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Review
. 2011 Apr;155(4):1566-77.
doi: 10.1104/pp.110.170399. Epub 2011 Mar 4.

Regulation of starch biosynthesis in response to a fluctuating environment

Peter Geigenberger  1
Affiliations
Review

Regulation of starch biosynthesis in response to a fluctuating environment

Peter Geigenberger. Plant Physiol. 2011 Apr.
No abstract available

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Figures

Figure 1.
Figure 1.
Schematic representation of the pathway of starch biosynthesis, its subcellular compartmentation, and distribution of flux control in photosynthetic leaves (A) and heterotrophic tissues (B). The reactions of the pathway of starch biosynthesis are catalyzed by the following enzymes: 1, phosphoglucoisomerase; 2, PGM; 3, AGPase; 4, SS; 5, SBE; 6, starch-debranching enzyme; 7, inorganic pyrophosphatase; 8, Suc synthase; 9 UDP-Glc pyrophosphorylase; 10, fructokinase; 11, ATP/ADP translocator; 12, Glc-6-P/Pi translocator; 13, cytosolic AGPase; and 14, ADP-Glc/ADP translocator (steps 13 and 14 are highlighted to be specific for cereal endosperm). The false color symbols represent the relative flux control coefficients (C) of the constituent enzymes, defined as the relation between the fractional change in enzyme activity (Ei) and starch flux (J). Missing symbols represent reactions for which experimental data are missing. Data were taken from Neuhaus and Stitt (1990) and Stitt et al. (2010) for leaves and from Geigenberger et al. (2004) for growing potato tubers.
Figure 2.
Figure 2.
Regulation of plastidial AGPase by multiple mechanisms allows starch synthesis to respond across a range of time scales to a variety of physiological and environmental stimuli. Plastidial AGPase is a heterotetramer that contains two large (APL; 51 kD) and two slightly smaller (APS; 50 kD) subunits, which both have regulatory functions. Top, Allosteric regulation by 3PGA and Pi operates in a time frame of seconds to adjust the rate of starch synthesis to the balance between photosynthesis and Suc synthesis in leaves in the light and Suc breakdown and respiration in tubers. Left, Posttranslational redox modulation involves reversible disulfide bond formation between Cys-82 of the two small APS1 subunits, leading to changes in AGPase activity in response to light and sugar signals in a time frame of minutes to hours. The signaling components leading to redox modulation of AGPase involve Trx and NTRC, which are linked to photoreduced Fdx and interact with different sugar signals. Right, In Arabidopsis leaves, APS1 and APL1 have been identified as potential targets for reversible protein phosphorylation. More studies are needed to investigate the in vivo relevance of this mechanism and the underlying plastidial kinase network. Bottom, Transcriptional regulation in response to changes in carbon and nutrient supply allows more gradual changes in AGPase activity, which may require up to days to develop. Red font indicates inhibition, blue font indicates activation, and question marks indicate unknown (see main text for references).
Figure 3.
Figure 3.
Posttranslational redox regulation of starch biosynthesis in response to light and sugar signals. Light activation of starch synthesis involves posttranslational redox activation of AGPase in the chloroplast (Hendriks et al., 2003) and resembles the light activation of enzymes of the Calvin-Benson cycle and related photosynthetic processes. Photosynthetic electron transport leads to reduction of Fdx, and reducing equivalents are transferred by FTR to Trx f or m, which activate target enzymes by the reduction of regulatory disulfides. NTRC, containing both an NADP-Trx reductase and a Trx in a single polypeptide, serves as an alternate system for transferring reducing equivalents from NADPH to AGPase, thereby enhancing storage starch synthesis (Michalska et al., 2009). In the light, NTRC is mainly linked to photoreduced Fdx via Fdx-NADP reductase (identified with the dashed arrow) and complements the FTR/Trx system in activating AGPase. In the dark or under conditions in which light reactions are impaired, NTRC is primarily linked to sugar oxidation via the initial reactions of the oxidative pentose phosphate pathway (OPP) and in this way regulates AGPase independently of the Fdx/Trx system. Redox activation of AGPase is also induced by Suc, which operates in leaves in the light and in nonphotosynthetic tissues (Tiessen et al., 2002; Hendriks et al., 2003). Tre-6-P acts an intracellular signal, linking Suc in the cytosol with AGPase in the plastid (Kolbe et al., 2005; Lunn et al., 2006). In the working model, an increase in Suc is sensed in the cytosol, leading to an increase in the level of Tre-6-P by modulating Tre-6-P synthase (TPS) and/or Tre-6-P phosphatase (TPP). Tre-6-P is taken up into the plastid and promotes NTRC- and/or FTR/Trx-dependent redox activation of AGPase by a yet unresolved mechanism. SnRK1 is also implicated in this Suc signaling pathway, although its specific role in signal transduction is not fully resolved yet (Tiessen et al., 2003; Jossier et al., 2009; Zhang et al., 2009). How SnRK1 and Tre-6-P interact in this signaling cascade is unclear and may depend on the tissue.
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