New growth inhibitors more effective in plants, less toxic to people

March 2, 2011

WEST LAFAYETTE, Ind. - A Purdue University scientist and researchers in Japan have produced a new class of improved plant growth regulators that are expected to be less toxic to humans.

Angus Murphy, a professor of horticulture, said the growth inhibitors block the transport of auxin, a plant hormone that, when transported throughout the plant, controls growth processes. Current growth regulators that inhibit auxin transport are inefficient because they also have hormonelike activity or affect other important plant processes. Current growth inhibitors also are often toxic.

Growth regulators are important in ornamental plants and horticultural crops that would require labor-intensive manipulation and pruning. The inhibitors are used to keep plants a desired size and shape and control fruit formation.

"These regulators would be used primarily on ornamental plants, flowers and trees that aren't going to be genetically changed easily," Murphy said. "Growth regulators are used regularly on this type of plant. Inhibition of auxin transport with these new compounds is also an alternative to the use of more toxic regulators like 2,4-D."

The toxicity of growth regulators can be an environmental concern and add safety and monitoring costs to commercial growing operations. They are generally not applied to edible portions of plants or are applied early enough that there is little or no residue on edible portions of plants.

The new plant growth inhibitors are derived from natural and artificial auxins but have a bulky benzoyl group - a chemical conjugate derived from benzoic acid - attached that prevents movement of the inhibitor out of the cell.

"Since it looks like auxin, it will open the door, but it can't get through," Murphy said. "However, these new growth regulators have no hormonal activity themselves."

Murphy worked with scientists from several universities in Japan, including Okayama University of Science, Tokyo Metropolitan University, Niigata University and the Nara Institute of Science and Technology. Their findings were reported in the Journal of Biological Chemistry.

Murphy said he would continue studying how to regulate other hormonal pathways in plants and use the new regulator to understand hormonal transport in plants. Companies licensed by the Japanese institutes will continue environmental and toxicity testing of the regulators in greenhouse and field trials.

The Office of Basic Energy Sciences of the U.S. Department of Energy funded the research.

Writer:  Brian Wallheimer, 765-496-2050, 

Source:  Angus Murphy, 765-496-7956,

Ag Communications: (765) 494-2722;
Keith Robinson,
Agriculture News Page



Alkoxy-auxins Are Selective Inhibitors of Auxin Transport
Mediated by PIN, ABCB, and AUX1 Transporters

Etsuko Tsuda, Haibing Yang, Takeshi Nishimura, Yukiko Uehara, Tatsuya Sakai, Masahiko Furutani, Tomokazu Koshiba, Masakazu Hirose, Hiroshi Nozaki, Angus S. Murphy and Ken-ichiro Hayashi

Polar auxin movement is a primary regulator of programmed and plastic plant development. Auxin transport is highly regulated at the cellular level and is mediated by coordinated transport activity of plasma membrane-localized PIN, ABCB, and AUX1/LAX transporters. The activity of these transporters has been extensively analyzed using a combination of pharmacological inhibitors, synthetic auxins, and knock-out mutants in Arabidopsis. However, efforts to analyze auxin-dependent growth in other species that are less tractable to genetic manipulation require more selective inhibitors than are currently available. In this report, we characterize the inhibitory activity of 5-alkoxy derivatives of indole 3-acetic acid and 7-alkoxy derivatives of naphthalene 1-acetic acid, finding that the hexyloxy and benzyloxy derivatives act as potent inhibitors of auxin action in plants. These alkoxy-auxin analogs inhibit polar auxin transport and tropic responses associated with asymmetric auxin distribution in Arabidopsis and maize. The alkoxy-auxin analogs inhibit auxin transport mediated by AUX1, PIN, and ABCB proteins expressed in yeast. However, these analogs did not inhibit or activate SCFTIR1 auxin signaling and had no effect on the subcellular trafficking of PIN proteins. Together these results indicate that alkoxy-auxins are inactive auxin analogs for auxin signaling, but are recognized by PIN, ABCB, and AUX1 auxin transport proteins. Alkoxy-auxins are powerful new tools for analyses of auxin-dependent development.