There were two phosphorus NMR signals because the resonant frequency of a chemical species depends upon the local chemical environment and the plant vacuole is significantly more acidic than the cytoplasm.
Within 24 hours, resistant horseweed had managed to shuttle 85 percent of the glyphosate into the vacuole. Sensitive horseweed, on the other hand, had disposed of only 15 percent of the glyphosate in this way.
Scientists believe resistant horseweed has a pump in the tonoplast (the membrane surrounding the vacuole) that actively shuttles glyphosate into this storage compartment where it can no longer interfere with the critical biological reactions taking place in the chloroplast (the small green organelle to the upper left).
Meanwhile, the glyphosate remaining in the cytoplasm was being transported together with sugars to rapidly growing parts of the plant, such as young leaves and root tips.
Once glyphosate reaches such "metabolic sinks," it interrupts the critical shikimate pathway and kills the plant. Within 24 hours, sensitive plants had translocated 35 percent of the glyphosate from the source leaves to these sink tissues, whereas resistant plants had allowed only 15 percent to move to the sinks and much of this was shuttled into the vacuole of the sink tissue, thus further reducing chloroplast exposure.
"It's really a race," says d'Avignon. "Once glyphosate gets to the vacuole it is trapped," he says. "Because resistant horseweed rapidly shuttles glyphosate into the vacuole, there's less of it available for translocation to rapidly growing parts of the plants."
The scientists believe resistant horseweed has a pump, or transporter, that actively moves glyphosate across the tonoplast (the vacuole membrane).
"The existence of a glyphosate transporter is a surprise," says d'Avignon. "People had thought glyphosate moved into plant cells and
|Contact: Diana Lutz|
Washington University in St. Louis