Polymeric bio molecules produced by the living organisms are termed as bio polymers. In a polymeric structure, the monomers join together by some kind of bonding such as covalent bond.
Bio polymers are of two types:
- Naturally occurring that includes silk, pectin, rubber, DNA and Proteins
- Synthetic Bio polymers such as polyethylene, Teflon, epoxy and nylon etc
Plants naturally produce bio polymers in small quantities. They include DNA, silk, wool, pectin, rubber, proteins and cellulose/ligno-celluloses. All of these consists monomeric sub units joined together by specific type of bond. For example, proteins have amino acids joined together by peptide bonds.
Naturally Occurring Plant Polymers
Rubber
It is naturally occurring plant polymer in the form of sticky substance. This source is latex (milky liquid material) and consisting of isoprene units. Naturally occurring rubber is processed by addition of some essential materials in it to make it durable. For this purpose, heating rubber with sulfur improves its durability, reliance and other chemical and physical properties. This process is vulcanization which links several polyisoprene chains together by chemical bonds. This produces hardened rubber that is more durable and smooth rather than soft and sticky material like that of plants.
Pectin
It’s a naturally occurring plant polymer consisting of pectic and pectinic acid molecules as monomers. Because of these acids this polymer is considered as polysaccharide that is extracted from citrus plants and apple plants. It is one of the extracellular materials in plants that join the cells together. Crystallization of chains of pectin produces a 3D structure forming a gel like substance, which can be used as thickener in food industry in production of jams and jellies.
Synthetic Plant Polymers
The advances in the field of genetic engineering have enabled the scientist to find a way of production of bio polymers in plants. Synthetic polymers that are synthesized preciously are petroleum based. Also some of them are not environment friendly as they are not easily degrade so a cause of pollution as well. So the production of bio polymers by genetically engineered plants will be less expensive and environment friendly as well as these bio polymers is biodegradable.
Plant Based Bio polymers
There are two main types of bio polymers that are produced by transgenic or genetically engineered plants:
- Polyhydroxy alkanoate Homopolymers (PHAs)
- Protein Based Polymers
Polyhydroxy alkanoate Homopolymers
These bio polymers are the polyesters of 3-hydroxy acids having properties of biodegradable plastics as they consist of elastomers (elastic polymers). Some of the microbes such as some bacterial species produce these polymers as carbon and energy source. So they also have the ability to degrade these polymers to yield water and carbon dioxide gas. This polymer has better physical and chemical qualities than PBA. Synthesis of this polymer in plant Arabidopsis thaliana has been successful.
About 100 different types of hydroxy acid are part of these bio polymers and the physical and chemical properties of various types of bio polymers derived from polyhydroxy alkanoates can vary.
The timing for degradation of these PHA polymers ranges from 3-9 months. But their problem is high cost of PHA bio polymers. Moreover, these are not sufficient to replace other petroleum based polymers. Therefore, the bio polymers produced by the plants will fulfill this purpose.
Polyhydroxy Butyrate (PHB) Bio polymer
Polyhyroxy Butyrate (PHB), a plant based homopolymer that is compose of acetyl-CoA is one of the polyhydroxyalkanoate biopolymer. Acetyl-CoA is naturally present in plant cells as substrate of energy-producing pathways such as Kreb cycle. It is also present in plastids, cytoplasm and peroxisomes.
The level of production of polyhydroxy butyrate by plant cells is 0.1-0.2% of its dry weight. The production of this type of bio polymer was possible because of expression of genes that codes for enzymes, PHA synthase and acetoacetyl-CoA reductase. These genes were isolated from bacterial genome of alcaligenes eutrophus and ralstonia eutropha species. Genetically engineer plant cells produced PHB bio polymers. On the contrary, this resulted in low growth rate of plants possibly due to production of PBA polymers. Probably, it happened because of short supply of acetyl –CoA in plant cells for flavonoid and isoprenoid biosynthesis. Production of bio polymers in transgenic Arabidopsis thaliana plant shoots was observed to be 14% of its dry weight.
Production of polyhydroxy butyrate in plant cells
Different organelles of plant cell synthesize these bio polymers. The pathway for production of these polymers provides high yield of monomeric units. For example, in cytoplasm of cell for production of iosprenoid unit. While in plastids, fatty acids biosynthesis and in peroxisomes, β-oxidation of certain fatty acids yields 3-hydroxy butyrate and acetyl CoA as intermediate products occurs for PBH biosynthesis.
Experimentation reveals that diverting the pathway of fatty acid biosynthesis yields acetyl-coA. This is the way of the plant cell for biopolymer production. In leucoplast cell of transgenic plants, the %age of PBA was 8% of dry weight by means of these pathways.
Production of Polyhydroxy Alkanoate Biopolymers
For the commercial production of this bio polymer by R.eutropha plant, the overall yield of propionyl CoA is enhaced by adding propionate in growth media. Later, this propionate condenses with acetyle CoA to produce 3-ketovaleryl CoA.
This pathway for production of polyhydroxy alkanoate is modification of bio-synthetic pathway of branched chain amino acids. The expression of genes that codes for acetyl CoA reducatase and PHA synthase enzyme along with bktB gene from a plant genome, results in production of PHA polymer in plant cells. The %age of PHA synthesis in plant cells was 0.2-0.8%. The production of PHA bio polymer still needs improvements to replace other synthetic polymers that lack some of the good qualities as this. The problem is that we need to control two separate pathways for monomers production for this type of bio polymer in plants.
Medium chain length PHAs consists of 3-hydroxyacid monomers whose number ranges from 6 to 16 carbon atoms. The source of PHA synthase gene was bacterial species pseudomonas aeruginosa strain. It was successfully incorporated in plant genome of A.thaliana. The target is production of PHA in peroxisomes by pathway of β-oxidation of fatty acid that yields 3-hydroxyacids monomers for PHA synthesis.
The production rate was 0.4% of dry weight. And the monomers were of complex nature; both saturated and unsaturated fatty acids. So by controlling the pathway of β-oxidation of fatty acids, PHA synthesis in plants can be enhanced.
PROTEIN-BASED POLYMERS
Proteins are bio polymers of amino acids as monomers, joined together by peptide bonds. Collagen and elastin are examples of proteins having a highly repetitive amino acid structure.
In aspergillus ridullans and E.coli strains the gene for synthetic bovine elastin protein expressed itself. Plant polymers based on proteins sequences, show elastic properties, non-toxic, and are biodegradable as well. However, the expression of genes for synthesis of protein based polymers is high in some plants while in some cases, its expression is low.
These protein based bio polymers can improve some qualities of plant fibers. For example, water absorption, strength of plan fibers, the ability of cotton to bind to certain dyes etc.
Why Plant Based Bio polymers?
- Plant bio polymers are of vital importance when it comes to improvement and introduction of certain traits in plant fibers. These polymers can act as:
- The replacement of polymers that lack some good qualities such as toughness, flexibility and most importantly their biodegradability.
- In cytoplasm of cotton fibers, plant polymers have their utility to improve its qualities such as its ability to absorb certain dyes.
- PBA production in plants can provide it insulating properties against heat as it enhances the heat enduring capacity of plant cells and fibers.
- Researchers are finding ways to improve the qualities of natural plant polymers such as rubber and pectin by addition of these PBA & PHAs.
- Plant cells produce PHA bio polymers by modification of some metabolic pathways that yield its monomeric units. So this pathway has utility for qualitative and quantitative biochemical analysis.
FUTURE PERSPECTIVE
Transgenic plants can develop a variety of bio polymers. This will be of great significance in future as these plant based polymers can prove to be inexpensive, sufficient in quantity and surly of better qualities as well.
The challenge is to enhance the production of bio polymers in plants for their sufficient production to replace the petroleum based polymers. Development of extraction techniques is also a challenge. The bio polymers such as PHAs and protein based polymers are of good qualities, but their rate of production is less. It needs to be 15% of the dry weight of plant.
Recent advances in field of genetic engineering for production of bio polymers by transgenic plants have enabled the scientists for multigene transfer in plant genome and to modify the bio-synthetic pathway as well. The idea of production of novel compounds by transgenic plants is feasible now and in future, the probability to use plants as source of bio energy as a renewable source of energy. Bio polymers such as PHA, PBA and protein based polymers are vital to replace the harmful and non-biodegradable synthetic polymers such as polyester, polystyrene etc.
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