News & Events

Recent Discoveries

1. Two Laforin-Like phosphoglucan phosphatases are required for starch degradation and control starch phosphate content.

In the past decade, the importance of glucan-bound phosphate in starch metabolism has been realized. Two enzymes (glucan, water dikinases) phosphorylate starch at either the 6- or 3- positions of the glucose molecules that make up starch. Mutants lacking these proteins cannot break down their starch properly. Our recent work, led by Dr Diana Santelia and Dr Oliver Kötting has identified two more enzymes – SEX4 (for Starch EXcess 4) and LSF2 (for Like Starch-excess Four 2) that dephosphorylate starch and are also necessary its degradation in Arabidopsis. Collectively the data show that there is a cycle of transient phosphorylation during starch breakdown. Phosphorylation disrupts the crystalline structures of starch, allowing access by the glucan hydrolases like amylases. SEX4 and LSF2 then remove the phosphates to enable complete degradation. The phosphate content of starch also affects the functional properties of extracted starches. By controlling the amounts of phosphorylating and dephosphorylating enzymes, crops with improved starches could be developed.

Santelia D., Kötting O., Seung D., Schubert M., Thalmann M., Bischof S., Meekins D.A., Lutz A., Patron N., Gentry M. S., Allain F.H.-T., Zeeman S.C. (2012) The phosphoglucan phosphatase LSF2 (Like Sex Four 2) dephosphorylates starch at the C3-position in Arabidopsis. Plant Cell, 23, 4096-4111. [pdf]

Kötting O., Santelia D., Edner C., Eicke,S. Marthaler T, Gentry M.S., Comparot- Moss S., Chen J., Smith A.M., Steup M., Ritte G., and Zeeman S.C. (2009) STARCH-EXCESS4 Is a Laforin-Like Phosphoglucan Phosphatase Required for
Starch Degradation in Arabidopsis thaliana. The Plant Cell, 21, 334-346. [pdf]

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Other relevant literature

Niittylä, T., Comparot-Moss, S., Lue, W.-L., Messerlie, G., Trevisan, M., Seymour, M.D.J., Gatehouse, J.A., Villadsen, D., Smith, S.M., Chen, J., Zeeman, S.C., and Smith, A.M. (2006) Similar protein phosphatases control starch metabolism in plants and glycogen metabolism in mammals. J Biol Chem, 281, 11815-11818. [pdf]

Kötting, O., Pusch, K., Tiessen, A., Geigenberger, P., Steup, M., and Ritte, G. (2005) Identification of a Novel Enzyme Required for Starch Metabolism in Arabidopsis Leaves. The Phosphoglucan, Water Dikinase. Plant Physiol., 137, 242-252. [PubMed]

Hejazi
M., Fettke J., Kötting O., Zeeman S.C., Steup M. (2010) The laforin-like
dual-specificity phosphatase SEX4 from Arabidopsis thaliana hydrolyses both C6-
and C3-phosphate esters introduced by starch-related dikinases and thereby
affects phase transition of α-glucans. Plant Physiology, 152, 711-722. [pdf]

2. A potential new player in the metabolic control of gene expression

We recently discovered that some of the genes in plants annotated as β-amylases (BAMs; enzymes usually associated with starch breakdown) are in fact nuclear transcription factors controlling plant growth and development. The two Arabidopsis thaliana β-amylase-like proteins, BAM7 and BAM8, possess DNA-binding domains also found in transcription factors mediating brassinosteroid hormone responses (BZR1, for BrassinaZole Resistant 1). These so-called ‘BZR1-BAMs’ bind a DNA sequence in the promoters of genes to activate expression. The BAM domain may still bind a glucan, like a normal β-amylase, but our analyses reveal little or no activity. Altering BZR1-BAM levels in the plant (the bam7bam8 double mutant and BAM8 over-expressing plants) causes altered growth and development. Of the genes up-regulated in plants over-expressing BAM8 and down-regulated in bam7bam8 plants, many carry the regulatory DNA element in their promoters. Many brassinosteroid-responsive genes are inversely regulated by BZR1-BAMs, suggesting crosstalk with hormone signaling. We currently believe that BZR1-BAMs may transmit metabolic signals by binding a ligand in their BAM domain, and work to identify this ligand is ongoing.

Reinhold H., Soyk S., Simkova K., Hostettler C., Marafino J., Samantha M., Vaughan C.K., Monroe J.D., Zeeman S.C. (2011) Beta-amylase–like proteins function as transcription factors in Arabidopsis, controlling shoot growth and development. Plant Cell, 23, 1391-1403. [pdf]

3. Cytosolic phosphoglucomutase is required for gametophyte development in Arabidopsis.

Cytosolic phosphoglucomutase (cPGM) interconverts glucose 6-phosphate and glucose 1-phosphate which, together with fructose 6-phosphate, comprise the hexose phosphate pool – an important node in the central metabolism. A number of important biosynthetic and respiratory pathways lead out of the hexose phosphate pool and it is critical that the pool is maintained in equilibrium to balance carbohydrate supply with demand. Our recent experiments show that Arabidopsis has two genes (PGM2 and PGM3) encoding cPGM proteins with redundant functions. Loss of both genes severely impairs gametophyte function. Double mutant pollen completes development but fails to germinate. Double mutant ovules also develop normally but half remain unfertilised after pollination. We attribute these severe phenotypes to an inability to effectively distribute carbohydrate from imported or stored substrates (e.g. sucrose). This would have dramatic consequences for germinating pollen grains, which have high metabolic and biosynthetic activities. Residual cPGM mRNA or protein derived from the mother plant may enable double mutant gametophytes to develop to maturity and for some ovules to be fertilised. Fertilisation by pollen carrying a functional cPGM gene would rescue the ovule.

Egli B., Kölling K., Köhler C., Zeeman S.C., Streb S. (2010) Loss of cytosolic phosphoglucomutase compromises gametophyte development in Arabidopsis. Plant Physiology, in press. [pdf]

4. Blocking the metabolism of starch breakdown products in Arabidopsis leaves triggers chloroplast degradation – a possible link between carbohydrate metabolism and autophagy.

We have uncovered an unexpected link between transitory starch metabolism and autophagy – a process by which cellular components are broken down and recycled. In a healthy leaf, part of the carbon dioxide fixed through photosynthesis is stored as starch in chloroplasts. The starch is used at night to support metabolism. Mutations affecting starch utilisation reduce plant growth and, in some cases, lead to the accumulation of metabolic intermediates. One mutant, maltose excess 1 (mex1), lacks the chloroplast envelope maltose exporter and so maltose produced from starch is trapped in its chloroplasts. Interestingly, mex1 leaves become pale green (chlorotic) due to the induction of autophagy-like chloroplast degradation. Introduction of additional mutations to prevent starch synthesis, or block maltose release from starch, prevent mex1 chlorosis. In contrast, introduction of mutations that cause additional intermediates to accumulate, greatly increase chlorosis. Our results suggest that the accumulation of intermediates of starch mobilisation causes chloroplast dysfunction, which in turn triggers chloroplast autophagy.

Stettler M., Eicke S., Mettler T., Messerli G., Hörtensteiner S., Zeeman S.C. (2009) Blocking the Metabolism of Starch Breakdown Products in Arabidopsis Leaves Triggers Chloroplast Degradation. Molecular Plant, 2, 1233-1246. [pdf]

Niittylä, T., Messerli, G., Trevisan, M., Chen, J., Smith, A.M. and S.C. Zeeman (2004) A novel maltose transporter essential for starch degradation in leaves. Science 303, 87-89. [pdf]

5. The debate on the pathway of starch synthesis: a closer look at low-starch mutants lacking plastidial phosphoglucomutase supports the chloroplast-localised pathway

Recently, the widely accepted pathway of transient starch synthesis in chloroplast was questioned and an “alternative” pathway postulated in which sucrose is an intermediate of starch synthesis (Muñoz et al., 2006). The “alternative” model also suggests that starch is continuously turned over as it accumulates. Using genetics in the model plant Arabidopsis, we show that this scenario is unlikely. However, the observation that plants lacking an enzyme crucial in the classical pathway (PGM: chloroplastic phosphoglucomutase) still synthesis tiny amounts of starch cannot be fully explained. Further research is required to explain the discrepancies between experimental data and metabolic models.

Streb S., Egli B., Eicke S., Zeeman S.C. (2009) The debate on the pathway of starch synthesis: a closer look at low-starch mutants lacking plastidial phosphoglucomutase supports the chloroplast-localised pathway. Plant Physiology, 151, 1769-1772. [pdf]

Other relevant literature

Barratt DHP, Derbyshire P, Findlay K, Pike M, Wellner N, Lunn J, Feil R, Simpson C, Maule AJ, Smith AM (2009) Normal growth of Arabidopsis requires cytosolic invertase but not sucrose synthase. Proc Natl Acad Sci USA, 106: 13124–13129. [PubMed]

Muñoz FJ, Zorzano MTM, Alonso-Casajus N, Baroja-Fernandez E, Etxeberria E, Pozueta-Romero J (2006) New enzymes, new pathways and an alternative view on starch biosynthesis in both photosynthetic and heterotrophic tissues of plants. Biocatalysis Biotransformation 24, 63-76.

Caspar T, Huber SC, Somerville C (1985) Alterations in growth, photosynthesis, and respiration in a starchless mutant of Arabidopsis thaliana (L.) deficient in chloroplast phosphoglucomutase activity. Plant Physiol 79, 11-17.