Plant cell elongation and wall formation are coupled processes driven by exocytosis, a process whereby the cell wall matrix contained inside Goigi vesicles is delivered to the existing cell wall while at the same time the vesicle membrane fuses with the plasma membrane.Vesicle fusion occurs in a non-random pattern; the sites of exocytosis have to be temporally and spatially controlled.One aspect of exocytosis site determination is directed Golgi body transport by the actin cytoskeleton, but another aspect is targeting a membrane domain for exocytosis.In yeast, this preparation is being done by the exocyst, an octameric protein complex involved specifically in tethering post Golgi vesicles to the plasma membrane prior to vesicle membrane fusion with the plasma membrane.Since almost all plant cells grow in a polar fashion either by the so-called intercalary, or diffuse, growth or by tip growth, a plant complex like the exocyst (Li et al.2010) could be an important determinant of the orientation of plant cell elongation.SEC3A is one of the subunits of the exocyst complex, a conserved, octameric protein complex that is essential for polarized secretion in eukaryotes.The plant exocyst has not been studied in great detail.We have characterized a T-DNA insertion mutant in Arabidopsis SEC3A, which is embryo lethal.