Auxin as an intercellular signal - Intercellular Communication in Plants - page 3

Figure 1.2 Nonpolar and polar auxin transport pathways. Auxin is distributed throughout plant by a
nonpolar pathway in phloem and by a slow polar movement from the apex to the base (basipetal) and
further to the root tip (acropetal) through vasculature and from the tip backwards (basipetal) through
outer layers.
existence of specific auxin uptake and efflux proteins (Rubery & Sheldrake, 1974;
Raven, 1975). Indeed, both saturable cellular auxin efflux and influx were demon-
strated in single plant cells and tissue segments (Benning, 1986; Lomax et al ., 1995).
Cellular efflux readily transports, besides naturally occurring IAA, also the synthetic
analogue NAA. On the other hand, 2,4-D is a preferred substrate for auxin uptake car-
riers but not, in most species, for auxin efflux carriers (Delbarre et al ., 1996). Auxin
influx and efflux pathways can also be physiologically distinguished using auxin
efflux inhibitors (AEIs). These pharmacological tools arose from correlative explo-
ration of structure-activity profiles of chemicals with auxin-like activity (Katekar
& Geissler, 1977) and were demonstrated to inhibit efflux of auxin from cells and
shoots segments. The most widely recognized AEIs are 1- N -naphthylphthalamic