Phloem translocation of a reduced oligogalacturonide inRicinus communis L

Abstract

We have re-examined the evidence against the phloem mobility of oligogalacturonide elicitors using a reduced oligogalacturonide in the phloem translocation system ofRicinus communis var. Gibsonii. A tritium-labelled end-reduced oligogalacturonide of degree of polymerisation 6 was injected into the hollow centre of the petiole of four- to five-week-old plants. Two experimental procedures were followed. In the first, the whole plant was harvested and dissected after 5 h incubation. In the second, phloem sap was collected from an incision in the main stem below the injected petiole; collection started 2 h after incubation and continued for a further 3 h. Determination of the total radiolabel present in the dissected plant showed that at least 8% of the applied activity was exported from the injected leaf, most of this being recovered from the main stem below the injected petiole and the roots. The activity in the phloem exudate showed that the rate of export of radiolabel was already at its maximum by the end of the 2-h incubation period. Radiolabelled material recovered from the main stem was found to be highly comparable to starting material when subjected to thin-layer chromatography. These results demonstrate the phloem mobility of reduced oligogalacturonides of low degree of polymerisation and therefore re-establish the potential for oligogalacturonides to act as systemic signals.