Radial transport of abscisic acid conjugates in maize roots: its implication for long distance stress signals

Conjugated, alkaline hydrolysable ABA (predominantly abscisic acid glucose ester, ABA-GE), which is transported in the xylem from roots to shoots of Z ea mays L. plants, has its origin in the root symplast rather than from soi l, although it was detectable in soil solution with concentrations up to 30 nM. External ABA glucose ester cannot be dragged with the water flow acros s the exodermis and the endodermis because of its hydrophobic properties. E xperimental evidence is presented that enzymes in the cortical apoplast cle ave ABA-GE thus releasing ABA from its conjugates. Liberated ABA can then b e translocated apoplastically and symplastically to the xylem vessels. Endo genous ABA-GE can be released from isolated cortical and stelar tissues to the surrounding media, with rates that are up to 5-fold higher from stelar tissues than those from cortical tissues. Release of ABA-GE is highest unde r conditions of inhibited ABA-metabolism.