Wheat and Cereals

The Wheat Genome

The growth characteristics of wheat are controlled by the genes within the cells of the plant. The cells are linked into long chains, the chromosomes, that together irrespective of the number of chromosomes that the organism possesses, form the cell genome. These chromosomes are the location of the genes that control the way the plant grows. Bread wheat has a large genome of 42 chromosomes, created during the evolution of wheat from three wild species. When the organism reproduces itself, copies of the chromosomes are transferred to the progeny.

The Corn Harvesters

Wheat was originally a very tall plant, as seen in "The Corn Harvesters" painted by Pieter Bruegel the Elder in 1565. It shows wheat growing up to the heads of the men harvesting it with scythes.

Most modern combines are unable to harvest tall cereals with long straw due to blockage problems. After the discovery of shorter landraces in Japan, wheat breeders transferred selected height reducing (Rht) genes from different landraces to create the shorter but higher yielding wheat varieties more suited to modern farming.

What are Landraces?

A landrace is a collection of plants that have adapted to growing under particular local conditions, such as the soil type and the weather. They are typically created by farmers saving and re-sowing their own seeds for many years. Plants that are unable to survive the conditions will die, leaving the adapted plants to produce seeds. After adaptation to a particular locality, the seed stock may be unsuitable for growing at locations with different conditions.

Height Reducing Genes

The picture below shows the wheat variety "Mercia" with a control and four lines with different Rht genes. The Rht (Control) plant on the left, without height reducing genes, is used as the control for height comparison.

Most wheat varieties now include height reducing genes, usually Rht-1 or Rht-2, to control the height of the plants. Rht-1 (Rht-B1b) and Rht-2 (Rht-D1b) typically produce semi-dwarf plants, about two-thirds the height of the control. Rht-3 (Rht-B1c), Rht-10 (Rht-D1c) and Rht-12 typically produce dwarf plants one-third the height of the control.

Plants with the Rht-12 gene also have awns on the ears. The awn and Rht genes are believed to be located very close together on the same chromosome and at the time of writing this article, wheat geneticists have been unable to separate the two genes.

Wild Einkorn and Goat Grasses

Wild Einkorn and some Goat Grasses are diploid plants, the cells having 14 chromosomes, or seven matching pairs. When any pollen and ovules are created, one chromosome from each pair transfers to the new cell, giving seven chromosomes in each of the pollen or ovule cells. If pollinated by the same species, the pollen and ovule will have matching chromosomes and the plant will remain a diploid plant with 14 chromosomes. If they are from two different species, the chromosomes will be unmatched and any progeny would normally be sterile.

Wheat Hybrids

In the case of wheat hybrids, chromosome doubling or amphiploidy occurred. Each chromosome could then combine with the replicate to form matching paired chromosomes, similar to a normal pollination. Naturally occurring amphiploidy and mutations have created many tetraploid wheats with 28 chromosomes and hexaploid wheats with 42 chromosomes. You can see a table of these here.

Chromosome Diagrams

These diagrams shows the origin of the chromosomes that created the genomes of modern wheats, Durum Wheat, a tetraploid with 28 chromosomes, and Spelt and Bread Wheat, hexaploids with 42 chromosomes.

Modern Diversification

Scientists can now create Synthetic Hexaploid Wheats from tetraploid wheats and diploid goat grasses by using colchicine to induce chromosome doubling. These can be used to diversify the wheat genome by transferring selected genes, such as those responsible for grain yield or disease resistance into modern wheat varieties.