In Vivo 31P-NMR Studies of Glyphosate Uptake, Vacuolar Sequestration, and Tonoplast Pump Activity in Glyphosate Resistant Horseweed (Conyza canadensis)

Abstract

ABSTRACT Horseweed is considered a significant glyphosate-resistant (GR) weed in agriculture, spreading to 21 states in the United States and is now found globally on five continents. This laboratory previously reported rapid vacuolar sequestration of glyphosate as the mechanism of resistance in GR horseweed. The observation of vacuole sequestration is consistent with the existence of a tonoplast-bound transporter. 31P-NMR experiments performed in vivo with GR horseweed leaf tissue show glyphosate entry into the plant cell (cytosolic compartment) is: (i) first order in extracellular glyphosate concentration, independent of pH, and dependent upon ATP, (ii) competitively inhibited by alternate substrates [aminomethyl phosphonate (AMPA), N-methyl glyphosate, (NMG)], which themselves enter the plant cell, and (iii) blocked by vanadate, a known inhibitor/blocker of ATP-dependent transporters. Vacuole sequestration of glyphosate is: (i) first order in cytosolic glyphosate concentration and dependent upon ATP, (ii) competitively inhibited by alternate substrates (AMPA, NMG), which themselves enter the plant vacuole, and (iii) saturable. 31P-NMR findings with GR horseweed are consistent with active transport of glyphosate and alternate substrates (AMPA, NMG) across the plasma membrane and tonoplast in a manner characteristic of ABC-binding cassette (ABC) transporters similar to those that have been identified in mammalian cells.