findings are reported in the July 15 issue of Genes andDevelopment by first author Dr. Christian Luschnig and his colleagues Ms. PaulaGrisafi, Dr. Roberto Gaxiola, and Dr. Gerald R. Fink, director of the WhiteheadInstitute.

"These findings provide important new insights into age-old mysteriesabout root growth, and they also may have tremendous implications for theagricultural and pharmaceutical industries," says Dr. Fink. "Currently, mostherbicides are developed by trial and error. Compounds first are tested fortheir ability to kill weeds, and then later tested--often for years--to ensuretheir 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 inanimals. Our findings suggest that one can design new classes of compoundstargeted at plant-specific genes like EIR1 such that they would automatically beharmful to plants but safe for humans."

The Fink lab findings have additional implications for the agriculturalindustry. The genetic makeup of Arabidopsis is similar to that of food cropslike rice and corn, so understanding genetic pathways that regulate the growthof this weed will lead to new approaches for the genetic improvement ofagriculturally important crops. For example, since root development is oneimportant way plants obtain nutrients, understanding the genetics of root growthcould lead to new strategies for enhancing food production, particularly in aridclimates. (Scientists use Arabidopsis as a model to study plant genetics becauseof its small size, short generation time, and abundant seed production.)

Plant Growth And Tropisms

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

Since Darwinian times, scientists have tried to get a handle on howplants 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'sdirective that if a root is reoriented to lie horizontal to the surface of theearth, or in other words, turned 90 degrees with respect to gravity, it respondsby altering its direction of growth, curving downward again until it finds itsway into the earth.

Scientists have known that during root growth, the redistribution of aplant hormone called indole acetic acid (IAA) to the root tip is responsible forgravitropism. When the root tip is cut off, the plant no longer is able to growdownward. When roots are oriented horizontally, IAA accumulates along the lowerside of the elongating zone. Cells on the top part of the root elongate, causingthe downward curving of the root.

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

"When we studied the EIR1 gene, we found that it was very similar tobacterial genes that pump out toxins from bacterial cells," says Dr. Luschnig.And, when the scientists inserted the EIR1 gene into yeast cells, the yeastcells became resistant to fluorinated indolic compounds, suggesting that theEIR1 gene was helping yeast cells pump out the toxins. This suggests that EIR1functions as an efflux pump in roots, and because EIR1 is expressed only in theroots and not other parts of the plant, it suggests that the gene is responsiblefor the root's response to gravity.

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

The title of the Genes and Development paper is "EIR1, a Root Specific ProteinInvolved in Auxin Transport, is required for Gravitropism in Arabidopsisthaliana," and the authors are:

• Dr. Christian Luschnig, Postdoctoral Associate, Whitehead Institute forBiomedical Research
• Dr. Roberto A. Gaxiola, Postdoctoral Associate, Whitehead Institute forBiomedical Research
• Ms. Paula Grisafi, Technical Associate, Whitehead Institute for BiomedicalResearch
• Dr. Gerald R. Fink, Director, Whitehead Institute, Professor of Biology andAmerican Cancer Society Professor of Genetics.