Regulation of Cell Division in the Shoot Apex

The aim of this paper is to explore how changes in the plane, and in the rate, of cell division affect primordium initiation in the shoot apex. Leaf initiation is characterised by a change in the plane of cell division in incipient primordia or changes in the rate of cell division, or both. We refer to published work on the preprophase band (PPB) and the phragmosome to indicate how planes of cell division are predicted in plant cells and argue that the presence of F-actin in PPBs may be a potential substrate for key regulatory cell cycle protein kinases. However the key signalling molecules that may cause a repositioning of PPBs during leaf initiation are unknown. We also attempt to make a link between homeotic, or organ identity genes and the regulation of cell size during floral morphogenesis. Here, emphasis is placed on a model by EM Lord proposing that homeotic genes have heterochronic function. In keeping with a timer mechanism would be genes which regulate cell size at division. The timing mechanism could act first, through homeotic genes determining where and when flower primordia are initiated and second, through genes which cause cell size to alter in cells which are determined as a particular floral primordium.