Auxin Immunolocalization Implicates Vesicular Neurotransmitter-Like Mode of Polar Auxin Transport in Root Apices

Immunolocalization of auxin using a new specific antibody revealed, besides the expected diffuse cytoplasmic signal, the enrichment of auxin at cross-walls (end-poles, plant-synapses), within endosomes, and within nuclei of those root apex cells which accumulate abundant F-actin at their synapses. In Brefeldin A (BFA) treated roots, a strong auxin signal was scored within BFA compartments of cells having abundant actin and auxin at their synapses, as well as within adjacent endosomes, but not in other root cells. Interestingly, several types of polar auxin transport (PAT) inhibitors exert the same inhibitory effects on endocytosis, vesicle recycling, and on enrichment of F-actin at the plant-synapses. These findings indicate that auxin is secreted across F-actin-enriched plant synapses via neurotransmitter-like secretion. This new concept finds genetic support via the semaphore1, rum1, and rum1/lrt1 mutants of maize which are impaired in PAT, endocytosis and vesicle recycling, as well as in recruitments of F-actin and auxin to these auxin transporting plant synapses.