Tobacco BY-2 Cells as a Model for Differentiation in Heterotrophic Plant Cells

Abstract

Starch is the principal form of carbon storage in many plants, and is synthesized in both photosynthetic and non-photosynthetic tissues. Starch found in chloroplasts is termed transitory starch; it is synthesized during the day from photosynthetically fixed carbon and mobilized at night, primarily in the form of sucrose or one of its derivates. It is also synthesized from sugars such as sucrose, in specialized synthesis/storage plastids termed amyloplasts (Kirk 1978). Due to the nutritional and economic importance of starch, many studies have been undertaken to investigate the enzymes involved in its synthesis, as well as the structure of the starch itself (reviewed in Ball and Morel 2003; Geigenberger et al. 2004; Jobling 2004). In addition, the patterns of mRNA expression involved in carbohydrate metabolism including starch synthesis during storage organ development have been analyzed in several plant species (Visser et al. 1994; Ainsworth et al. 1995; Burton et al. 1995; Bachem et al. 1996; Weber et al. 1997). These studies have shed light on the enzymatic properties and expression profiles of individual genes during storage organ development. Typically, storage organ development occurs as a series of specific temporal and spatial steps, involving cell division and subsequent differentiation. These steps often occur simultaneously, and thus are difficult to separate and characterize individually. Moreover, the complex structure of storage organs makes analysis of the cellular mechanisms of storage difficult. Even the rootcap cells, where amyloplast-containing columella cells can be distinguished from their original meristem, are not suitable for analyses on amolecular basis because these cells are too fragile, small and complex. In starch-accumulating tissues, carbohydrate metabolism begins with the cleavage of mobilized sucrose to produce hexoses for use in starch formation, cell wall construction, and energy release. However, pools of metabolic intermediates are shared by several pathways, with the direction of metabolite flow being determined by the needs of the cell. These metabolite fluxes, as well as the regulation of transcription and activity of carbohydrate metabolizing enzymes, vary according to both the