I have been asked to talk today about the influence of new technologies on plant improvement. As a geneticist, I will discuss chromosomes, genes, and DNA, and the usefulness and potential of molecular genetics to plant breeding. As a wheat geneticist, I may tend to emphasise wheat and its relatives.

Plant Genomes

The genome of any organism is mainly comprised of nuclear DNA arranged into chromosomes. The size and total number of chromosomes varies markedly from species to species so the total amount of DNA present in the nucleus of a single cell also differs from species to species (Table 1). However, whether it is a 10 cm tall plant of Arahidopsis (much used by geneticists), a 2 m tall maize plant, or a human being, the actual number of genes that are present and necessary to fulfil the functions of life is probably in the vicinity of 20,000. As diploid (2x) species have at least two copies of each gene and as the average length of each gene is about 1200 nucleotide base pairs (bp), the amount of essential DNA is then the product of 20,000 x 2 x 1200 = 48 x 10⁶ bp.

As some genes can be present in larger numbers - for example, we shall see that there are thousands of copies of ribosonal- RNA (rRNA) genes in wheat - we can assume that at least 50 x 10⁶ bp of DNA are essential for the life of any plant or animal. From this, we can estimate that the genome of Arabidopsis probably contains about 20 x 10 bp of DNA with no obvious function. This so-called junk DNA or non-gene DNA comprises about 30% of the total DNA present in this plant. As the other diploid species listed require the same amount of gene DNA, we can see that in many organisms non-gene DNA comprises a massive proportion of the total amount of DNA present (Table 1).

Table 1. The amounts of DNA in different organisms.

Thus, if we are trying to find a particular gene, say one concerned with photosynthesis, it may be more easily found in Arahidopsis than in a cereal. Once isolated, that gene may then be used to locate the similar gene in wheat. Such techniques have been given the acronym “MAGIC” - for ‘Molecular Assisted Genetic Improvement of Plants’.

Genes and their Location

Even though modern day wheat is the result of thousands of years of selection, its evolutionary history can be read in its chromosomes. Wheat contains 21 pairs of chromiosomes, seven pairs derived from each of the diploid species that intercrossed to produce hexaploid wheat. Because the three species were related by evolution, so also are their individual chromosomes and the genes carried on these chromosomes. This means that if a pair of chromosomes or chromosome arms is deleted by chromosome engineering, the loss is generally compensated for by related genes on related chromosomes. If the genes are not identical, i.e. are allelic variants, we can find out which genes or alleles are deleted by the removal of chromatin and therefore place these genes onto particular chromosomes. To determine the position of a gene, we must, of course, be able to detect a genetic difference in the first place. In plants, we can generally detect:

(a) obvious morphological differences, such as differences in height, the presence or absence of awns, degrees of leaf waxiness, maturity, or flower colour;

(b) the presence or absence of genes for disease resistance;

(c) biochemical markers, such as proteins and enzymes; or

(d) some molecular markers.

In very few plants are the locations of more than 200 or so genes actually known and these comprise a very small percentage of the total number of genes present. In general, we do not know where the majority of genes are located and, furthermore, we do not know how most genes actually operate. The situation is further complicated in that many agronomic characters appear to be controlled by two or more genes, or by interactions between different genes. Such polygenic characters are even more difficult to locate. Yet it is these characters, such as yield, quality, adaptation to differing environments, response to nutrients or tillage conditions, that are equally important to a plant breeder. In contrast to characters controlled by single dominant or recessive genes, the gene locations of multigenic characters are described as Quantitative Trait Loci (QTL). How can the genes controlling these traits be found, and how can desirable combinations of these genes be incorporated into newplant varieties?

In an ideal situation, a breeder knows the characters that are wanted in a variety. The breeder may know the most desirable source of these attributes and the crosses that should be made to incorporate new combinations of desirable attributes into new cultivars. Consequently, to be of use to the applied breeder, ‘MAGIC’ must help the breeder to achieve these aims. There are a number of ways in which this might be done.