Molecular Maps of Crop Plants

Arabidopsis thaliana had been used as a model plant for mutagenic and genetic studies in the 1970s. The advantages of this species as a model for molecular studies became apparent in the 1980s because of its small nuclear genome size, low repetitive DNA content, short life-cycle, large seed production and later its amenability to transformation. As a result, through international collaborations, a genomic sequencing programme was established and the complete DNA sequence of the Arabidopsis genome was completed in 2000. The rice genome is only about four times the size of the Arabidopsis genome and, as a model cereal and important food crop, sequencing of this genome is also well advanced. Massive sequencing efforts of ESTs of wheat, barley, soybean, rice, Medicago truncatula and other crops are also in progress, mainly driven by major life sciences companies.
Much interesting information on genome organisation has resulted from this work, including knowledge of location of genes, gene clustering and repetitive and non-coding sequences. For cereals, gene arrangements show ‘synteny’ in that major blocks of genes are arranged in similar sequences in rice, maize, barley, wheat, etc. The major differences in genome size is the result of different amounts of repetitive/non-coding sequences and, for wheat, the fact that it is hexaploid and contains three sets of progenitor genomes.
For Arabidopsis, it emerges that there are about 22 000 genes required to contain all the information for this organism. For other plants, we might expect the number of genes present to be between this figure and about 50 000 genes.