George Acquaah - Principles of Plant Genetics and Breeding

Plants reproduce sexually or asexually. Clonesare 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.

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 7.1List of crops where desirable and heritable somaclonal variations have been reported.

Seed production in higher plants that are sexually propagated species normally occurs after a sexual union in which male and female gametes fuse to form a zygote, which then develops into an embryo. However, some species have the natural ability to develop seed without fertilization, a phenomenon called apomixis. The consequence of this event is that apomictically produced seeds are clones of the mother plant. That is, apomixis is the asexual production of seed. Unlike sexual reproduction, there is no opportunity in apomixis for new recombination to occur to produce diversity in the offspring. However, the seeds have the same advantages as non‐apomictic seeds: they can be stored and sown, and have less chance to be virus infected than whole plants or plant organs.

Apomixis is widespread in nature, and occurs in several unrelated plant families. About 10% of the estimated 400 plant families and a mere 1% of the estimated 40 000 plant species they comprise exhibit apomixis. The plant families with the highest frequency of apomixis are Gramineae (Poaceae), Compositae, Rosaceae, and Asteraceae. Many species of Citrus , mango, perennial forage grasses, and guayule reproduce apomictically.

Some species can produce both sexual and apomictic seeds and are called partial apomicts(e.g. bluegrass, Poa pratensis ). Species such as bahiagrass ( Paspalum notatum ) reproduce exclusively or nearly so by apomixis and are called complete apomicts. There are several indicators of apomixis. When the progeny from a cross in a cross‐pollinated species fails to segregate, appearing uniform and identical to the mother plant, this could indicate apomixis. Similarly, when plants expected to exhibit high sterility (e.g. aneuploids, triploids) instead show significantly high fertility, apomixis could be the cause. Obligate apomicts may display multiple floral features (e.g. multiple stigmas and ovules per floret, double or fused ovaries) or multiple seedlings per seed. Facultative apomixis may be suspected if the progeny of a cross shows an unusually high number of identical homozygous individuals that resemble the mother plant in addition to the presence of individuals that are clearly different (hybrid products).

The indicators suggested are by no means conclusive evidence of apomixis. To confirm the occurrence of apomixis and discovery of its mechanisms requires an additional progeny test as well as cytological tests of megasporogenesis and embryo sac development.