Purdue University researchers have discovered a fine-tuning mechanism¹ involved in plant root growth that has them questioning whether a popular herbicide may have unintended consequences, causing some plants to need more water or nutrients². Angus Murphy, a professor of horticulture（园艺） , and Wendy Peer, an assistant professor of horticulture, study the movement of auxin（植物生长素） , a plant hormone³ essential for plant development. They showed that ABCB4, a protein responsible for moving auxin into cells, also removes the hormone when too much has accumulated.

"We knew that the protein took auxin up, but found that it switched to removing auxin when a threshold is reached," said Murphy, whose findings appeared in the early online version of The Plant Journal. "It starts transporting the hormones⁴ out."

That fine-tuning mechanism is integral to proper development of plant root hairs, which extend from the main plant root and are where most water and minerals enter.

"The root hairs are doing all the heavy lifting for bringing the water into the plant," Peer said. "And ABCB4 maintains the right auxin levels to keep root hairs growing optimally⁵."

The herbicide 2,4-D, a synthetic⁶ form of auxin, could have unintended consequences for the protein, Murphy and Peer said.

The herbicide is used to kill broadleaf weeds, which are dicots（双子叶植物） , while monocot（单子叶植物） grasses, such as sorghum⁷（高粱） and corn, are more resistant⁸. That's because grasses inactivate⁹ 2,4-D inside the plant, while broadleaf dicots do not.

But ABCB4 is located on the root surface and can be switched into intake-only mode, disabling its ability to remove excess auxin from cells, before 2,4-D can be inactivated¹⁰ inside the plant. This results in shorter root hairs.

"This suggests that ABCB4 is an unexpected target of 2,4-D action," Murphy said. "It's something that we have to be aware of with the commercial introduction of 2,4-D resistant soybeans and other dicot crops."

Murphy said laboratory testing of ABCB4 in yeast¹¹, tobacco and human cells subjected to 2,4-D all showed that ABCB4 could be locked into the uptake-only mode. The root hairs of mutant plants that had ABCB4 removed were not affected¹² by application of 2,4-D.

"It was very clear that what was happening in the plant was what was happening in the cell cultures," Murphy said.

Murphy said the findings suggest that application techniques that limit 2,4-D entry into soils are important to ensure that production with engineered 2,4-D resistant crop plants does not require additional fertilizer and/or water inputs¹³.

The Department of Energy funded the study. Murphy and Peer partnered with scientists at the Institute of Experimental Botany at the Academy of Sciences of the Czech Republic.