Psuedobulbs In the Dendrobium orchid, for example, roots form at the base of the offshoots. These offshoots may be removed and transplanted. 10.14.3 SUCKERS Suckers are adventitious shoots that arise from roots. In raspberry, the shoots are harvested from horizontal roots and used to propagate the plant. 10.14.4 OFFSHOOTS The variety of adventitious shoots (suckers, crown division, slips, and offsets) that arise from the stems or roots of plants are called offshoots. The pineapple plant is quite versatile in terms of parts that may be used to propagate the plant. The crown on top of the fruit, slips arising from axillary buds at the base, and suckers that originate from the lower part of the stem may all be used for propagation. 10.14.5 PLANT PATENT ACT A patent is a tool used by the government to provide incentive for discovery and invention of products. A plant patent may be awarded to an applicant who has “invented” or discovered and asexually reproduced a distinct and new variety of plant (other than a tuber propagated or a plant found in an uncultivated state). A plant patent holder is given the right to exclude others from asexually reproducing the plant and from using, offering for sale, or selling the plant so reproduced, or any of its parts, throughout the United States, or from importing the plant so reproduced, or any parts thereof, into the United States. Such rights are enforceable for a period of twenty years. Asexual (clonal) reproduction is specifically mentioned in the act because it is a means of preserving the genetic integrity of the product from one generation to the next. The owner of the patent is encouraged to reproduce it asexually and make adequate amounts of such materials available to the public. Plant produces that qualify for patenting are sports, mutants, and hybrids. A sport is a new and distinct variety that originates from a bud. Such spontaneous variations are distinguishable from the appearance or characteristic of a normal plant. Seedling variation by self-pollination may give rise to mutants, while a hybrid is a product of the crosspollination of two unidentical plants (e g., two species, varieties). Plant seedlings discovered, asexually reproduced, and proved to be stable, uniform, and to have new characteristics distinct from other known plants, are patentable. The law, however, specifically excludes plants found in an uncultivated state. The plant must not have been introduced to the public, sold, or offered for sale, more than one year prior to the patent application. To file for a plant patent, the patentee is required to clearly document his or her claim in writing, describing and defining the new plant, showing its unique and distinguishing features from known varieties, supplementing with drawings and or photographs, and making an oath or declaration. The applicant must declare that the plant has been reproduced asexually, or found in a cultivated area. Descriptions of the plant should be made in standard botanical terms and include the origin or parentage of the plant, geographic location, and the method of asexual reproduction (e.g., budding, cutting). If color is a distinguishing feature, it must be described using a color atlas or dictionary, preferably, The Royal Horticulture Society Color Chart. Offshoot The generic term for adventitious shoots that arise from various parts of the plant. SUMMARY Apart from roots, certain species develop a variety of underground swollen structures that are, in some cases, the economic, edible, or usable part of the plant. These modifications may be in the roots or stem. Bulbs such as onion, daffodil, and tulip have modified leaves that are scaly; corms such as gladiolus and crocus have modified stems that Summary 339
have assumed a globelike shape. In some plants, including ginger and banana, modified underground stems grow horizontally and are called rhizomes; in the Irish potato, the underground stem is swollen. However, in the sweet potato, the root is swollen into a tuberous root that is edible. The modified underground structures are used in propagating the respective plants. MODULE 6 MICROPROPAGATION (TISSUE CULTURE) 10.15 THE TECHNIQUE Micropropagation The technique of producing new plants from single cells, tissue, or small pieces of vegetative material. Callus A mass of undifferentiated cells that can be induced or arise naturally as a result of wounding. Explant Generic term for the living vegetative plant material extracted for tissue culturing on an appropriate medium. Somaclonal Variation Heritable variation that arises spontaneously as callus forms on a tissue culture medium. Micropropagation (or tissue culture) is a technique by which tissue obtained from a plant is cultured in an artificial medium. This tissue first changes into a mass of undifferentiated cells called callus, from which differentiation into shoot and roots or embryos may later occur (Figure 10–39). Molecular biotechnological procedures often incorporate tissue culture as one of their critical methods. Micropropagation may be used in herbaceous (e.g., strawberry, gladiolus, tobacco, carnation, and gloxinia) and woody plants (e.g., apple, rose, kalmia, and rhododendron). Tissue culture requires a completely sterile environment to be successful. Propagation can be initiated with any part of the plant: leaf, stem, root, pollen grain, embryo, and others. These excised plant parts used to initiate propagation by tissue culture are called explants. Both immature and mature plant tissue may be used. Explants are aseptically prepared by surface sterilization before being placed in a sterile medium. The medium is fortified with all of the nutrients required for plant growth and development, including mineral salts (major and minor elements), sugar, vitamins, and growth regulators (auxins and cytokinins). One of the most commonly used tissue culture media is the Murashige and Skoog medium. By modifying the composition of the medium, with respect to growth regulators, one can cause it to sustain normal plant growth or induce the explant to develop callus tissue, the result of repeated mitotic cell division. Sometimes scientists are able to manipulate callus tissue through the introduction of chemicals into the growth medium that induce heritable variations (mutations). Such variations occasionally arise spontaneously even in normal media culture and are called somaclonal variations. Some of these variants have agronomic value and are utilized in crop improvement by plant breeders. FIGURE 10–39 Callus formation in tissue culture. (Source: George Acquaah) 340 Chapter 10 Asexual Propagation
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