Alternative Agriculture: Molecular Approaches to Produce Recombinant Proteins and to Isolate Novel Compounds

8 June, 1999

Alternative Agriculture: Molecular Approaches to Produce Recombinant Proteins and to Isolate Novel Compounds

Plants are efficient producers of food, fiber, traditional medicines, and construction materials. More recently a new notion "alternative agriculture," which expands the uses of plants well beyond the conventional uses, is beginning to change the picture of plant biology. The production of useful compounds in plants is an obvious first step, and the foremost in this trend is that of animal proteins. As plants are capable of carrying out acetylation, phosphorylation, and glycosylation as well as other post-translational protein modifications required for the biological activity of many eukaryotic proteins, numerous recombinant proteins have been produced in plant leaves, fruits, roots, tubers, and seeds.
One example is the production of "plantibodies," a new jargon for monoclonal antibodies produced (in large amounts) in plants. Currently standard non-plant methods are capable of producing 5 -10 kg of a therapeutic monoclonal antibody (mAb) per year. An ambitious notion is now being entertained that the plant produces 10,000 kg of a key mAb per year, with the cost of only $10 - 100/g compared with current $ 200 - 1000/g. When mAbs are available in such a large quantity, they may be used in various ways as to coat the surfaces of microbial attachment such as tooth enamel to block Streptococcus mutans (Ma JK et al, 1998: uid=9585235), and as to prevent sexually transmitted diseases (STDs, annual cost $12 billion) such as in the vaginal application of antibodies against herpes simplex viruses (Zeitlin L et al, 1998: uid=9853620).

EPIcyte, Inc, together with ReProtect, LLC, actually plan to produce, in the above scheme, recombinant secretory immunoglobulin A mAbs in corn rather than in conventional tobacco plants (Hiatt A et al, 1989: uid=2509938). The mAbs may be "stored" dry in the seed for a long time and prepared easily by grinding corn seeds when necessary for use.

Another line of interest is the use of plant roots for molecular farming (secretion into rhizosphere). Plant roots can continuously produce and secrete biologically active compounds. Rhizosecretion is a way of phytomanufacturing, and may be designed to produce and secrete valuable natural products (organic molecules) and recombinant proteins from roots. At present, most of the recombinant proteins or valuable natural products used as fine chemicals, pharmaceuticals, crop protection compounds, and cosmetic ingredients are extracted from plants by using solvents, making downstream processing and purification of individual components difficult and costly.

Up to 10% of photosynthetically fixed carbon is secreted from the roots and give rise to a variety of compounds. An increasing number of interesting compounds have been identified in root exudates, and these are, for instance, Isoflavonoids, flavonoids, strigol, herbicidal allelochemicals and phytosiderophores. However, root exudates are a rather unexplored place to search for novel biologically active compounds. The analysis of root exudates from 120 plant species further indicates that, a) each plant species exudes a distinct set of compounds, b) root exudates are relatively simple mixtures, c) root exudates are devoid of pigments and tannins, and do not contain large quantities of biologically inert structural compounds, and d) the chemical composition of root exudates is very different from that of conventional methanolic extracts of root tissue (Gleba D et al, 1999: uid=10339526). These are the features that would make downstream processing and purification cost-effective.

For illustration of rhizosecretion, recent findings by Borisjuk NV et al (1999: uid=10331806) may be referred here. They addressed the large-scale production of recombinant proteins by engineering tobacco plants to continuously secrete proteins from their roots into a simple hydroponic medium. Three heterologous proteins of diverse origins (green fluorescent protein of jellyfish, human placental secretory alkaline phosphatase, and bacterial xylanase) were produced using the root secretion method. The daily secretion of green fluorescent protein reached 2.4 mg/g root dry weight, while secretion of human placental secretory alkaline phosphatase reached 20 mg/g root dry weight/day. Protein secretion was dependent on the presence of the endoplasmic reticulum signal peptide fused to the recombinant protein sequence. All three secreted proteins retained their biological activity and, as shown for secretory alkaline phosphatase, accumulated in much higher amounts in the medium than in the root tissue. It is expected that protein secretion is further enhanced with the system elaborations. In addition, it should be noted that recombinant proteins from root exudates are free from possible contamination with pathogenic viruses.

Antimicrobial compounds such as phytoalexins are elicited in plant by physical and chemical treatments. Exudates from 480 species were shown to contain moderate to strong activities against a variety of bacteria and fungi. These findings warrant systematic study of root exudates to discover new chemical entities for pharmaceutical industry. Whereas plant tissue culture is expensive, slow and relatively deficient of secondary metabolites, rhizosecretion may provide diverse sources for identifying novel chemical compounds that may be potential leads to develop useful therapeutics such as antibiotics and anticancer agents.

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