Production of the renewable biopolymer cyanophycin in transgenic potato plants as a source for polyaspartate and arginine

Autor: Waldvogel, E.P.; Hausmann, T.; Hühns, M.; Klemke, F.; Lockau, W.; Wohlleben, W.; Broer, I.
In:

7. International Symposium
Materials made of renewable Resources
Autor: naro.tech
Herausgeber: Forschungsvereinigung Werkstoffe aus nachwachsenden Rohstoffen e.V., Rudolstadt
Jahr: 2009

Einordung:
Institut: Professur Agrobiotechnologie

Abstract:
Certain biopolymers are intended to serve as substitutes for fuel-derived compounds. Polyaspartate for example is a soluble, non-toxic and biodegradable polycarboxylate that can be used to replace non-biodegradable polyacrylates in numerous industrial, agricultural and medical applications. Because no polyaspartate-producing organism has been identified so far, the polymer is chemically synthesized. However, polyaspartate can be obtained from the cyanobacterial reserve polymer cyanophycin (multi-L-arginyl-poly-L-aspartic acid), which is composed of a poly-α-aspartic acid backbone to which arginine residues are connected via isopeptide bonds. Mild hydrolysis of cyanophycin yields homo- and copolymers of polyaspartate and L-arginine. Besides the application of polyaspartate as dispersing agent both products could serve as a starting point for the synthesis of a range of chemicals. To avoid the high costs of bacterial fermentation, we aim for a CO2-neutral cyanophycin production in transgenic plants. Potato tubers are suitable for the production of biopolymers since they allow for cost-effective fabrication as a byproduct of starch. To this end, the cyanophycin-synthetase gene cphA of Thermosynechococcus elongatus BP-1 was expressed in potato plants under the control of the tuber-specific class 1 promoter B33. Tuber specific plastidic (pB33-PsbY-cphATe) expression resulted in significant polymer accumulation exclusively in amyloplasts of tubers with maximal cyanophycin content of up to 7.5% of dry weight. These tubers were normal in size and able to germinate despite some stress symptoms like brown sunken stainings. The biopolymer cyanophycin consists of the two amino acids aspartate and arginine. For optimal cyanophycin production in transgenic potato plants, the amino acid metabolism of the plant must be capable of synthesizing these substrates in sufficient amounts. The amino acid pool and its composition were measured by reversed phase HPLC-analysis. With this method, we detected a significantly reduced pool of free and bound glutamate in the transgenic tubers compared to the control plants. This decreased glutamate pool seems to be a strong limiting factor for cyanophycin synthesis since arginine is synthesized via glutamate. However, it may be possible to alleviate this effect by metabolic engineering of the nitrogen metabolism.

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