Plasma-membrane rosettes involved in localized wall thickening during xylem vessel formation of Lepidium sativum L.

Developing xylem vessel elements in roots of cress, Lepidium sativum L., were freeze-fractured after rapid freezing in nitrogen slush (without cryoprotection). With the double-replica technique, both the plasmatic fracture face (PF) and the extraplasmatic fracture face (EF) of the plasma membrane were exposed. The EF revealed abundant, but rather indistinct "terminal globules"; whereas the PF showed numerous "rosettes". Terminal globules and rosettes were localized, restricted to regions of secondary wall thickening only, and showed comparale frequencies per μm(2), supporting the assumption that they are part of the same synthase complex. The abundance of rosettes in regions of high cellulose production supports their postulated involvement in cellulose microfibril formation. With up to 191 rosettes per μm(2), the rosettes appear to be too densely arranged to be directly aligned on individual microtubules. This favors the channelling hypothesis of synthase movement in the plasma membrane.