Pattern formation in plant development: four vignettes.

In their development, plants precisely control the patterns of cell size, cell shape and cell division, the position of differentiation of cell types, and the position and number of organs. In the past year, progress has been made in understanding four of these five levels of control of plant pattern formation, including specification of cell fate by cell size in the alga Volvox carteri, genetic control of cell shape in the leaf hairs of the flowering plant Arabidopsis thaliana, precise control of cell division and cellular enlargement in Arabidopsis root meristems, and control of cell number in meristems of Arabidopsis, tobacco, and maize. These examples indicate that both the fate and shape of differentiated plant cells can be determined by the size of their undifferentiated precursors, and that the primary effect of mutations that alter cell fate and cellular shape can be on cell size. Furthermore, specific genes have been found that are necessary for normal patterns and numbers of cell divisions. Molecular cloning of these genes is revealing the molecular basis of plant cell division control.