Dynamics and Control of Phloem Loading of Indoleacetic Acid in Seedling Cotyledons ofRicinus CommunisDuring Germination

During seed germination sugars and auxin are produced from stored precursors or conjugates respectively and transported to the seedling axis. To elucidate the mode of travel of IAA into the phloem a solution of [ 3 H]indoleacetic acid (IAA), together with [ 14 C]sucrose, was injected in the endosperm cavity harboring the cotyledons of germinating seedlings of Ricinus communis . Phloem exudate from the cut hypocotyl was collected and the radioactivity recorded. Sucrose loading in the phloem was inhibited at higher IAA levels, and the rate of filling of the transient pool(s) was reduced by IAA. IAA was detected within 10 minutes, with the concentration increasing over 30 min and reaching a steady-state by 60 min. The kinetics indicated that phloem loading of IAA involving both an active, carrier-based, and a passive, diffusion-based component, with IAA traveling along a pathway containing an intermediary pool, possibly the protoplasts of mesophyll cells. Phloem loading of IAA was altered by sucrose, K + and a range of non-specific and IAA-specific analogues and inhibitors in a manner that showed that IAA moves into the phloem from the extra cotyledonary solution by multiple pathways, with a carrier mediated pathway playing a principal role. Summary Statement Indoleacetic acid is transported from the peri-cotyledonary space into the phloem of germinating Ricinus seedlings by both trans-membrane carriers and diffusive pathways, with the cells of the cotyledons forming an intermediate reservoir.