The herbicide 2,4-D, a synthetic 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, while monocot grasses, such as sorghum and corn, are more resistant. That's because grasses inactivate 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 inactivated 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 yeast, 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 affected 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 inputs.

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.