The Physiology of Flowering Plants - KHAM PHA MOI

224 VEGETATIVE DEVELOPMENTis possible to track which layers give rise to which cell types in themature organ by marking them. Exposure of the SAM to colchicinewill make some cells polyploid whilst other mutations can preventthe development of chlorophyll (this occurs naturally in variegatedplants). One can then recognize the progeny of particular cells bytheir ploidy or pigmentation. Such experiments show that the cells inL 1 give rise to the epidermis whilst the rest of the leaf is derived fromL 2 and L 3 (L 2 only in monocotyledons). These cell layers are largelyclonally isolated from each other, because of the orientation of celldivisions, and hence the term cell lineage is sometimes used todefine which cell types develop from which layer. However, thislineage is not fixed; rather cells develop according to their positions.This position-dependent development is an important concept inmany aspects of plant development.Cell marking experiments show that very occasionally a cell will bepushed from one layer into another. In such circumstances the cellwill develop according to its new position rather than according to thetype characteristic of the layer from which it originated, which is why,perhaps, the term lineage is misleading. Such cell displacements arequite rare but demonstrate the position-dependent nature of celldevelopment. This flexibility of development extends to later stagesof organ formation. If cells within layer L 2 are marked by polyploidytheir growth rate is essentially the same as that of cells with normalploidy. Under these conditions it is possible to demonstrate that aconsiderable number of cells within the leaf are derived from L 2 .However, if cells are marked by making them achlorophyllous, thenthey grow more slowly. Although proportionately fewer cells withinthe mature leaf will be derived from layer L 2 , increased growth from L 3will have compensated (reviewed by Meyerowitz 1997).9.2.2 What regulates the size of the shoot apical meristem?Clearly a balance must exist between cells dividing to form neworgans and maintaining the size of the original meristem. The SAMwill continue to produce cells throughout the plant’s lifetime (exceptduring periods of dormancy or if damaged) but it remains the samesize regardless of the eventual size and age of the mature plant. Thisis quite remarkable given that trees may reach over a hundred metresin height and grow for hundreds of years. Our understanding of thisbalance between cell recruitment into organ primordia, the PZ andCZ, has been revolutionized by studying mutant plants – e.g.Arabidopsis, maize (Zea mays) and petunia (Petunia hybrida) – wherethis process has been perturbed. At the heart of the balance appearsto be communication between the different zones of the meristem. Inthe clavata1 (clv1) mutation of Arabidopsis, the meristem enlarges as aresult of the accumulation of undifferentiated cells whilst in the shootmeristemless 1 (stm1) mutant, cells are not maintained in the meristemwhich is lost during embryogenesis. In the wuschel (wus) mutant(German – tousled hair) the meristem is lost after a few organs havebeen formed. In the wild-type plant the gene which has been inactivated