We have interest in studying the role of ADP-glucose (ADPG) and UDP-glucose (UDPG) producing enzymes in plants. The former compound is the key precursor of starch synthesis, whereas the latter serves as a direct or indirect precursor to all other polysaccharides, including sucrose, cellulose and hemicelluloses. In some nonphotosynthetic tissues (e.g. cereal seeds), the production of ADPG is tightly coupled to UDPG metabolism, but in leaves the origins of these nucleotide sugars are distinct. In the latter tissue, the major control of starch production is thought to be exerted via ADP-glucose pyrophosphorylase (AGPase), a two-gene-encoded enzyme which produces ADPG and which, in most plants and tissues, is regulated by phosphoglycerate (PGA) and phosphate (Pi). Leaf AGPase serves as the key determinant of starch production during photosynthesis via regulation by a PGA/Pi ratio rather than PGA or Pi alone. The PGA/ Pi- dependent activation states of leaf AGPase are directly related to a given starch level in chloroplasts, indicating that AGPase exerts very tight control over starch production. At a transcriptional level, genes for AGPase are powerfully regulated by sugars and/ or osmotica. Similar effects were found for genes of sucrose synthesis/ metabolism (e.g. Sus1). Light effects were implicated in AGPase gene regulation, whereas Sus genes were stress and/ or osmoticum responsive via an ABA-independent pathway. The results are consistent with the existence of distinct sensing mechanisms and distinct signal transduction pathways involved in carbohydrate metabolism in plants. They also suggest that the processes of starch synthesis and sucrose formation/ metabolism may be regulated, depending on physiological conditions, by a net result of activities of several homologous genes rather than by a given locus. 

 

In an effort to understand the complexities of regulation of nucleotide sugar synthesis/ metabolism in plants, particular emphasis is placed on elucidation of mechanisms of enzymatic/ metabolic control and those governing the operation of distinct signal transduction pathways that are involved in regulation of AGPase, Sus and other genes responsible for carbohydrate synthesis. Site-directed mutagenesis studies are under way, using plant proteins produced in bacteria/ yeast, to identify amino acids responsible for catalysis/ regulation. Details of sugar/ osmoticum signal transduction pathway(s) are elucidated using plants with impaired sensing mechanism (e.g. with altered levels of hexokinase or impaired phytohormone level), and using inhibitors of protein kinases/ phosphatases. Also, plants transformed with specific promoters coupled to a suitable reporter gene will help to investigate details of specific tissue and/or developmental stage and/or stress-responsiveness of a given gene. Detailed biochemical and physiological analysis of mutants and transgenic plants will be undertaken to establish the consequences of modifying sugar signalling systems. Whereas we use Arabidopsis as a model plant system, we also have interest in the developing wood tissues, using resources of the Poplar EST project within the UPSC. In the latter tissues, we focus on regulation of the biosynthesis of sugar nucleotide precursors to cellulose and hemicelluloses, the primary determinants of wood quality.

Martz, F., Wilczynska, M., Kleczkowski, L.A. () Oligomerization status, with the monomer as active species, defines catalytic efficiency of UDP-glucose pyrophosphorylase.  Biochemical Journal 367: 295–300.

Igamberdiev, A.U., Kleczkowski, L.A.  ()  Implications of adenylate kinase-governed equilibrium of adenylates on contents of free magnesium in plant cells and compartments.  Biochemical Journal 360: 225-231.

Ciereszko, I., Johansson, H., Kleczkowski, L.A.  ()  Sucrose regulation of a cold-inducible UDP-glucose pyrophosphorylase gene via a hexokinase-independent and ABA-insensitive pathway  in Arabidopsis.  Biochemical Journal 354: 67-72.

Déjardin, A., Sokolov, L.N., Kleczkowski, L.A. () Sugar/ osmoticum levels modulate differential ABA-independent expression of two stress-responsive sucrose synthase genes in Arabidopsis. Biochemical Journal 344: 503-509.

Kleczkowski, L.A. () A phosphoglycerate to inorganic phosphate ratio is the major factor controlling starch levels in chloroplasts via ADP-glucose pyrophosphorylase regulation. FEBS Letters 448: 153-156.

Sokolov, L.N., Déjardin, A., Kleczkowski, L.A. () Sugars and light/ dark exposure trigger differential regulation of ADP-glucose pyrophosphorylase genes in Arabidopsis thaliana. Biochemical Journal 336: 681-687.

Thorbjörnsen, T., Villand, P., Kleczkowski, L.A., Olsen, O.A. () A single gene encodes two different transcripts for the ADP-glucose pyrophosphorylase small subunit from barley (Hordeum vulgare) Biochemical Journal 313:149-154.

Kleczkowski, L.A. () Back to the drawing board: redefining starch synthesis in cereals. Trends in Plant Science 1: 363-364.

Kleczkowski, L.A. () Inhibitors of photosynthetic enzymes/ carriers and metabolism. Annual Review of Plant Physiology and Plant Molecular Biology 45: 339- 367.