Box 2.6:  New Approaches to Plant Disease Resistance

The new developments in genomics offer means for the more targeted development of host resistance to pathogens, based on understanding of the genomic make up of the host and its pathogens, and their inter-actions in different environments.

In developing new resistant cultivars of crop plants, a desired approach is where disease resistance genes control pathogens at low metabolic cost by inducing defense responses only in those cells that are challenged by the pathogen, thus minimizing any yield penalty in the cultivated crop.

Genomic approaches are increasing understanding of the genetic basis of plant disease resistance, through greater understanding of resistance genes themselves and other genes and the pathways that they regulate in the plant.

For example, through structural genomics, large-scale sequencing is being used to reveal the detailed organization of resistance gene clusters and the genetic mechanisms involved in generating new resistance to specific pathogens. Global functional analysis is being used to understand the complex regulatory networks and the diversity of proteins involved in resistance and susceptibility in plants.

Comparative genomics: Early studies are showing that significant blocks of genetic material are shared among genomes of related species (eg cereals). As the genomes of more plant species are sequenced and compared, it may become possible to predict the position of some genes in various parts of the genome. A few studies are looking at the sharing (synteny) of resistance genes within cereals, grasses, and Solaneaceous species (potato, tomato). Early results suggest that there is relatively little sharing of disease resistance genes even amongst related species.

Functional genomics: It is anticipated that catalogues of genes expressed under a range of different conditions, in different organs, or in different individuals, will become available. The global analysis of plant gene expression is still in its infancy and its full potential is far from being realised. Genes that have altered expression in compatible and incompatible plant-pathogen interactions are being characterised, through microarray analysis. For example, in maize, over 100 genes may be involved in the response to a single fungus.

These genes need to be further examined on a gene by gene basis to determine which genes produce proteins that are important in causing resistance in the plant to the pathogen. This may be done by various techniques (such as viral-induced gene silencing, viral over-expression, gene knock-out and promoter-trap strategies). In additional to testing the function of individual genes, these strategies can also be used with libraries of unknown sequences for gene discovery. It is likely that each of these approaches will be able to demonstrate the function of some but not all genes, and a combination of strategies will be required.

Discovery and cloning of disease resistance genes: About twenty disease resistance genes have now been cloned. This has required expensive map-based cloning or transposon-tagging of individual genes. Resistance gene discovery will become much faster when resistant phenotypes are matched to candidate sequences identified by genomic sequencing. The present limiting step in the discovery of disease resistance genes is the confirmation of the function of individual genes. New approaches are speeding up the introduction and evaluation of individual genes in crops, for example through the use of viral vectors to directly introduce candidate genes into the target plant for evaluation, rather than introducing the genes through a time-consuming transformation and regeneration process.

Source: Michelmore 2000.

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