Cell wall secretion in the green alga Micrasterias

The monoclonal antibodies JIM 1 against non-arabinogalactan epitopes, JIM 5 and JIM 7 recognizing unesterified and methyl-esterified pectins, and JIM 8 specific for arabinogalactan proteins (AGPs) are used for investigating different stages of cell wall formation in high pressure frozen and freeze substituted Micrasterias cells by means of immunoelectron microscopy. The results show that the septum-forming vesicles and the septum wall consist mainly of methyl-esterified pectins which become only partly de-esterified in the septum wall. Arabinogalactan proteins appear at the septum rim at the end of septum growth and are main constituents of the primary cell wall, together with esterified pectins. Only the outermost layer of the primary cell wall is labelled by JIM 5, indicating the presence of unesterified pectins. AGPs, non-AGP epitopes indicated by JIM 1, and pectins are transported together in the contents of the primary wall, forming 'dark vesicles' from the site of their production at the dictyosomes to the plasma membrane. Labelling of exclusively the plasma membrane of the non-growing semicell by JIM 1 and JIM 8 points towards a regulatory mechanism of membrane glycoproteins for vesicle fusion. The secondary cell wall of Micrasterias is not labelled by any of the antibodies used. JIM 4, JIM 15, JIM 84 and MAC 207 do not produce any specific staining in Micrasterias.