Secretion marker proteins and cell wall polysaccharides move through different secretory pathways.

The building up of the cell wall is tightly dependent on the functionality of the secretory pathway. Syntaxins as well as other SNARE proteins play important roles during vesicle secretion and fusion. We have compared the secretion of newly synthesised cellwall polysaccharides to that of secretory marker proteins such as secreted green-Xuorescent protein (sec- GFP) and secreted rat preputial -glucuronidase (secRGUS) in leaf protoplasts and roots of wild-type and transgenic Nicotiana tabacum plants, overexpressing a syntaxin homologue NtSyr1 (Sp1) and its soluble variant Sp2 that interferes speciWcally with Sp1 function, aVecting post-Golgi transport. In protoplasts transiently transformed with secGFP and Sp1, no variation was observed in the pattern of Xuorescence with respect to control; on the contrary, GFP Xuorescence accumulate within the cells in protoplasts co-transformed with secGFP and Sp2. Sp2 reduced the percentage of marker protein secretion to 53% as quantiWed with secRGUS. In protoplasts obtained from leaves of wild-type and transformed tobacco plants expressing Sp1, Sp2 and Sp1 plus Sp2, no remarkable diVerences in the percentage of newly synthesised polysaccharides incorporated into the regenerating cell walls were observed. The same results were conWrmed in roots of whole transformed seedlings. Tests with cytochalasin D (CD) showed a marked decrease in the amount of newly synthesised polysaccharides into the wall and a simultaneous sharp increase in membrane-associated polysaccharides. SecRGUS secretion was also inhibited by CD. The data indicate that marker proteins and matrix polysaccharides, as well as cellulose synthase complexes, are secreted through the involvement of diVerent secretory machineries.