A new method for rapid screening of xenobiotic phloem mobility in plants

Abstract

The deposition of hydrophobic polymers in the xylem of wheat grain floral axes prevents water and solute movement into grains via the xylem (xylem discontinuity). The only pathway for translocation of photo- synthate or externally applied xenobiotics into wheat grains is via the phloem. We have developed a new method based on the xylem discontinuity for rapidly screening phloem mobility of xenobiotics. By quantifying xenobiotic concentration in grains and excised plants after the compounds were applied through the cut stems, the phloem mobility can be estimated quantitatively. The phloem mobility obtained with our new grain-based method was correlated to xenobiotic chemical properties such as log Kow, pKa and electrical charge, and is consistent with pub- lished literature. Phloem mobility values determined by the grain-based assay were correlated to those from the direct phloem sap (aphid stylet exudate) assay of excised and intact plants, indicating that the grain-based assay is as reliable as the direct assay with aphid stylectomy. The new grain-based method is simple, quick, and can be scaled up for rapid screening of xenobiotic phloem mobility in plants. Similar seed (fruit)-based assay could also be developed with wide ranges of plant species that use the phloem as the only pathway for supplying water and nutrients into their seeds or fruits.