February, 2000: New Jersey (United States)
Plant tissue produces a complex mixture of chemicals that, if extracted, have many valuable uses for humans. In the past, extraction and purification were expensive, time-consuming, and low-yielding processes. But today, plants can be genetically engineered to secrete proteins and other pharmaceutically desirable chemicals from their roots, making other extraction methods obsolete.
The term that describes root cell secretion is rhizosecretion. Although secretion is a basic function of all plant cells, some scientists believe roots have perfected the process because they are responsible for interactions with soil organisms, nutrient uptake, and allelopathic reactions. Allelopathy is a form of rhizosecretion in which roots secrete chemicals that prevent other plants from growing in close proximity. It is a plant-plant interference mechanism believed to be a response to resource competition.
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Recombinant DNA technology.
Researchers at Rutgers University have genetically engineered two types of tobacco plants, Nicotiana tabaccum and N. plumbagnifolia, to continuously secrete recombinant proteins from their roots. Recombination is a naturally occurring process between homologous DNA molecules of closely related plant species. However, in genetic engineering, the process is manipulated in vitro between completely different species. Thus, the desirable gene is selected from one plant and inserted into a bacterial strand called a plasmid, which serves as a vector to transfer the gene into another plant. The second plant is the genetically engineered, or
In one experiment, the genetically engineered tobacco plants were grown in a water-based nutrient solution rather than in soil. This method, known as hydroponics, was originally practiced in the mid-1800s and reintroduced in 1937. Researchers either purchased transgenic tobacco plants or transformed the tobacco plants with DNA from one of three different sources by using a standard Agrobacterium-mediated leaf-disc transformation method (protocol by R.B. Horsch et. al., illustrated in a 1985 issue of Science 227). Three proteins were introduced: (1) green fluorescent protein of jellyfish, (2) human placental alkaline phosphatase, or (3) bacterial xylanase. (Remember that the suffix -ase denotes an enzyme, all of which are proteins.)
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Hydroponics. (Photo © 1999 Papi.)
Hydroponic techniques used with tomato plants.
The resulting protein secretion was dependent on whether or not an endoplasmic reticulum (ER) signal peptide was present. The ER target signals, which were fused to the recombinant protein sequence, fooled the tobacco plants into shuttling the introduced genetic information through the plant and out the roots as if it were the plant's own genetic material. In addition, plants with the ER target signals showed a higher concentration of protein in the hydroponic medium than in the root cells compared to plants without the ER target signals. Conversely, plants without ER target signals maintained higher concentration in the root cells and did not leach the chemicals into the hydroponic medium.
Because plants secrete few proteins from their roots, isolation of the desirable proteins was easier, faster, and less expensive from the hydroponic medium than traditional plant extraction and isolation methods. When the secreted proteins are separated from the hydroponic medium and analyzed, they are found to retain their own biological activity! Also, rhizosecretion can run continuously without harming the plant, leading to a higher yield of pharmaceutically favorable proteins in the transgenic plants.
Nicotiana is a genus in the Solanaceae family. Members of Solanaceae are dicots with stamen adnate to the fused petals. They have a superior ovary that typically develops into either a dry fruit, such as a capsule, or a fleshy fruit, such as a berry. The leaves and/or stems of the plants are commonly covered with prickles. Economically, tobacco plants have been grown as ornamentals and for the alkaloids they produce. Since 1664, several types of alkaloids, including nicotine, have been used for their insecticidal properties. In addition, different cultivars of N. tabaccum have been commonly used in cigarettes and cigars, ground and flavored for snuff, or mixed with molasses for chewing tobacco. Thus, through advances in genetic engineering, hydroponics, and rhizosecretion, scientists may have found yet another, and more beneficial, use for tobacco plants--as mini drug manufacturers.
References, Websites, and Further Reading
"Harvesting Proteins," New York Times, 4 May 1999, Tuesday science edition.
Hopkins, W. G. 1999. Introduction to plant physiology. 2d edition. New York: John Wiley & Sons, Inc.
Raskin, I., et. al. 1999. Production of recombinant proteins in plant root exudates. Nature Biotechnology 17:466-70.
Raskin, Ilya. Use of plant roots for biochemical manufacturing. Lecture presented at XVI International Botanical Congress, St. Louis, MO, 3 August 1999.
List of articles published by researchers at Cook College, Rutgers University
Plant Protein Club, including articles and workshops
Stern, Introductory Plant Biology, 8th Edition
Chapter 2: The Nature of Life
Proteins, pp. 24-25
Enzymes, p. 26
Chapter 3: Cells
Endoplasmic reticulum (ER), p. 40
Chapter 4: Tissues
Plant tissues in general, pp. 52-61
Chapter 5: Roots and Soils
Roots, pp. 63-76
Chapter 9: Water in Plants
Mineral requirements for growth, p. 160
Chapter 24: Flowering Plants and Civilization
Solanaceae (nightshade family), pp. 450-51
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