Cell Wall Extension

Plant cells are usually bounded by a cell wall. One has only to drop a naked protoplast into water and watch it swell up and burst to realize that the wall limits the size of a plant cell. If a cell is to enlarge, it can only do so by enlarging the area of the cell wall. As a result, any review of cell elongation is, by necessity, a review of cell wall extension and vice versa. This review then will concern itself with the twin processes of cell enlarge­ ment and cell wall extension. The importance of cell enlargement has long been recognized, and the papers dealing with the process number in the tens of thousands. From these papers there has been distilled a body of basic information concerning the process, to wit: (a) cell enlargement involves a stretching of the wall already present as well as synthesis of new wall; (b) the driving force for extension is turgor pressure; (c) cell enlargement is an active process that normally requires respiration; (d) continuous synthesis of RNA and protein are needed for cell enlargement; (e) the rate of enlargement is regulated in many higher plant cells by auxin. The problem is that it is unclear as to how to fit these pieces together into any coherent picture of cell enlargement. Surprisingly, there have been only a few reviews of either cell enlarge­ ment or cell wall extension (107, 153, 185, 215), although various aspects of the problem have been well covered, such as the ultrastructure of the wall (3, 58, 133, 154, 155, 175, 176), wall composition (102, 140, 204) and biosynthesis (4, 71, 72, 102), RNA synthesis and cell enlargement (94, 206), the mechanism of auxin action (59, 60, 194), and the theory of cell elonga­ tion (21, 106, 107). The two reviews that stand out are Heyn's classic of 1940 (81) and Wilson's of 1964 (215), which seems to be the most recent major one. In addition, Ray (166) has just written an excellent short account of this subject. Before commencing this review, certain reservations and restrictions need to be listed. First, since one can only cover a small portion of the immense literature on this subject, this review will concentrate on certain aspects of cell wall extension; namely, the mechanical properties of primary walls and their relation to cell enlargement, the role of turgor pressure in growth, and the biochemical mechanism of cell wall loosening. Secondly, this review will pay particular attention to the knowledge obtained from two systems : Avena coleoptile sections and Nitella internode cells; not be-