Plants Reveal Sophisticated Machinery
The most prevalent plant hormones, auxins, play a fundamental role in everything from pointing plants toward sunlight to helping them develop leaves and roots. Now researchers have made a serendipitous discovery about the way this hormone is transported in plants cells--revealing a much more sophisticated pumping system than was previously appreciated.
Auxins are produced in the tip of growing shoots and transported to the rest of the plant through a long column of cells. Auxin enters a cell through "influx" pumps nestled in the membrane. The auxin is packaged into vesicles and passed along internal tightropes to "efflux" pumps that shoot the hormone out of the other end of the cell. This picture was suddenly complicated in 1999, when Gerd Jürgens and his colleagues at the University of Tübingen in Germany discovered a mutant of the model plant Arabidopsis with efflux pumps everywhere in the cell. Because the mutated gene was involved in vesicle transport, the team suspected that the pumps might not be permanently bound to the membrane.
To see if the efflux pumps wander through the cell, Jürgens's team treated the cells with a transport inhibitor called brefeldin A that immobilized the pumps. In the 27 September issue of Nature, they report that the inhibitor caused the pumps to accumulate inside the plant cell. The pumps returned to the membrane when the inhibitor was removed, leading the researchers to conclude that efflux pumps cycle back and forth between the cell membrane and the cell's interior.
The experiment also challenges long-held assumptions about how these inhibitors work. Instead of just disrupting the efflux pumps, the compounds have a broader impact on intracellular transportation. A related inhibitor, TIBA, also affected the cell's entire transportation network, rather than a single component.
The findings are "very surprising" and will lead to a revision of basic ideas about auxin biology, says Mark Estelle, a biochemist at the University of Texas, Austin. "I think it will change how people think about how these compounds function," he says.