Jens Kossmann

Bio: Jens Kossmann is an academic researcher from Max Planck Society. The author has contributed to research in topics: Starch & Amylopectin. The author has an hindex of 22, co-authored 26 publications receiving 4075 citations. Previous affiliations of Jens Kossmann include Institut national de la recherche agronomique & Stellenbosch University.

Starch: its metabolism, evolution, and biotechnological modification in plants.

Abstract: Starch is the most widespread and abundant storage carbohydrate in plants. We depend upon starch for our nutrition, exploit its unique properties in industry, and use it as a feedstock for bioethanol production. Here, we review recent advances in research in three key areas. First, we assess progress in identifying the enzymatic machinery required for the synthesis of amylopectin, the glucose polymer responsible for the insoluble nature of starch. Second, we discuss the pathways of starch degradation, focusing on the emerging role of transient glucan phosphorylation in plastids as a mechanism for solubilizing the surface of the starch granule. We contrast this pathway in leaves with the degradation of starch in the endosperm of germinated cereal seeds. Third, we consider the evolution of starch biosynthesis in plants from the ancestral ability to make glycogen. Finally, we discuss how this basic knowledge has been utilized to improve and diversify starch crops.

The starch-related R1 protein is an α-glucan, water dikinase

Abstract: To determine the enzymatic function of the starch-related R1 protein it was heterologously expressed in Escherichia coli and purified to apparent homogeneity. Incubation of the purified protein with various phosphate donor and acceptor molecules showed that R1 is capable of phosphorylating glucosyl residues of α-glucans at both the C-6 and the C-3 positions in a ratio similar to that occurring naturally in starch. Phosphorylation occurs in a dikinase-type reaction in which three substrates, an α-polyglucan, ATP, and H2O, are converted into three products, an α-polyglucan-P, AMP, and orthophosphate. The use of ATP radioactively labeled at either the γ or β positions showed that solely the β phosphate is transferred to the α-glucan. The apparent Km of the R1 protein for ATP was calculated to be 0.23 μM and for amylopectin 1.7 mg⋅ml−1. The velocity of in vitro phosphorylation strongly depends on the type of the glucan. Glycogen was an extremely poor substrate; however, the efficiency of phosphorylation strongly increased if the glucan chains of glycogen were elongated by phosphorylase. Mg2+ ions proved to be essential for activity. Incubation of R1 with radioactively labeled ATP in the absence of an α-glucan showed that the protein phosphorylates itself with the β, but not with the γ phosphate. Autophosphorylation precedes the phosphate transfer to the glucan indicating a ping-pong reaction mechanism.

Inhibition of a starch-granule-bound protein leads to modified starch and repression of cold sweetening.

Abstract: We have cloned a gene involved in starch metabolism that was identified by the ability of its product to bind to potato starch granules. Reduction in the protein level of transgenic potatoes leads to a reduction in the phosphate content of the starch. The complementary result is obtained when the protein is expressed in Escherichia coli, as this leads to an increased phosphate content of the glycogen. It is possible that this protein is responsible for the incorporation of phosphate into starch-like glucans, a process that is not understood at the biochemical level. The reduced phosphate content in potato starch has some secondary effects on its degradability, as the respective plants show a starch excess phenotype in leaves and a reduction in cold-sweetening in tubers.

One of two different ADP-glucose pyrophosphorylase genes from potato responds strongly to elevated levels of sucrose.

Abstract: The key regulatory step in starch biosynthesis is catalyzed by the tetrameric enzyme ADP-glucose pyrophosphorylase (AGPase). In leaf and storage tissue, the enzyme catalyzes the synthesis of ADP-glucose from glucose-l-phosphate and ATP. Using heterologous probes from maize, two sets (B and S) of cDNA clones encoding potato AGPase were isolated from a tuber-specific cDNA library. Sequence analysis revealed homology to other plant and bacterial sequences. Transcript sizes are 1.9 kb (AGPase B) and 2.1 kb (AGPase S). Northern blot experiments show that the two genes differ in their expression patterns in different organs. Furthermore, one of the genes (AGPase S) is strongly inducible by metabolizable carbohydrates (e.g. sucrose) at the RNA level. The accumulation of AGPase S mRNA was always found to be accompanied by an increase in starch content. This suggests a link between aGPase S expression and the status of a tissue as either a sink for or a source of carbohydrates. By contrast, expression of AGPase B is much less variable under various experimental conditions.

The Arabidopsis sex1 Mutant Is Defective in the R1 Protein, a General Regulator of Starch Degradation in Plants, and Not in the Chloroplast Hexose Transporter

Abstract: Starch is the major storage carbohydrate in higher plants and of considerable importance for the human diet and for numerous technical applications. In addition, starch can be accumulated transiently in chloroplasts as a temporary deposit of carbohydrates during ongoing photosynthesis. This transitory starch has to be mobilized during the subsequent dark period. Mutants defective in starch mobilization are characterized by high starch contents in leaves after prolonged periods of darkness and therefore are termed starch excess (sex) mutants. Here we describe the molecular characterization of the Arabidopsis sex1 mutant that has been proposed to be defective in the export of glucose resulting from hydrolytic starch breakdown. The mutated gene in sex1 was cloned using a map-based cloning approach. By complementation of the mutant, immunological analysis, and analysis of starch phosphorylation, we show that sex1 is defective in the Arabidopsis homolog of the R1 protein and not in the hexose transporter. We propose that the SEX1 protein (R1) functions as an overall regulator of starch mobilization by controlling the phosphate content of starch.

Genome sequence and analysis of the tuber crop potato.

Abstract: Potato (Solanum tuberosum L.) is the world's most important non-grain food crop and is central to global food security. It is clonally propagated, highly heterozygous, autotetraploid, and suffers acute inbreeding depression. Here we use a homozygous doubled-monoploid potato clone to sequence and assemble 86% of the 844-megabase genome. We predict 39,031 protein-coding genes and present evidence for at least two genome duplication events indicative of a palaeopolyploid origin. As the first genome sequence of an asterid, the potato genome reveals 2,642 genes specific to this large angiosperm clade. We also sequenced a heterozygous diploid clone and show that gene presence/absence variants and other potentially deleterious mutations occur frequently and are a likely cause of inbreeding depression. Gene family expansion, tissue-specific expression and recruitment of genes to new pathways contributed to the evolution of tuber development. The potato genome sequence provides a platform for genetic improvement of this vital crop.

Carbohydrate-modulated gene expression in plants.

Abstract: Plant gene responses to changing carbohydrate status can vary markedly Some genes are induced, some are repressed, and others are minimally affected As in microorganisms, sugar-sensitive plant genes are part of an ancient system of cellular adjustment to critical nutrient availability However, in multicellular plants, sugar-regulated expression also provides a mechanism for control of resource distribution among tissues and organs Carbohydrate depletion upregulates genes for photosynthesis, remobilization, and export, while decreasing mRNAs for storage and utilization Abundant sugar levels exert opposite effects through a combination of gene repression and induction Long-term changes in metabolic activity, resource partitioning, and plant form result Sensitivity of carbohydrate-responsive gene expression to environmental and developmental signals further enhances its potential to aid acclimation The review addresses the above from molecular to whole-plant levels and considers emerging models for sensing and transducing carbohydrate signals to responsive genes

Drought, salt, and temperature stress-induced metabolic rearrangements and regulatory networks

Abstract: Plants regularly face adverse growth conditions, such as drought, salinity, chilling, freezing, and high temperatures. These stresses can delay growth and development, reduce productivity, and, in extreme cases, cause plant death. Plant stress responses are dynamic and involve complex cross-talk between different regulatory levels, including adjustment of metabolism and gene expression for physiological and morphological adaptation. In this review, information about metabolic regulation in response to drought, extreme temperature, and salinity stress is summarized and the signalling events involved in mediating stress-induced metabolic changes are presented.

Orchestrated Transcription of Key Pathways in Arabidopsis by the Circadian Clock

Abstract: Like most organisms, plants have endogenous biological clocks that coordinate internal events with the external environment. We used high-density oligonucleotide microarrays to examine gene expression in Arabidopsis and found that 6% of the more than 8000 genes on the array exhibited circadian changes in steady-state messenger RNA levels. Clusters of circadian-regulated genes were found in pathways involved in plant responses to light and other key metabolic pathways. Computational analysis of cycling genes allowed the identification of a highly conserved promoter motif that we found to be required for circadian control of gene expression. Our study presents a comprehensive view of the temporal compartmentalization of physiological pathways by the circadian clock in a eukaryote.