Polyhydroxyalknoate synthesis in plants as a tool for biotechnology and basic studies of lipid metabolism

PHA is not just a bioplastic!

Polyhydroxyalkanoates (PHA) have evolved into versatile biopolymers, transcending their origins as mere bioplastics. This extensive review delves into the multifaceted landscape of PHA applications, shedding light on the diverse industries that have harnessed their potential. PHA has proven to be an invaluable eco-conscious option for packaging materials, finding use in films foams, paper coatings and even straws. In the textile industry, PHA offers a sustainable alternative, while its application as a carbon source for denitrification in wastewater treatment showcases its versatility in environmental remediation. In addition, PHA has made notable contributions to the medical and consumer sectors, with various roles ranging from 3D printing, tissue engineering implants, and cell growth matrices to drug delivery carriers, and cosmetic products. Through metabolic engineering efforts, PHA can be fine-tuned to align with the specific requirements of each industry, enabling the customization of material properties such as ductility, elasticity, thermal conductivity, and transparency. To unleash PHA's full potential, bridging the gap between research and commercial viability is paramount. Successful PHA production scale-up hinges on establishing direct supply chains to specific application domains, including packaging, food and beverage materials, medical devices, and agriculture. This review underscores that PHA's future rests on ongoing exploration across these industries and more, paving the way for PHA to supplant conventional plastics and foster a circular economy.

Leads and hurdles to sustainable microbial bioplastic production

Indiscriminate usage, disposal and recalcitrance of petroleum-based plastics have led to its accumulation leaving a negative impact on the environment. Bioplastics, particularly microbial bioplastics serve as an ecologically sustainable solution to nullify the negative impacts of plastics. Microbial production of biopolymers like Polyhydroxyalkanoates, Polyhydroxybutyrates and Polylactic acid using renewable feedstocks as well as industrial wastes have gained momentum in the recent years. The current study outlays types of bioplastics, their microbial sources and applications in various fields. Scientific evidence on bioplastics has suggested a unique range of applications such as industrial, agricultural and medical applications. Though diverse microorganisms such as Alcaligenes latus, Burkholderia sacchari, Micrococcus species, Lactobacillus pentosus, Bacillus sp., Pseudomonas sp., Klebsiella sp., Rhizobium sp., Enterobacter sp., Escherichia sp., Azototobacter sp., Protomonas sp., Cupriavidus sp., Halomonas sp., Saccharomyces sp., Kluyveromyces sp., and Ralstonia sp. are known to produce bioplastics, the industrial production of bioplastics is still challenging. Thus this paper also provides deep insights on the advancements made to maximise production of bioplastics using different approaches such as metabolic engineering, rDNA technologies and multitude of cultivation strategies. Finally, the constraints to microbial bioplastic production and the future directions of research are briefed. Hence the present review emphasizes on the importance of using bioplastics as a sustainable alternative to petroleum based plastic products to diminish environmental pollution.

Sustainable applications of polyhydroxyalkanoates in various fields: A critical review

PHA is one of the most promising candidates in bio-polymer family which is biodegradable and environment-friendly in nature. In recent years, it has been applied as a biodegradable alternative for petroleum-based plastic across different domains. In literature, several research groups have scrutinised the biocompatibility and biodegradability of PHA in both in vivo settings as well as in in vitro conditions. Microbial yield polyhydroxyalkanoates (PHAs) are promoted at present as biodegradable plastics. On the other hand, only a limited number of products is being commercially manufactured out of PHAs (e.g., bottles). A succession of microbes (prokaryotes in addition to eukaryotes) has been identified as potential candidates that can disintegrate PHAs. These materials have been successfully employed in packaging industry, medical devices and implants, moulded goods, paper coatings, adhesives, performance additives, mulch films, non-woven fabrics, etc. The present paper reviews and focuses on the potential applications of PHA and its derivatives in different industries.

Sustainable Production of the Cyanophycin Biopolymer in Tobacco in the Greenhouse and Field

The production of biodegradable polymers as coproducts of other commercially relevant plant components can be a sustainable strategy to decrease the carbon footprint and increase the commercial value of a plant. The biodegradable polymer cyanophycin granular polypeptide (CGP) was expressed in the leaves of a commercial tobacco variety, whose seeds can serve as a source for biofuel and feed. In T0 generation in the greenhouse, up to 11% of the leaf dry weight corresponded to the CGP. In T1 generation, the maximum content decreased to approximately 4% dw, both in the greenhouse and first field trial. In the field, a maximum harvest of 4 g CGP/plant could be obtained. Independent of the CGP content, most transgenic plants exhibited a slight yield penalty in the leaf biomass, especially under stress conditions in greenhouse and field trials. After the harvest, the leaves were either Sun dried or ensiled. The resulting material was used to evaluate the extraction of CGP compared to that in the laboratory protocol. The farm-level analysis indicates that the extraction of CGP from tobacco plants can provide alternative income opportunities for tobacco farmers. The CGP yield/ha indicates that the CGP production in plants can be economically feasible depending on the cultivation and extraction costs. Moreover, we analyzed the consumer acceptance of potential applications associated with GM tobacco in four European countries (Germany, Finland, Italy and the Netherlands) and found unexpectedly high acceptance.

Photoautotrophic and Mixotrophic Cultivation of Polyhydroxyalkanoate-Accumulating Microalgae Consortia Selected under Nitrogen and Phosphate Limitation

Microalgae consortia were photoautotrophically cultivated in sequencing batch photobioreactors (SBPRs) with an alteration of the normal growth and starvation (nutrient limitation) phases to select consortia capable of polyhydroxyalkanoate (PHA) accumulation. At the steady state of SBPR operation, the obtained microalgae consortia, selected under nitrogen and phosphate limitation, accumulated up to 11.38% and 10.24% of PHA in their biomass, which was identified as poly(3-hydroxybutyrate) (P3HB). Photoautotrophic and mixotrophic batch cultivation of the selected microalgae consortia was conducted to investigate the potential of biomass and PHA production. Sugar source supplementation enhanced the biomass and PHA production, with the highest PHA contents of 10.94 and 6.2%, and cumulative PHA productions of 100 and 130 mg/L, with this being achieved with sugarcane juice under nitrogen and phosphate limitation, respectively. The analysis of other macromolecules during batch cultivation indicated a high content of carbohydrates and lipids under nitrogen limitation, while higher protein contents were detected under phosphate limitation. These results recommended the selected microalgae consortia as potential tools for PHA and bioresource production. The mixed-culture non-sterile cultivation system developed in this study provides valuable information for large-scale microalgal PHA production process development following the biorefinery concept.

Process optimization, metabolic engineering interventions and commercialization of microbial polyhydroxyalkanoates production - A state-of-the art review

Microbial polyhydroxyalkanoates (PHAs) produced using renewable resources could be the best alternative for conventional plastics. Despite their incredible potential, commercial production of PHAs remains very low. Nevertheless, sincere attempts have been made by researchers to improve the yield and economic viability of PHA production by utilizing low-cost agricultural or industrial wastes. In this context, the use of efficient microbial culture or consortia, adoption of experimental design to trace ideal growth conditions, nutritional requirements, and intervention of metabolic engineering tools have gained significant attention. This review has been structured to highlight the important microbial sources for PHA production, use of conventional and non-conventional substrates, product optimization using experimental design, metabolic engineering strategies, and global players in the commercialization of PHA in the past two decades. The challenges about PHA recovery and analysis have also been discussed which possess indirect hurdle while expanding the horizon of PHA-based bioplastics. Selection of appropriate microorganism and substrate plays a vital role in improving the productivity and characteristics of PHAs. Experimental design-based bioprocess, use of metabolic engineering tools, and optimal product recovery techniques are invaluable in this dimension. Optimization strategies, which are being explored in isolation, need to be logically integrated for the successful commercialization of microbial PHAs.

Designing Novel Interfaces via Surface Functionalization of Short-Chain-Length Polyhydroxyalkanoates

Polyhydroxyalkanoates (PHA), a microbial plastic has emerged as promising biomaterial owing to the broad range of mechanical properties. However, some studies revealed that PHA is hydrophobic and has no recognition site for cell attachment and this is often a limitation in tissue engineering aspects. Owing to this, the polymer is tailored accordingly in order to enhance the biocompatibilityin vivoas well as to suit the intended application. Thus far, these surface modifications have led to PHA being widely used in various biomedical and pharmaceutical applications such as cardiac patches, wound management, nerve, bone, and cartilage repair. This review addresses the surface modification on biomedical applications focusing on short-chain-length PHA such as poly(3-hydroxybutyrate) [P(3HB)], poly(3-hydroxybutyrate-co-4-hydroxybutyrate) [P(3HB-co-4HB)] and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)].

Effect of mcl-PHA synthesis in flax on plant mechanical properties and cell wall composition

The high demand for new biomaterials makes synthesis of polyhydroxyalkanoates (PHA) in plants an interesting and desirable achievement. Production of polymers in plants is an example of application of biotechnology for improving the properties of plants, e.g. industrial properties, but it can also provide knowledge about plant physiology and metabolism. The subject of the present study was an industrially important plant: flax, Linum usitatissimum L., of a fibre cultivar (cv Nike). In the study the gene encoding PHA synthase from Pseudomonas aeruginosa, fused to a peroxisomal targeting signal, was expressed in flax plants with the aim of modifying the mechanical properties of plants. Medium-chain-length (mcl) hydroxy acids in flax plants from tissue cultures were detected by GC-FID and FTIR method. The introduced changes did not affect fatty acid content and composition in generated flax plants. Since mcl-PHA are known as elastomers, the mechanical properties of created plants were examined. Modified plants showed increases in the values of all measured parameters (except strain at break evaluated for one modified line). The largest increase was noted for tensile stiffness, which was 2- to 3-fold higher than in wild-type plants. The values estimated for another parameter, Young's modulus, was almost at the same level in generated flax plants, and they were about 2.7-fold higher when compared to unmodified plants. The created plants also exhibited up to about 2.4-fold higher tensile strength. The observed changes were accompanied by alterations in the expression of selected genes, related to cell wall metabolism in line with the highest expression of phaC1 gene. Biochemical data were confirmed by spectroscopic methods, which also revealed that crystallinity index values of cellulose in modified flax plants were increased in comparison to wild-type flax plants and correlated with biomechanical properties of plants.

Production of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) under photoautotrophy and heterotrophy by non-heterocystous N2-fixing cyanobacterium

The photoautotrophically grown cyanobacterium Oscillatoria okeni TISTR 8549 was found to produce bioplastic poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). This PHBV production occurred under nitrogen deprivation (-N) that yielded PHBV accumulation of 14±4% (w/w DW) in which 3-hydroxyvalerate accounted for 5.5mol%. The heterotrophically grown (-N condition with acetate supplementation) cells under light showed no increase of PHBV storage, but under dark condition these cells increased PHBV accumulation to 42±8% (w/w DW) with 6.5mol% of 3-hydroxyvalerate. Compared to poly-3-hydroxybutyrate (PHB), the PHBV from O. okeni had a lower melting temperature by 5-7°C, a higher % elongation at break by 4-7times and a greater Young's elastic modulus by 2.3-2.5times.

Chlamydomonas carries out fatty acid β-oxidation in ancestral peroxisomes using a bona fide acyl-CoA oxidase

Peroxisomes are thought to have played a key role in the evolution of metabolic networks of photosynthetic organisms by connecting oxidative and biosynthetic routes operating in different compartments. While the various oxidative pathways operating in the peroxisomes of higher plants are fairly well characterized, the reactions present in the primitive peroxisomes (microbodies) of algae are poorly understood. Screening of a Chlamydomonas insertional mutant library identified a strain strongly impaired in oil remobilization and defective in Cre05.g232002 (CrACX2), a gene encoding a member of the acyl-CoA oxidase/dehydrogenase superfamily. The purified recombinant CrACX2 expressed in Escherichia coli catalyzed the oxidation of fatty acyl-CoAs into trans-2-enoyl-CoA and produced H₂ O₂ . This result demonstrated that CrACX2 is a genuine acyl-CoA oxidase, which is responsible for the first step of the peroxisomal fatty acid (FA) β-oxidation spiral. A fluorescent protein-tagging study pointed to a peroxisomal location of CrACX2. The importance of peroxisomal FA β-oxidation in algal physiology was shown by the impact of the mutation on FA turnover during day/night cycles. Moreover, under nitrogen depletion the mutant accumulated 20% more oil than the wild type, illustrating the potential of β-oxidation mutants for algal biotechnology. This study provides experimental evidence that a plant-type FA β-oxidation involving H₂ O₂ -producing acyl-CoA oxidation activity has already evolved in the microbodies of the unicellular green alga Chlamydomonas reinhardtii.

Evaluation of short-chain-length polyhydroxyalkanoate accumulation in Bacillus aryabhattai

This study was focused on the polyhydroxybutyrate (PHB) accumulation property of Bacillus aryabhattai isolated from environment. Twenty-four polyhydroxyalkanoate (PHA) producers were screened out from sixty-two environmental bacterial isolates based on Sudan Black B colony staining. Based on their PHA accumulation property, six promising isolates were further screened out. The most productive isolate PHB10 was identified as B. aryabhattai PHB10. The polymer production maxima were 3.264 g/L, 2.181 g/L, 1.47 g/L, 1.742 g/L and 1.786 g/L in glucose, fructose, maltose, starch and glycerol respectively. The bacterial culture reached its stationary and declining phases at 18 h and 21 h respectively and indicated growth-associated PHB production. Nuclear Magnetic Resonance (NMR) spectra confirmed the material as PHB. The material has thermal stability between 30 and 140 °C, melting point at 170 °C and maximum thermal degradation at 287 °C. The molecular weight and poly dispersion index of the polymer were found as 199.7 kDa and 2.67 respectively. The bacterium B. aryabhattai accumulating PHB up to 75% of cell dry mass utilizing various carbon sources is a potential candidate for large scale production of bacterial polyhydroxybutyrate.

Growth-related Metabolism of the Carbon Storage Poly-3-hydroxybutyrate in Legionella pneumophila

Legionella pneumophila, the causative agent of Legionnaires disease, has a biphasic life cycle with a switch from a replicative to a transmissive phenotype. During the replicative phase, the bacteria grow within host cells in Legionella-containing vacuoles. During the transmissive phenotype and the postexponential (PE) growth phase, the pathogens express virulence factors, become flagellated, and leave the Legionella-containing vacuoles. Using (13)C labeling experiments, we now show that, under in vitro conditions, serine is mainly metabolized during the replicative phase for the biosynthesis of some amino acids and for energy generation. During the PE phase, these carbon fluxes are reduced, and glucose also serves as an additional carbon substrate to feed the biosynthesis of poly-3-hydroxybuyrate (PHB), an essential carbon source for transmissive L. pneumophila. Whole-cell FTIR analysis and comparative isotopologue profiling further reveal that a putative 3-ketothiolase (Lpp1788) and a PHB polymerase (Lpp0650), but not enzymes of the crotonyl-CoA pathway (Lpp0931-0933) are involved in PHB metabolism during the PE phase. However, the data also reflect that additional bypassing reactions for PHB synthesis exist in agreement with in vivo competition assays using Acanthamoeba castellannii or human macrophage-like U937 cells as host cells. The data suggest that substrate usage and PHB metabolism are coordinated during the life cycle of the pathogen.

Production of polyhydroxybutyrate in oil palm (Elaeis guineensis Jacq.) mediated by microprojectile bombardment of PHB biosynthesis genes into embryogenic calli

Biodegradable plastics, mainly polyhydroxybutyrate (PHB), which are traditionally produced by bacterial cells, have been produced in the cells of more than 15 plant species. Since the production of biodegradable plastics and the synthesis of oil in plants share the same substrate, acetyl-coenzyme A (acetyl-CoA), producing PHB in oil bearing crops, such as oil palm, will be advantageous. In this study, three bacterial genes, bktB, phaB, and phaC, which are required for the synthesis of PHB and selectable marker gene, bar, for herbicide Basta resistant, were transformed into embryogenic calli. A number of transformed embryogenic lines resistant to herbicide Basta were obtained and were later regenerated to produce few hundred plantlets. Molecular analyses, including polymerase chain reaction (PCR), Southern blot, and real-time PCR have demonstrated stable integration and expression of the transgenes in the oil palm genome. HPLC and Nile blue A staining analyses confirmed the synthesis of PHB in some of the plantlets.

Optimization of enzymatic ring-opening copolymerizations involving δ-gluconolactone as monomer by experimental design

Enzymatic incorporation of carbohydrate-derived monomer units into hydrophobic polyester backbones represents a promising alternative to obtain new biodegradable oligomers and polymers. Immobilized lipases are efficient biocatalysts for copolymerization of β-butyrolactone and δ-gluconolactone, but only a systematic optimization study was able to highlight the influence of the main reaction parameters on the polymerization degree and on the relative copolymer content of the product. Therefore, experimental design was employed for determination of the optimal ring-opening copolymerization conditions in solventless reaction systems, at temperatures up to 80 °C. The obtained products, cyclic and linear polyesters, have been characterized by FT-IR, MALDI-TOF MS, NMR, and TG analysis, demonstrating the incorporation of gluconolactone unit(s) into the hydrophobic backbone of the polyester and the formation of new bio-based products.

Microbial degradation of linseed oil-based elastomer and subsequent accumulation of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer

The microbial synthesis of environment-friendly poly(3-hydroxybutyrate--co-3-hydroxyvalerate), PHBV, has been performed by using an alkaliphilic microorganism, Alkaliphilus oremlandii OhILAs strain (GenBank Accession number NR_043674.1), at pH 8 and at a temperature of 30-32 °C through the biodegradation of linseed oil-based elastomer. The yield of the copolymer on dry cell weight basis is 90 %. The elastomers used for the biodegradation have been synthesized by cationic polymerization technique. The yield of the PHBV copolymer also varies with the variation of linseed oil content (30-60 %) in the elastomer. Spectroscopic characterization ((1)H NMR and FTIR) of the accumulated product through biodegradation of linseed oil-based elastomers indicates that the accumulated product is a PHBV copolymer consisting of 13.85 mol% of 3-hydroxyvalerate unit. The differential scanning calorimetry (DSC) results indicate a decrease in the melting (T m) and glass transition temperature (T g) of PHBV copolymer with an increase in the content of linseed oil in the elastomer, which is used for the biodegradation. The gel permeation chromatography (GPC) results indicate that the weight average molecular weight (M w) of PHBV copolymer decreases with an increasing concentration of linseed oil in the elastomer. The surface morphology of the elastomer before and after biodegradation is observed under scanning electron microscope (SEM) and atomic force microscope (AFM); these results indicate about porous morphology of the biodegraded elastomer.

ATP citrate lyase activity is post-translationally regulated by sink strength and impacts the wax, cutin and rubber biosynthetic pathways

Cytosolic acetyl-CoA is involved in the synthesis of a variety of compounds, including waxes, sterols and rubber, and is generated by the ATP citrate lyase (ACL). Plants over-expressing ACL were generated in an effort to understand the contribution of ACL activity to the carbon flux of acetyl-CoA to metabolic pathways occurring in the cytosol. Transgenic Arabidopsis plants synthesizing the polyester polyhydroxybutyrate (PHB) from cytosolic acetyl-CoA have reduced growth and wax content, consistent with a reduction in the availability of cytosolic acetyl-CoA to endogenous pathways. Increasing the ACL activity via the over-expression of the ACLA and ACLB subunits reversed the phenotypes associated with PHB synthesis while maintaining polymer synthesis. PHB production by itself was associated with an increase in ACL activity that occurred in the absence of changes in steady-state mRNA or protein level, indicating a post-translational regulation of ACL activity in response to sink strength. Over-expression of ACL in Arabidopsis was associated with a 30% increase in wax on stems, while over-expression of a chimeric homomeric ACL in the laticifer of roots of dandelion led to a four- and two-fold increase in rubber and triterpene content, respectively. Synthesis of PHB and over-expression of ACL also changed the amount of the cutin monomer octadecadien-1,18-dioic acid, revealing an unsuspected link between cytosolic acetyl-CoA and cutin biosynthesis. Together, these results reveal the complexity of ACL regulation and its central role in influencing the carbon flux to metabolic pathways using cytosolic acetyl-CoA, including wax and polyisoprenoids.

Recent advances in high performance poly(lactide): from "green" plasticization to super-tough materials via (reactive) compounding

Due to its origin from renewable resources, its biodegradability, and recently, its industrial implementation at low costs, poly(lactide) (PLA) is considered as one of the most promising ecological, bio-sourced and biodegradable plastic materials to potentially and increasingly replace traditional petroleum derived polymers in many commodity and engineering applications. Beside its relatively high rigidity [high tensile strength and modulus compared with many common thermoplastics such as poly(ethylene terephthalate) (PET), high impact poly(styrene) (HIPS) and poly(propylene) (PP)], PLA suffers from an inherent brittleness, which can limit its applications especially where mechanical toughness such as plastic deformation at high impact rates or elongation is required. Therefore, the curve plotting stiffness vs. impact resistance and ductility must be shifted to higher values for PLA-based materials, while being preferably fully bio-based and biodegradable upon the application. This review aims to establish a state of the art focused on the recent progresses and preferably economically viable strategies developed in the literature for significantly improve the mechanical performances of PLA. A particular attention is given to plasticization as well as to impact resistance modification of PLA in the case of (reactive) blending PLA-based systems.

An engineered plant peroxisome and its application in biotechnology

Plant metabolic engineering is a promising tool for biotechnological applications. Major goals include enhancing plant fitness for an increased product yield and improving or introducing novel pathways to synthesize industrially relevant products. Plant peroxisomes are favorable targets for metabolic engineering, because they are involved in diverse functions, including primary and secondary metabolism, development, abiotic stress response, and pathogen defense. This review discusses targets for manipulating endogenous peroxisomal pathways, such as fatty acid β-oxidation, or introducing novel pathways, such as the synthesis of biodegradable polymers. Furthermore, strategies to bypass peroxisomal pathways for improved energy efficiency and detoxification of environmental pollutants are discussed. In sum, we highlight the biotechnological potential of plant peroxisomes and indicate future perspectives to exploit peroxisomes as biofactories.

PHA bioplastics, biochemicals, and energy from crops

Large scale production of polyhydroxyalkanoates (PHAs) in plants can provide a sustainable supply of bioplastics, biochemicals, and energy from sunlight and atmospheric CO(2). PHAs are a class of polymers with various chain lengths that are naturally produced by some microorganisms as storage materials. The properties of these polyesters make them functionally equivalent to many of the petroleum-based plastics that are currently in the market place. However, unlike most petroleum-derived plastics, PHAs can be produced from renewable feedstocks and easily degrade in most biologically active environments. This review highlights research efforts over the last 20 years to engineer the production of PHAs in plants with a focus on polyhydroxybutryrate (PHB) production in bioenergy crops with C(4) photosynthesis. PHB has the potential to be a high volume commercial product with uses not only in the plastics and materials markets, but also in renewable chemicals and feed. The major challenges of improving product yield and plant fitness in high biomass yielding C(4) crops are discussed in detail.

Enhanced polyhydroxybutyrate production in transgenic sugarcane

Polyhydroxybutyrate (PHB) is a bacterial polyester that has properties similar to some petrochemically produced plastics. Plant-based production has the potential to make this biorenewable plastic highly competitive with petrochemical-based plastics. We previously reported that transgenic sugarcane produced PHB at levels as high as 1.8% leaf dry weight without penalty to biomass accumulation, suggesting scope for improving PHB production in this species. In this study, we used different plant and viral promoters, in combination with multigene or single-gene constructs to increase PHB levels. Promoters tested included the maize and rice polyubiquitin promoters, the maize chlorophyll A/B-binding protein promoter and a Cavendish banana streak badnavirus promoter. At the seedling stage, the highest levels of polymer were produced in sugarcane plants when the Cavendish banana streak badnavirus promoter was used. However, in all cases, this promoter underwent silencing as the plants matured. The rice Ubi promoter enabled the production of PHB at levels similar to the maize Ubi promoter. The maize chlorophyll A/B-binding protein promoter enabled the production of PHB to levels as high as 4.8% of the leaf dry weight, which is approximately 2.5 times higher than previously reported levels in sugarcane. This is the first time that this promoter has been tested in sugarcane. The highest PHB-producing lines showed phenotypic differences to the wild-type parent, including reduced biomass and slight chlorosis.

Remodeling the isoprenoid pathway in tobacco by expressing the cytoplasmic mevalonate pathway in chloroplasts

Metabolic engineering to enhance production of isoprenoid metabolites for industrial and medical purposes is an important goal. The substrate for isoprenoid synthesis in plants is produced by the mevalonate pathway (MEV) in the cytosol and by the 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway in plastids. A multi-gene approach was employed to insert the entire cytosolic MEV pathway into the tobacco chloroplast genome. Molecular analysis confirmed the site-specific insertion of seven transgenes and homoplasmy. Functionality was demonstrated by unimpeded growth on fosmidomycin, which specifically inhibits the MEP pathway. Transplastomic plants containing the MEV pathway genes accumulated higher levels of mevalonate, carotenoids, squalene, sterols, and triacyglycerols than control plants. This is the first time an entire eukaryotic pathway with six enzymes has been transplastomically expressed in plants. Thus, we have developed an important tool to redirect metabolic fluxes in the isoprenoid biosynthesis pathway and a viable multigene strategy for engineering metabolism in plants.

Peroxisomal polyhydroxyalkanoate biosynthesis is a promising strategy for bioplastic production in high biomass crops

Polyhydroxyalkanoates (PHAs) are bacterial carbon storage polymers with diverse plastic-like properties. PHA biosynthesis in transgenic plants is being developed as a way to reduce the cost and increase the sustainability of industrial PHA production. The homopolymer polyhydroxybutyrate (PHB) is the simplest form of these biodegradable polyesters. Plant peroxisomes contain the substrate molecules and necessary reducing power for PHB biosynthesis, but peroxisomal PHB production has not been explored in whole soil-grown transgenic plants to date. We generated transgenic sugarcane (Saccharum sp.) with the three-enzyme Ralstonia eutropha PHA biosynthetic pathway targeted to peroxisomes. We also introduced the pathway into Arabidopsis thaliana, as a model system for studying and manipulating peroxisomal PHB production. PHB, at levels up to 1.6%-1.8% dry weight, accumulated in sugarcane leaves and A. thaliana seedlings, respectively. In sugarcane, PHB accumulated throughout most leaf cell types in both peroxisomes and vacuoles. A small percentage of total polymer was also identified as the copolymer poly (3-hydroxybutyrate-co-3-hydroxyvalerate) in both plant species. No obvious deleterious effect was observed on plant growth because of peroxisomal PHA biosynthesis at these levels. This study highlights how using peroxisomal metabolism for PHA biosynthesis could significantly contribute to reaching commercial production levels of PHAs in crop plants.

Identification of gene disruptions for increased poly-3-hydroxybutyrate accumulation in Synechocystis PCC 6803

Inverse metabolic engineering (IME) is a combinatorial approach for identifying genotypes associated with a particular phenotype of interest. In this study, gene disruptions that increase the biosynthesis of poly-3-hydroxybutyrate (PHB) in the photosynthetic bacterium Synechocystis PCC6803 were identified. A Synechocystis mutant library was constructed by homologous recombination between the Synechocystis genome and a mutagenized genomic plasmid library generated through transposon insertion. Using a fluorescence-activated cell sorting-based high throughput screen, high PHB accumulating mutants from the library grown in different nutrient conditions were isolated and characterized. While several mutants isolated from the screen had increased PHB accumulation, transposon insertions in only two ORFs could be linked to increased PHB production. Disruptions of sll0461, coding for gamma-glutamyl phosphate reductase (proA), and sll0565, a hypothetical protein, resulted in increased accumulation in standard growth media and acetate supplemented media. These genetic perturbations have increased PHB accumulation in Synechocystis and serve as markers for engineering increased polymer production in higher photosynthetic organisms.

Biodegradable Polymers

Biodegradable materials are used in packaging, agriculture, medicine and other areas. In recent years there has been an increase in interest in biodegradable polymers. Two classes of biodegradable polymers can be distinguished: synthetic or natural polymers. There are polymers produced from feedstocks derived either from petroleum resources (non renewable resources) or from biological resources (renewable resources). In general natural polymers offer fewer advantages than synthetic polymers. The following review presents an overview of the different biodegradable polymers that are currently being used and their properties, as well as new developments in their synthesis and applications.

A transient assay to evaluate the expression of polyhydroxybutyrate genes regulated by oil palm mesocarp-specific promoter

The promoter of the oil palm metallothionein-like gene (MT3-A) demonstrated mesocarp-specific activity in functional analysis using transient expression assay of reporter gene in bombarded oil palm tissue slices. In order to investigate the tissue-specific expression of polyhydroxybutyrate (PHB) biosynthetic pathway genes, a multi-gene construct carrying PHB genes fused to the oil palm MT3-A promoter was co-transferred with a construct carrying GFP reporter gene using microprojectile bombardment targeting the mesocarp and leaf tissues of the oil palm. Transcriptional analysis using RT-PCR revealed successful transcription of all the three phbA, phbB, and phbC genes in transiently transformed mesocarp but not in transiently transformed leaf tissues. Furthermore, all the three expected sizes of PHB-encoded protein products were only detected in transiently transformed mesocarp tissues on a silver stained polyacrylamide gel. Western blot analysis using polyclonal antibody specific for phbB product confirmed successful translation of phbB mRNA transcript into protein product. This study provided valuable information, supporting the future engineering of PHB-producing transgenic palms.

Production of renewable polymers from crop plants

Plants produce a range of biopolymers for purposes such as maintenance of structural integrity, carbon storage, and defense against pathogens and desiccation. Several of these natural polymers are used by humans as food and materials, and increasingly as an energy carrier. In this review, we focus on plant biopolymers that are used as materials in bulk applications, such as plastics and elastomers, in the context of depleting resources and climate change, and consider technical and scientific bottlenecks in the production of novel or improved materials in transgenic or alternative crop plants. The biopolymers discussed are natural rubber and several polymers that are not naturally produced in plants, such as polyhydroxyalkanoates, fibrous proteins and poly-amino acids. In addition, monomers or precursors for the chemical synthesis of biopolymers, such as 4-hydroxybenzoate, itaconic acid, fructose and sorbitol, are discussed briefly.

Assessment of technological options and economical feasibility for cyanophycin biopolymer and high-value amino acid production

Major transitions can be expected within the next few decades aiming at the reduction of pollution and global warming and at energy saving measures. For these purposes, new sustainable biorefinery concepts will be needed that will replace the traditional mineral oil-based synthesis of specialty and bulk chemicals. An important group of these chemicals are those that comprise N-functionalities. Many plant components contained in biomass rest or waste stream fractions contain these N-functionalities in proteins and free amino acids that can be used as starting materials for the synthesis of biopolymers and chemicals. This paper describes the economic and technological feasibility for cyanophycin production by fermentation of the potato waste stream Protamylassetrade mark or directly in plants and its subsequent conversion to a number of N-containing bulk chemicals.

Beta-oxidation in fatty acid degradation and beyond

The degradation of fatty acids in plants occurs primarily in the peroxisomes through the beta-oxidation cycle. Enzymes that are involved in various aspects of beta-oxidation have been identified recently and shown to act biochemically on a diversity of fatty acids and derivatives. Analysis of several mutants has revealed essential roles for beta-oxidation in the breakdown of reserve triacylglycerols, seed development, seed germination and post-germinative growth before the establishment of photosynthesis. Beta-oxidation has also a considerable importance during the vegetative and reproductive growth phases, and plays a role in plant responses to stress, particularly in the synthesis of jasmonic acid.

Genetic and ultrastructural analysis of different mutants of Pseudomonas putida affected in the poly-3-hydroxy-n-alkanoate gene cluster

Functional analyses of the different proteins involved in the synthesis and accumulation of polyhydroxyalkanoates (PHAs) in P. putida U were performed using a mutant in which the pha locus had been deleted (PpUDeltapha). These studies showed that: (i) Pha enzymes cannot be replaced by other proteins in this bacterium, (ii) the transformation of PpDeltapha with a plasmid containing the locus pha fully restores the synthesis of bioplastics, (iii) the transformation of PpDeltapha with a plasmid harbouring the gene encoding the polymerase PhaC1 (pMCphaC1) permits the synthesis of polyesters (even in absence of phaC2ZDFI); however, in this strain (PpUDeltapha-pMCphaC1) the number of PHAs granules was higher than in the wild type, (iv) the expression of phaF in PpUDeltapha-pMCphaC1 restores the original phenotype, showing that PhaF is involved in the coalescence of the PHAs granules. Furthermore, the deletion of the phaDFI genes in P. putida U considerably decreases (> 70%) the biosynthesis of PHAs consisting of hydroxyalkanoates with aliphatic constituents, and completely prevents the synthesis of those ones containing aromatic monomers. Additional experiments revealed that the deletion of phaD in P. putida U strongly reduces the synthesis of PHA, this effect being restored by PhaF. Moreover, the overexpression of phaF in P. putida U, or in its DeltafadBA mutant, led to the collection of PHA over-producer strains.

Biotechnological approaches for the production of polyhydroxyalkanoates in microorganisms and plants - a review

The increasing effect of non-degradable plastic wastes is a growing concern. Polyhydroxyalkanoates (PHAs), macromolecule-polyesters naturally produced by many species of microorganisms, are being considered as a replacement for conventional plastics. Unlike petroleum-derived plastics that take several decades to degrade, PHAs can be completely bio-degraded within a year by a variety of microorganisms. This biodegradation results in carbon dioxide and water, which return to the environment. Attempts based on various methods have been undertaken for mass production of PHAs. Promising strategies involve genetic engineering of microorganisms and plants to introduce production pathways. This challenge requires the expression of several genes along with optimization of PHA synthesis in the host. Although excellent progress has been made in recombinant hosts, the barriers to obtaining high quantities of PHA at low cost still remain to be solved. The commercially viable production of PHA in crops, however, appears to be a realistic goal for the future.

Identification and functional characterization of a monofunctional peroxisomal enoyl-CoA hydratase 2 that participates in the degradation of even cis-unsaturated fatty acids in Arabidopsis thaliana

A gene, named AtECH2, has been identified in Arabidopsis thaliana to encode a monofunctional peroxisomal enoyl-CoA hydratase 2. Homologues of AtECH2 are present in several angiosperms belonging to the Monocotyledon and Dicotyledon classes, as well as in a gymnosperm. In vitro enzyme assays demonstrated that AtECH2 catalyzed the reversible conversion of 2E-enoyl-CoA to 3R-hydroxyacyl-CoA. AtECH2 was also demonstrated to have enoyl-CoA hydratase 2 activity in an in vivo assay relying on the synthesis of polyhydroxyalkanoate from the polymerization of 3R-hydroxyacyl-CoA in the peroxisomes of Saccharomyces cerevisiae. AtECH2 contained a peroxisome targeting signal at the C-terminal end, was addressed to the peroxisome in S. cerevisiae, and a fusion protein between AtECH2 and a fluorescent protein was targeted to peroxisomes in onion cells. AtECH2 gene expression was strongest in tissues with high beta-oxidation activity, such as germinating seedlings and senescing leaves. The contribution of AtECH2 to the degradation of unsaturated fatty acids was assessed by analyzing the carbon flux through the beta-oxidation cycle in plants that synthesize peroxisomal polyhydroxyalkanoate and that were over- or underexpressing the AtECH2 gene. These studies revealed that AtECH2 participates in vivo to the conversion of the intermediate 3R-hydroxyacyl-CoA, generated by the metabolism of fatty acids with a cis (Z)-unsaturated bond on an even-numbered carbon, to the 2E-enoyl-CoA for further degradation through the core beta-oxidation cycle.

Analysis of the contribution of the β-oxidation auxiliary enzymes in the degradation of the dietary conjugated linoleic acid 9-cis-11-trans-octadecadienoic acid in the peroxisomes of Saccharomyces cerevisiae

Beta-oxidation of the conjugated linoleic acid 9-cis,11-trans-octadecadienoic acid (rumenic acid) was analyzed in vivo in Saccharomyces cerevisiae by monitoring polyhydroxyalkanoate production in the peroxisome. Polyhydroxyalkanoate is synthesized by the polymerization of the beta-oxidation intermediates 3-hydroxyacyl-CoAs via a bacterial polyhydroxyalkanoate synthase targeted to the peroxisome. The amount of polyhydroxyalkanaote synthesized from the degradation of rumenic acid was found to be similar to the amount synthesized from the degradation of 10-trans,12-cis-octadecadienoic acid, oleic acid or 10-cis-heptadecenoic acid. Furthermore, the degradation of 10-cis-heptadecenoic acid was found to be unaffected by the presence of rumenic acid in the media. Efficient degradation of rumenic acid was found to be independent of the Delta(3,5),Delta(2,4)-dienoyl-CoA isomerase but instead relied on the presence of Delta(3),Delta(2)-enoyl-CoA isomerase activity. The presence of the unsaturated monomer 3-hydroxydodecenoic acid in polyhydroxyalkanoate derived from rumenic acid degradation was found to be dependent on the presence of a Delta(3),Delta(2)-enoyl-CoA isomerase activity. Together, these data indicate that rumenic acid is mainly degraded in vivo in S. cerevisiae through a pathway requiring only the participation of the auxiliary enzymes Delta(3),Delta(2)-enoyl-CoA isomerase, along with the enzyme of the core beta-oxidation cycle.

Molecular identification and characterization of the Arabidopsis delta(3,5),delta(2,4)-dienoyl-coenzyme A isomerase, a peroxisomal enzyme participating in the beta-oxidation cycle of unsaturated fatty acids

Degradation of unsaturated fatty acids through the peroxisomal beta-oxidation pathway requires the participation of auxiliary enzymes in addition to the enzymes of the core beta-oxidation cycle. The auxiliary enzyme delta(3,5),delta(2,4)-dienoyl-coenzyme A (CoA) isomerase has been well studied in yeast (Saccharomyces cerevisiae) and mammals, but no plant homolog had been identified and characterized at the biochemical or molecular level. A candidate gene (At5g43280) was identified in Arabidopsis (Arabidopsis thaliana) encoding a protein showing homology to the rat (Rattus norvegicus) delta(3,5),delta(2,4)-dienoyl-CoA isomerase, and possessing an enoyl-CoA hydratase/isomerase fingerprint as well as aspartic and glutamic residues shown to be important for catalytic activity of the mammalian enzyme. The protein, named AtDCI1, contains a peroxisome targeting sequence at the C terminus, and fusion of a fluorescent protein to AtDCI1 directed the chimeric protein to the peroxisome in onion (Allium cepa) cells. AtDCI1 expressed in Escherichia coli was shown to have delta(3,5),delta(2,4)-dienoyl-CoA isomerase activity in vitro. Furthermore, using the synthesis of polyhydroxyalkanoate in yeast peroxisomes as an analytical tool to study the beta-oxidation cycle, expression of AtDCI1 was shown to complement the yeast mutant deficient in the delta(3,5),delta(2,4)-dienoyl-CoA isomerase, thus showing that AtDCI1 is also appropriately targeted to the peroxisome in yeast and has delta(3,5),delta(2,4)-dienoyl-CoA isomerase activity in vivo. The AtDCI1 gene is expressed constitutively in several tissues, but expression is particularly induced during seed germination. Proteins showing high homology with AtDCI1 are found in gymnosperms as well as angiosperms belonging to the Monocotyledon or Dicotyledon classes.

Analysis of the β-oxidation of trans-unsaturated fatty acid in recombinant Saccharomyces cerevisiae expressing a peroxisomal PHA synthase reveals the involvement of a reductase-dependent pathway

The degradation of fatty acids having cis- or trans-unsaturated bond at an even carbon was analyzed in Saccharomyces cerevisiae by monitoring polyhydroxyalkanoate production in the peroxisome. Polyhydroxyalkanaote is synthesized by the polymerization of the beta-oxidation intermediates 3-hydroxy-acyl-CoAs via a bacterial polyhydroxyalkanoate synthase targeted to the peroxisome. The synthesis of polyhydroxyalkanoate in cells grown in media containing 10-cis-heptadecenoic acid was dependent on the presence of 2,4-dienoyl-CoA reductase activity as well as on Delta3,Delta2-enoyl-CoA isomerase activity. The synthesis of polyhydroxyalkanoate from 10-trans-heptadecenoic acid in mutants devoid of 2,4-dienoyl-CoA reductase revealed degradation of the trans fatty acid directly via the enoyl-CoA hydratase II activity of the multifunctional enzyme (MFE), although the level of polyhydroxyalkanoate was 10-25% to that of wild type cells. Polyhydroxyalkanoate produced from 10-trans-heptadecenoic acid in wild type cells showed substantial carbon flux through both a reductase-dependent and a direct MFE-dependent pathway. Flux through beta-oxidation was more severely reduced in mutants devoid of Delta3,Delta2-enoyl-CoA isomerase compared to mutants devoid of 2,4-dienoyl-CoA reductase. It is concluded that the intermediate 2-trans,4-trans-dienoyl-CoA is metabolized in vivo in yeast by both the enoyl-CoA hydratase II activity of the multifunctional protein and the 2,4-dienoyl-CoA reductase, and that the synthesis of the intermediate 3-trans-enoyl-CoA in the absence of the Delta3,Delta2-enoyl-CoA isomerase leads to the blockage of the direct MFE-dependent pathway in vivo.