Role of metabolite transporters in source-sink carbon allocation

Abstract

Plants assimilate carbon dioxide during photosynthesis in chloroplasts. Assimilated carbon is subsequently allocated throughout the plant. Generally, two types of organs can be distinguished, mature green source leaves as net photoassimilate exporters, and net importers, the sinks, e.g., roots, flowers, small leaves, and storage organs like tubers. Within these organs, different tissue types developed according to their respective function, and cells of either tissue type are highly compartmentalized. Photoassimilates are allocated to distinct compartments of these tissues in all organs, requiring a set of metabolite transporters mediating this intercompartmental transfer. The general route of photoassimilates can be briefly described as follows. Upon fixation of carbon dioxide in chloroplasts of mesophyll cells, triose phosphates either enter the cytosol for mainly sucrose formation or remain in the stroma to form transiently stored starch which is degraded during the night and enters the cytosol as maltose or glucose to be further metabolized to sucrose. In both cases, sucrose enters the phloem for long distance transport or is transiently stored in the vacuole, or can be degraded to hexoses which also can be stored in the vacuole. In the majority of plant species, sucrose is actively loaded into the phloem via the apoplast. Following long distance transport, it is released into sink organs, where it enters cells as source of carbon and energy. In storage organs, sucrose can be stored, or carbon derived from sucrose can be stored as starch in plastids, or as oil in oil bodies, or – in combination with nitrogen – as protein in protein storage vacuoles and protein bodies. Here, we focus on transport proteins known for either of these steps, and discuss the implications for yield increase in plants upon genetic engineering of respective transporters.

Loss of Cytosolic Phosphoglucose Isomerase Affects Carbohydrate Metabolism in Leaves and Is Essential for Fertility of Arabidopsis

“…3) but apart from that appeared healthy. This seems to be a common feature of mutants impaired in primary carbohydrate metabolism, such as pgi1, pgm1, and adg1 (Caspar et al, 1985;Lin et al, 1988;Yu et al, 2000), or sugar transport, such as suc2 and sweet1 sweet2 (Gottwald et al, 2000;Chen et al, 2012), which results from the inability to properly allocate and distribute fixed carbon as energy or carbon skeletons to growing tissues (Ludewig and Flügge, 2013).…”

Crosstalk between Two bZIP Signaling Pathways Orchestrates Salt-Induced Metabolic Reprogramming in Arabidopsis Roots

“…Accordingly, the transcriptome data set shows b-AMYLASE gene activation in roots. Moreover, sink-driven transport processes to the root may occur (Ludewig and Flügge, 2013). SWEET4 and SWEET2 are examples of genes misregulated in bzip1 bzip53 that encode putative sugar transporters (Chen et al, 2012;Xuan et al, 2013).…”

Strategies of two tropical woody species to tolerate salt stress

“…The decreased g s in stressed plants of both species (Figure 2) was accompanied by a decrease in their SS concentration (Table 2). This reduction in g s directly affects the CO 2 concentration in the mesophyll and impairs the formation of trioses-P in chloroplasts, which are used in the synthesis of soluble sugars (Ludewig & Ingo-Flügge, 2013). In addition to total soluble sugars (SS), total soluble proteins (TSP) also decreased (Table 2), which may have been due to synthesis depletion caused by the loss of cell turgor (Hsiao, 1973).…”