The Arabidopsis exocyst subunit SEC3A is essential for embryo development and accumulates in transient puncta at the plasma membrane

The exocyst is a protein complex that is essential for polarized secretion in mammals and fungi. Although the exocyst is essential for plant development, its precise function has not been elucidated. We studied the role of exocyst subunit SEC3A in plant development and its subcellular localization. T-DNA insertional mutants were identified and complemented with a SEC3A-green fluorescent protein (GFP) fusion construct. SEC3A-GFP localization was determined using confocal microscopy. sec3a mutants are defective in the globular to heart stage transition in embryogenesis. SEC3A-GFP has similar cell plate localization to the other plant exocyst subunits. In interphase cells, SEC3A-GFP localizes to the cytoplasm and to the plasma membrane, where it forms immobile, punctate structures with discrete lifetimes of 2-40 s. These puncta are equally distributed over the cell surface of root epidermal cells and tip growing root hairs. The density of puncta does not decrease after growth termination of these cells, but decreases strongly when exocytosis is inhibited by treatment with brefeldin A. SEC3A does not appear to be involved in polarized secretion for cell expansion in tip growing root hairs. The landmark function performed by SEC3 in mammals and yeast is likely to be conserved in plants