Molecular Approaches to Sink-Source Interactions

Abstract

Growth and development of plants are dependent upon the energy gained by fixing carbon dioxide into carbohydrates during photosynthesis and the translocation of newly fixed photoassimilates from their site of synthesis to regions of utilization and/or storage. Following this scheme, plant organs can be divided into two kinds: (a) photosynthetically active source organs (defined as net exporters of photoassimilates), represented mainly by mature leaves, and (b) photosynthetically inactive sink organs (defined as net importers of fixed carbon). Sinks can be divided into at least two different classes (6): (a) 'utilization sinks,' highly metabolically active, rapidly growing tissues like meristems and immature leaves, and (b) 'storage sinks' such as tubers, seeds, or roots, which deposit the imported carbohydrates in the form of storage compounds (e.g. starch, sucrose, fatty acids, or proteins). The flow of carbon from the initial step of carbon dioxide fixation to the translocation to its final destination needs the interaction of different organelles and organs. The principal biochemical pathway of photoassimilate partitioning in source leaves is given in Figure 1. The primary products of carbon fixation are starch and sucrose. In source leaves, starch is synthesized within the chloroplast and serves mainly as an intermediate deposit for photoassimilates, whereas sucrose, synthesized within the cytosol, plays a central role in the distribution of photoassimilates throughout the plant. With respect to starch synthesis, although there are two possible pathways (via starch phosphorylase and/or starch synthase), the main route is believed to be catalyzed by starch synthase starting from ADP-glucose. The synthesis of starch is regulated at the level of the enzyme ADPG pyrophosphorylase,l catalyzing the formation of ADP-glucose from glucose-i-P. The enzyme ADPG pyrophosphorylase is inhibited by Pi and activated by 3-PGA; thus, the ratio between 3-PGA and Pi within the plastid will regulate the rate of starch synthesis (for review see ref. 12). Sucrose synthesis is most likely regulated at two steps: the interconversion of fructose-1,6-bisP to fructose-6-P and the synthesis of sucrose-6-P from fructose-6-P and UDP-glucose by the enzyme SPS. The first reaction is regulated via the signal metabolite 2,6-FBP. This metabolite inhibits the cytosolic FBPase and stimulates the enzyme pyrophosphate:fructose-6-P phosphotransferase, thus driving the flow of metabolites toward glycolysis. The third enzyme, phosphofructokinase, involved in this reaction is not regulated by 2,6-FBP. The enzyme responsible for the formation of 2,6FBP (2,6-FBP-kinase) is inhibited by 3-PGA and activated by fructose-6-P and Pi (for review see ref. 16). The synthesis of sucrose-6-P in many plant species is regulated by protein phosphorylation of the enzyme sucrose-6-P-synthase (7). Our knowledge of the biochemical pathways is based on: (a) isolation and characterization of the respective enzymes, (b) the use of chemical inhibitors, (c) analysis of isolated organelles, and (d) analysis of classical mutants. During the last 10 years, molecular tools have become available allowing for a more precise in vivo manipulation and analysis of postulated biochemical pathways. Prerequisites for molecular manipulations are: (a) the regeneration and transformation of plants, (b) the isolation of promoter sequences allowing the tissueand cell-specific expression of chimeric genes, (c) the isolation of genes encoding enzymes involved in biochemical pathways (either plant or heterologous enzymes), and (d) the targeting of newly synthesized proteins into different subcellular compartments (e.g. plastids, mitochondria, cell wall, vacuole, etc.). Genes encoding enzymes involved in biochemical pathways can be used either to inhibit cellular enzymic activities via the antisense approach (for review see ref. 17) or for the overexpression of enzymic activities. The ectopic expression of foreign genes allows the introduction of new enzymes that differ from the plant endogenous ones either with respect to regulatory and/ or kinetic properties or because they are alien to the plant environment in which they are expressed. Thus, molecular biology permits the creation of transgenic plants that have been specifically modulated with respect to a certain enzymic activity within a biosynthetic pathway. A subsequent analysis of the mutant plant with regard to several biochemical, physiological, and ecological parameters allows a causal relationship between a well-defined in vivo change and its subsequent effects to be established. Our review focuses specifically on this new approach with respect to our understanding of sink-source relations in higher plants. There are several excellent reviews covering the huge literature using nonmolecular genetic approaches (cf. 3, 4, 6).