Engineering the plant cell factory for secondary metabolite production

Plant secondary metabolism is very important for traits such as flower color, flavor of food, and resistance against pests and diseases. Moreover, it is the source of many fine chemicals such as drugs, dyes, flavors, and fragrances. It is thus of interest to be able to engineer the secondary metabolite production of the plant cell factory, e.g. to produce more of a fine chemical, to produce less of a toxic compound, or even to make new compounds, Engineering of plant secondary metabolism is feasible nowadays, but it requires knowledge of the biosynthetic pathways involved. To increase secondary metabolite production different strategies can be followed, such as overcoming rate limiting steps, reducing flux through competitive pathways, reducing catabolism and overexpression of regulatory genes. For this purpose genes of plant origin can be overexpressed, but also microbial genes have been used successfully. Overexpression of plant genes in microorganisms is another approach, which might be of interest for bioconversion of readily available precursors into valuable fine chemicals. Several examples will be given to illustrate these various approaches. The constraints of metabolic engineering of the plant cell factory will also be discussed. Our limited knowledge of secondary metabolite pathways and the genes involved is one of the main bottlenecks. This revised version was published online in August 2006 with corrections to the Cover Date.     

Chemically modified, immobilized trypsin reactor with improved digestion efficiency

Tryptic digestion followed by identification using mass spectrometry is an important step in many proteomic studies. Here, we describe the preparation of immobilized, acetylated trypsin for enhanced digestion efficacy in integrated protein analysis platforms. Complete digestion of cytochrome c was obtained with two types of modified-trypsin beads with a contact time of only 4 s, while corresponding unmodified-trypsin beads gave only incomplete digestion. The digestion rate of myoglobin, a protein known to be rather resistant to proteolysis, was not altered by acetylating trypsin and required a buffer containing 35% acetonitrile to obtain complete digestion. The use of acetylated-trypsin beads led to fewer interfering tryptic autolysis products, indicating an increased stability of this modified enzyme. Importantly, the modification did not affect trypsin's substrate specificity, as the peptide map of myoglobin was not altered upon acetylation of immobilized trypsin. Kinetic digestion experiments in solution with low-molecular-weight substrates and cytochrome c confirmed the increased catalytic efficiency (lower K(M) and higher k(cat)) and increased resistance to autolysis of trypsin upon acetylation. Enhancement of catalytic efficiency was correlated with the number of acetylations per molecule. The favorable properties of the new chemically modified trypsin reactor should make it a valuable tool in automated protein analysis systems.     

Sequential solubilization of proteins precipitated with trichloroacetic acid in acetone from cultured Catharanthus roseus cells yields 52% more spots after two-dimensional electrophoresis

Sample preparation is still the most critical step in two-dimensional gel electrophoresis (2-DE), and needs to be optimized for each type of sample. To analyze the proteome of the medicinal plant Catharanthus roseus, we developed and evaluated a sequential solubilization procedure for the solubilization of proteins after precipitation in trichloroacetic acid and acetone. The procedure includes solubilization with a conventional urea buffer followed by a stronger solubilizing buffer containing thiourea. The sequential solubilization of the precipitated proteins results in very different spot patterns following 2-DE. The number of protein spots which could be detected in both samples of the sequential solubilization was only about 10% of the total number of spots. Compared to solubilization in a single step, the total number of spots that could be detected in the sequential solubilization procedure was increased by 52%. The method described is simple and is applicable to different types of plant tissue.     

Chalcone synthase and its functions in plant resistance

Chalcone synthase (CHS, EC 2.3.1.74) is a key enzyme of the flavonoid/isoflavonoid biosynthesis pathway. Besides being part of the plant developmental program the CHS gene expression is induced in plants under stress conditions such as UV light, bacterial or fungal infection. CHS expression causes accumulation of flavonoid and isoflavonoid phytoalexins and is involved in the salicylic acid defense pathway. This review will discuss CHS and its function in plant resistance.     

NMR-based plant metabolomics: where do we stand, where do we go?

NMR-based metabolomics is an important tool for studying biological systems and has been applied in various organisms, including animals, plants and microbes. NMR is able to provide a 'holistic view' of the metabolites under certain conditions, and thus is advantageous for metabolomic studies. To maximize the use of the information obtained, it is also important to create a platform to measure, store and share data. Public databases for storing and sharing information are still lacking for NMR-based metabolomic analysis in plants. Such databases are urgently needed to make metabolic profiling a real omics technology. In addition, to understand metabolic processes in depth, single-cell analysis and the turnover of metabolites in pathways (fluxomics) should be measured.     

Improving the in vivo predictability of an on-line HPLC stable free radical decoloration assay for antioxidant activity in methanol-buffer medium

The pivotal role of ROS (reactive oxygen species) in various (patho)physiological processes has stimulated research on the potential of intervening in these processes with antioxidants (AO). In vitro model systems to investigate AO activity against the various ROS are a valuable tool in classifying antioxidants. To improve the in vivo predictability of the results obtained, we have modified and characterized the widely used DPPH (2,2'-diphenyl-1-picrylhydrazyl) on-line decoloration assay. Previous investigations using the DPPH reaction in a pure methanolic medium exhibit slow kinetics and a reaction going to completion. In this study, a medium which includes an aqueous buffer at physiological pH has been applied, resulting in the rapid establishment of equilibrium. The results obtained in an aqueous medium at physiological pH are expected to be more relevant for extrapolation to in vivo circumstances than previously published findings. The antioxidants investigated are classified according to the results obtained and the relevance of their behavior to in vivo situations is discussed. Special emphasis is put on the significance of the results for prediction of redox-cycling characteristics and structure-activity relationships.