Metabolic engineering of plants for the synthesis of polyhydroxyalkanoates

Synthesis of polyhydroxyalkanoates (PHAs) in crop is viewed as an attractive approach for the production of this family of biodegradable plastics in large quantities and at low costs. Synthesisof PHAs containing various monomers has so far been demonstrated in the cytosol, plastids, and peroxisomes of plants. Several biochemical pathways have been modifies to achieve this, including the isoprenois pathway, the fatty acid biosynthetic pathway, and the fatty aci

The high-carb economy

The recent attempt by a group of suicide bombers to drive a car bomb into one of Saudi Arabia's biggest oil refineries sent a shock wave through the industry, and crude oil prices on the world market shot up, even though the terrorists failed. In Blaire, Nebraska, Cargill (an international marketer, processor and distributor of agricultural, food, financial and industrial products) has an industrial plant that takes maize, converts it to sugars that feed microbes in fermentation vats, and each day pumps out 136 tonnes of polylactic acid (PLA)

AraGEM, a Genome-scale reconstruction of the primary metabolic network in arabidopsis

Genome-scale metabolic network models have been successfully used to describe metabolism in a variety of microbial organisms as well as specific mammalian cell types and organelles. This systems-based framework enables the exploration of global phenotypic effects of gene knockouts, gene insertion, and up-regulation of gene expression. We have developed a genome-scale metabolic network model (AraGEM) covering primary metabolism for a compartmentalized plant cell based on the Arabidopsis (Arabidopsis thaliana) genome. AraGEM is a comprehensive literature-based, genome-scale metabolic reconstruction that accounts for the functions of 1,419 unique open reading frames, 1,748 metabolites, 5,253 gene-enzyme reaction-association entries, and 1,567 unique reactions compartmentalized into the cytoplasm, mitochondrion, plastid, peroxisome, and vacuole. The curation process identified 75 essential reactions with respective enzyme associations not assigned to any particular gene in the Kyoto Encyclopedia of Genes and Genomes or AraCyc. With the addition of these reactions, AraGEM describes a functional primary metabolism of Arabidopsis. The reconstructed network was transformed into an in silico metabolic flux model of plant metabolism and validated through the simulation of plant metabolic functions inferred from the literature. Using efficient resource utilization as the optimality criterion, AraGEM predicted the classical photorespiratory cycle as well as known key differences between redox metabolism in photosynthetic and nonphotosynthetic plant cells. AraGEM is a viable framework for in silico functional analysis and can be used to derive new, nontrivial hypotheses for exploring plant metabolism

Genetic uniformity of international isolates of Leifsonia xyli subsp. xyli, causal agent of ratoon stunting disease of sugarcane

An international collection of the sugarcane ratoon stunting disease pathogen, Leifsonia xyli subsp. xyli, was analysed to assess genetic diversity. DNA fingerprinting using BOX primers was performed on 105 isolates, comprising 65 Australian isolates and an additional 40 isolates from Indonesia (n=8), Japan (n=1), USA (n=3), Brazil (n=2), Mali (n=2), Zimbabwe (n=13), South Africa (n=9) and Réunion (n=2). Sixty-two of these isolates were also screened using ERIC primers. No variation was found among any of the isolates. The intergenic spacer (IGS) region of the ribosomal RNA genes from 54 isolates was screened for sequence variation using singlestranded conformational polymorphism (SSCP), but none was observed. Direct sequencing of the IGS from a subset of nine isolates, representing all of the countries sampled in this study, confirmed the results of the SSCP analysis. Likewise, no sequence variation was found in the 16S ribosomal RNA genes of the same subset. Four Colombian isolates from sugarcane, morphologically similar to L. xyli subsp. xyli, were putatively shown to be an undescribed Agrococcus species of unknown pathogenicity. The lack of genetic variation among L. xyli subsp. xyli isolates, independent of time of sampling, cultivar of isolation, or country of origin, suggests the worldwide spread of a single pathogenic clone, and further suggests that sugarcane cultivars resistant to ratoon stunting disease in one area should retain this property in other regions

Reduced peroxisomal citrate synthase activity increases substrate availability for polyhydroxyalkanoate biosynthesis in plant peroxisomes.

Polyhydroxyalkanoates (PHAs) are bacterial carbon storage polymers used as renewable, biodegradable plastics. PHA production in plants may be a way to reduce industrial PHA production costs. We recently demonstrated a promising level of peroxisomal PHA production in the high biomass crop species sugarcane. However, further production strategies are needed to boost PHA accumulation closer to commercial targets. Through exogenous fatty acid feeding of Arabidopsis thaliana plants that contain peroxisome-targeted PhaA, PhaB and PhaC enzymes from Cupriavidus necator, we show here that the availability of substrates derived from the β-oxidation cycle limits peroxisomal polyhydroxybutyrate (PHB) biosynthesis. Knockdown of peroxisomal citrate synthase activity using artificial microRNA increased PHB production levels approximately threefold. This work demonstrates that reduction of peroxisomal citrate synthase activity may be a valid metabolic engineering strategy for increasing PHA production in other plant species