| Recently, we have discovered that cell wall pectins are internalized into root cells of apical meristem. In cells exposed to the fungal metabolite brefeldin A (BFA), all secretory pathways are inhibited while endocytic pathways remain intact, resulting in accumulation of internalized cell wall pectins within brefeldin A-induced compartments (Baluška et al. 2002). Here we report that in addition to the already published cell wall epitopes, RGI and xyloglucans also accomplish large-scale internalization into dividing root cells. Interestingly, multilamellar endosomes were identified as compartments internalizing 6D7-reactive arabinan cell wall pectins while large vacuole-like endosomes internalized 2F4-reactive homogalacturonans. As all endosomes belong topographically to the exocellular space, cell wall pectins disposed in these ‘cell wall islands’, enclosed by the plasma membrane-derived membrane, are ideally suited to act as temporary disposals for rapid formation of cell wall and generation of new plasma membrane. In accordance with this notion, we report that all cell wall pectins as well as xyloglucans that internalize into endosomes are highly enriched within cytokinetic cell plates and accumulate within BFA compartments. On the other hand, the JIM7-reactive pectins produced in Golgi apparatus localize to cell plates only in small amounts and do not accumulate within BFA compartments. In conclusion, meristematic root cells have developed pathways for internalization and recycling of cell wall molecules which are relevant for plant-specific cytokinesis. |
