Disease control

Science programme: Applied plant pathology

The Mt Albert-based mapping team is involved in four major projects:

• Cloning major disease resistance genes
• Development of the technologies to characterise large DNA insert libraries
• Developing genetic maps of apple and developing molecular markers for these maps
• DNA-based fingerprinting in apples and kiwifruit.

A fundamental hypothesis that has underpinned plant pathology for several decades is the gene-for-gene hypothesis first developed by Flor in the 1940s. He discovered that the basis of plant resistance to pathogens and pests can often be traced to a single major plant gene that appears to interact with a single gene in the pathogen or the pest. When both of these genes are present there is natural resistance; however if one of these genes is missing (in either in the plant or in the pathogen), then disease results.

The first of these major disease resistance genes was cloned from tomato and its DNA sequence identified in 1994. Since then more than 40 of these genes have been isolated from various species of plants. This information has increased considerably our understanding of how a plant mounts resistance against its enemies. It appears that these genes act as a kind of surveillance system that detects the presence of an attacker at an early stage and activates a series of reactions that give protection against that specific attacker. This knowledge has given rise to more recent theories such as the guard hypothesis.

This team has mined the HortResearch EST databases, identified disease resistance gene candidates in apple and used these candidates to identify where on the chromosomes of apple these genes are situated. The team has focused on identifying one of the specific genes responsible for protecting apples from a major fungal disease and has made significant progress in identifying the gene responsible.

Along the way they have developed many of the resources and technologies required to allow this to be done on a more routine basis in future. In particular, large DNA insert libraries and methodologies to work with these libraries have been developed, and these in turn have led to the team being involved in other projects that utilise these libraries.

The team also develops DNA-based markers for apple that are then combined to produce genetic maps of apple. These markers and maps allow us to identify where genes of interest are located on apple chromosomes, allow us to select for useful traits by using these markers, and are also a good starting point for identifying the specific genes responsible for the trait.

The genetic markers developed by the team can also be used for DNA-based fingerprinting techniques, and on a commercial level the team supplies clients with fingerprinting services for apple and kiwifruit.

The team interacts with other researchers at HortResearch in:

• Bioinformatics
• New varieties
• Gene cloning and sequencing
• Plant breeding
• Gene transformation
• Plant pathogen interaction
• Proteomics.