Chapter 4
The genetic code
All living plant cells contain a nucleus which controls every activity in the cell. Within the nucleus is the chemical deoxyribonucleic acid (DNA), a very large molecule made up of thousands of atoms. DNA contains hundreds of subunits (nucleotides), each of which contains a chemically active zone called a ‘base’. There are four different bases: guanine, cytosine, thymine and adenine. The sequence of these bases is the method by which genetic information is stored in the nucleus, and also the means by which information is transmitted from the nucleus to other cell organelles (this sequence is called the genetic code). A change in the base sequencing of a plant’s code will lead to it developing new characteristics. These very long molecules of DNA are called chromosomes. Each species of plant has a specific number of chromosomes. The cells of tomato (Lycopersicum esculentum) contain 24 chromosomes, the cells of Pinus and Abies species 24 and onions 16 (humans have 46). Each chromosome contains a succession of units, called genes, containing many base units. Each gene usually is the code for a single characteristic such as flower colour or disease resistance. Scientists have been able to correlate many gene locations with plant characteristics that they control. Microscopic observation of cells during cell division reveals two similar sets of chromosomes, e.g. in tomatoes a total of 12 similar or homologous pairs. The situation in a nucleus where there are two sets of chromosomes is termed the diploid condition. A gene for a particular characteristic, such as flower colour, has a precise location on one chromosome, and on the same location of the homologous chromosome. For each characteristic, therefore, there are at least two alleles (alternative forms of the gene), one on each chromosome in the homologous pair, which provide genetic information for that characteristic. The fact that every living plant cell which has a nucleus has a complete set of all genetic information (totipotency) means that cells have the information to become any specialized cell in the plant. Therefore, when organs are removed from their usual place, as in vegetative propagation, they are able to develop new parts, such as adventitious roots, using this information. Vegetative propagation is described in detail in Chapter 4.
Inheritance of characteristics
Genetic information is passed from parent to offspring when material from male and female parent comes together by fusion of the sex cells. Genes from each parent can, in combination, produce an intermediate form, a mixture of the parents’ characteristics in the offspring; e.g. a gene for red flowers inherited from the male parent, combined with a gene for white flowers from the female parent, could produce pink-flowered offspring, if both conditions are equal (see Figure 1).
If one of the genes, however, completely dominated the other, e.g. if the red gene inherited from the male parent was dominant over the white female gene, all offspring would produce red flowers. The non-dominating (recessive) white gene would still be present as part of the genetic make-up of the offspring cells and could be passed on to the next generation. If it then were to combine at fertilization with another white gene, the offspring would be white-flowered. This is the basis of more extensive principles of genetics used by plant breeders to develop new cultivars.
Figure 1 The pattern of inheritance genes
Mutations
Spontaneous changes in the content or arrangement of chromosomes (mutations), whether in the cells of the vegetative plant or in the reproductive cells, occur in nature at the rate of approximately one cell in a million. These changes to the plant DNA are one of the most important causes of new alleles leading to changes in the characteristics of the individual. Extreme chromosome alterations result in malformed and useless plants, but slight rearrangements may provide horticulturally desirable changes in flower colour or plant habit. Such desirable mutations have been seen in plants such as chrysanthemums, dahlias and Streptocarpus. Mutation breeding also produces these variations but using irradiation treatments with X-rays, gamma rays or mutagenic chemicals, which increase the mutation rate. In both situations (natural mutations and induced mutations), the mutation only becomes significant in the plant when the mutated cell originates in a meristem, where it proceeds to create a mass of novel genetic tissues (and organs).
When a shoot with a different coloured flower or leaf arises, it is often referred to as a sport. A more extreme example of a mutation is a chimaera (see Figure 1). This occurs when organs (and even whole plants) have two or more genetically distinct kinds of tissues existing together. This often results in variegation of the leaves, as seen in some Acer and Pelargonium species. Horticulturists use one form or other of vegetative propagation to preserve and increase the genetic novelty. These useful mutations may give rise to potential new cultivars in just one generation.
Polyploids
Polyploidy occurs when duplication of chromosomes fails to result in mitotic cell division.
Polyploids are plants with cells containing more than the usual (diploid) number of chromosomes, e.g. a triploid has three times the haploid number, a tetraploid four times and the polyploid series continues in many species up to octaploid (eight times haploid). An increase in size of cells, with a resultant increase in roots, fruit and flower size of many species of chrysanthemums, fuchsias, strawberries, turnips and grasses, is the result of polyploidy. There is a limit to the number of chromosomes that a species can contain within its nucleus. Polyploidy occurs when duplication of chromosomes (see mitosis) fails to result in mitotic cell division. The multiplication of a polyploid cell within a meristem may form a complete polyploid shoot that, after flowering and fertilization, may produce polyploid seed. Polyploidy can occur spontaneously, and has led to many variant types in wild plant populations. It can be artificially induced by the use of a mitosis inhibitor, such as colchicine.
Flower structure
Flower structure shorthand The flower can be described in shorthand using a floral formula or a floral diagram. K = calyx C = corolla A = androecium G = gynoecium = regular flower ./. = irregular flower For example, the flora formula, with the interpretation, for wallflower (Cheiranthus cheiri; Figure 2), a member of the Brassicaceae family, is as follows:
Other examples of floral formulae include:
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