Organogenesis in Graptopetalum paraguayense E. Walther: shifts in orientation of cortical microtubule arrays are associated with periclinal divisions.

The interior of a new lateral organ, such as a leaf, arises from the products of periclinal divisions of sub-epidermal cells. The biophysical basis of the elongation of such a new axis is transverse (hoop) reinforcement of the cells by cellulose in the primary walls. This structural polarity is associated with transverse alignment of cortical microtubules. We have brought the histological and biophysical views together by showing that the new, periclinal, divisions are a prerequisite for a corresponding change in the orientation of the microtubular array in the daughter cells. Investigation of this relationship required development of criteria for assessing the predominant orientation of a microtubule array in a single section of known orientation. By obtaining information about the predominant orientation of microtubule arrays in the sub-epidermal cells, we were able to study structural polarity shifts which occurred as a detached leaf of Graptopetalum produced a new shoot. During organogenesis, the new polarity is seen only in cells which have divided periclinally. Following single periclinal divisions, cells are seen with microtubules in the old or new orientation or in a mixture of different orientations. Cells with more than one orientation of microtubules are probably at intermediate stages in the shift to the new polarity. Among cells which have undergone two consecutive periclinal divisions, the old polarity is no longer seen, all cells having high frequencies of microtubules in the new orientation. Such cells are either polarized in the new direction or nonpolarized. The shifts in polarity of the cells in the interior anticipate the appearance of the first leaf primordia. However, contrary to the expectations from the histological view of organogenesis, these shifts do not dominate the process. Concurrent polarity changes in the epidermis appear at least as important.