Principles of Plant Genetics and Breeding. George Acquaah
but are more difficult to extract from the plant. Further, they have low survival rates. Meristems in general tend to be free of virus infection, even if the rest of the plant is infected. Meristems may therefore be the ideal explant to cure virus infected valuable clones.
7.12.2 Adventitious shoot production
Adventitious shoots originate from adventitious meristems. Non‐meristematic tissue can be induced to form plant organs (e.g. embryos, flowers, leaves, shoots, roots). Differentiated plant cells (with specific functional roles) can be induced to dedifferentiate from their current structural and functional state, and then embark upon a new developmental path to produce new structures. Adventitious shoot production through organogenesis occurs by one of two pathways – indirect or direct.
1 Indirect organogenesisThe indirect organogenetic pathway goes through a stage in which a mass of dedifferentiated cells (callus) forms (i.e. the explant forms a callus from which adventitious meristems are induced and from which plant regeneration is initiated). The callus consists of an aggregation of meristem‐like cells that are developmentally plastic (can be manipulated to redirect morphogenic end point). The negative side of this method is that the callus phase sometimes introduces mutations (somaclonal variation, making this not always a 100% clonal procedure). The callus phase also makes it more technically challenging than shoot tip micropropagation.
2 Direct organogenesisDirect organogenesis bypasses a callus stage in forming plant organs. The cells in the explant act as direct precursors of a new primordium. This pathway is less common than the callus mediated pathway.
7.12.3 Somatic adventitious embryogenesis
A zygote is formed after an egg has been fertilized by a sperm. The zygote then develops into an embryo (zygotic embryo). In vitro tissue culture techniques may be used to induce the formation of embryos from somatic tissue (non‐zygotic embryo or somatic embryogenesis) using growth regulators. Somatic embryos arise from a single cell rather than budding from a cell mass as in zygotic embryos. This option is very important in biotechnology since transgenesis in plants may involve the manipulation of single somatic cells. However, without successful regeneration, plant transformation cannot be undertaken. Somatic embryogenesis has been extensively studied in Apiaceae, Fabaceae, and Solanaceae.
7.13 Concept of totipotency
Plants reproduce sexually or asexually. Clones are identical copies of a genotype, derived from somatic tissue or cells of the source plant. Together, they are phenotypically homogeneous, since they all originate from the same source plant either in one or more clonal generations of reproduction. However, individually, they are highly heterozygous. Clonally propagated plants produce genetically identical progeny. Pieces of plant parts (leaf, stem, roots, tubers) can be used to grow full plants in the soil. In vitro (growing plants under sterile conditions) plant culture was first proposed in the early 1900s. By 1930s, cell culture had been accomplished. Each cell in a multicellular organism is theoretically totipotent (i.e. endowed with the full complement of genes to direct the development of the cell into a full organism). In theory, a cell can be taken from a root, leaf, or stem, and cultured in vitro into a complete plant.
7.14 Somaclonal variation
Clones, as previously stated, are exact replicas of the genotype from which their source tissue had been derived. Commonly, however, clonal propagation, occurring under a tissue culture environment, produces materials that are not exact replicas of the original material used to initiate the culture. Such variation resulting not from meiosis but from the culture of somatic tissue is referred to as somaclonal variation, and the variants somaclones. The variation observed may be transient (epigenetic) or heritable (genetic in origin). It is important to authenticate the presence of a true mutational event before using the somaclone in a breeding program as a valuable source of variation. Somaclonal variants can be recovered in tissue culture with selection pressure (e.g. deliberate inclusion of a toxic agent in the culture medium) or without selection pressure (the basic cultural medium).
A variety of mechanisms have been implicated in this phenomenon. Chromosomal changes, both polyploidy and aneuploidy have been observed in potato, wheat, and ryegrass. Some research suggests mitotic crossovers to be involved whereas cytoplasmic factors (mitochondrial genes) have been implicated by others. Further, point mutation, transposable elements, deoxyribonucleic acid (DNA) methylation, and gene amplification are other postulated mechanisms for causing somaclonal variation. One more trivial source of variation in plants derived from tissue culture is that they derived from mutated section of the explants. Somatic cells may have undergone mutations (leading to chimeras, see Chapter 25). Tissue from chimeric plants may lead to genetically different progeny.
As a breeding tool, breeders may deliberately plan and seek these variants by observing certain factors in tissue culture. Certain genotypes are more prone to genetic changes in tissue culture, polyploids generally being more so than diploids. Also, holding the callus in undifferentiated state for prolonged periods of time enhances the chances of somaclonal variation occurring. Not unexpectedly, the tissue culture environment (medium components) may determine the chance for heritable changes in the callus. The inclusion of auxin 2, 4‐D enhances the chances of somaclonal variation.
The value of somaclonal variation as a breeding tool is evidenced by the successes in various species (Table 7.1). These include disease resistance (e.g. Helminthosporium sacchari in sugarcane, and Fusarium in Apium graveolens) and resistance to various abiotic stresses.
Table 7.1 List of crops where desirable and heritable somaclonal variations have been reported.
| Species | Characters, which were modified |
| A. Monocotyledons | |
| 1. Allium sativa | Bulb size and shape; clove no.; aerial bulbit |
| 2. Avena sativa | Plant ht.; heading date; awns |
| 3. Hordeum spp. | Plant ht.; tillering |
| 4. Lolium hybrids | Leaf size; flower, vigor; survival |
| 5. Oryza sativa | Plant ht.; heading date; seed fertility: grain no and wt. |
| 6. Saccharum officinarum | Diseases (eye spot, fiji virus, downy mildew, leaf scald) |
| 7. Triticum aestivum | Plant and ear morphology; awns; gliadins; amylase; grain wt., yield |
| 8. Zea mays | T toxin resistance; male fertility; mtDNA |
| B. Dicotyledons | |
| 9. Lactuca sativa | Leaf wt., length, width, flatness, and color |
| 10. Solanum lycopersicum | Leaf morphology; branching habit; fruit color; pedicel; male fertility; growth |
| 11. Medicago sativa |
Multifoliate leaves; elongated petioles; growth; branch no.; plant ht.; |