MICROTUBULES AND THE ESTABLISHMENT OF APPARENT CELL-WALL INVAGINATIONS IN MESOPHYLL-CELLS OF PINUS-SILVESTRIS L

Ridges of cell wall material protruding into the cellular lumen are ch aracteristic of Pinus silvestris mesophyll cells. These ridges, e.g., appear to increase the inner surface for optimal dispersal of organell es, especially of chloroplasts. We show that the ridges are the result of local, brace-like wall reinforcements deposited during early turgo r-driven expansion growth of postmitotic cells. The reinforced sites r esist expansion and become the base of deep, narrow folds representing the ridges, while the intervening thin-walled areas evaginate as the cell volume increases. Cell wall material is deposited uniformly after expansion to stabilize the protrusions. There is a distinct correlati on between cell wall deposition and patterns of microtubules, known to be involved in determining the orientation of cellulose microfibril d eposition. It appears that this mode of establishing wall ridges is a variant of a comprehensive cell shaping strategy, recently shown to be responsible for modelling the outer contour of a number of different cell types.