Lateral ABA transport in maize roots (Zea mays): visualization by immunolocalization

The intensity of an ABA (abscisic acid) signal as a root-to-shoot signal, as well as its action on root hydraulic conductivity, strongly depends on the distribution of ABA during its radial transport across roots. Therefore ABA was visualized by immunolocalization with monoclonal ABA antibodies under conditions of lateral water flow induced by the application of a pressure gradient to the cut surface of the mesocotyl of maize seedlings. From the labelling of rhizodermis, hypodermis, cortical cells, and endodermis of roots of hydroponically (no exodermis) and aeroponically (with exodermis) grown seedlings it is concluded that the exodermis acts as a barrier to apoplastic transport that controls ABA uptake and efflux, but that the endodermis can easily be overcome via an apoplastic bypass. In longitudinal sections the strongest ABA signals originated from the root cap and the meristematic root tip, which is in agreement with the non-vacuolated cells of these tissues being an effective anion trap for ABA.