Why doesn’t plant genetic resistance last?
Genetic resistance is often only transient due to the constant evolution of disease-causing pathogens. Faced with these ever-changing pathogen populations, breeders are forced to constantly identify new resistance genes and introduce them into elite cultivars to maintain protection in commercial crops.
Resistance genes often recognise a corresponding molecule produced by a pathogen. Upon this recognition a defence response is activated in the plant to give resistance. However, pathogen populations can rapidly change or lose a recognised molecule to avoid recognition by a plant resistance gene product, causing resistance breakdown.
Stacked genes: A gene resistance sandwich
An ever-growing number of wheat rust resistance genes have been cloned, and most of these gene products each recognise a unique molecule produced by the rust pathogen. If these genes are used singularly they tend to breakdown rapidly due to the dispensable nature of the pathogen molecules being recognised. However, while the pathogen can readily lose one recognised molecule, it becomes progressively harder and harder to lose more and more of these molecules all at the same time.
This is why we have worked to combine resistance genes together, and give more durable resistance. Resistance genes to wheat rusts, and other diseases, can be combined by breeding. However, this is laborious to do and even harder to keep all the genes together in subsequent breeding. To overcome these difficulties we have combined (stacked) five different stem rust resistance genes at a single locus.
Longer lasting pathogen protection
Using cloned wheat stem rust resistance genes, we have physically combined five stem rust resistance genes into a single molecule, in addition to a 35S-hygromycin selectable marker gene2. This DNA construct we named the Big 5.
All these genes were introduced into a single location in the wheat genome using a bacterium, Agrobacterium, that naturally introduces DNA into plant cells. As these genes are physically linked, they will remain together as a single unit when used for breeding and the multigene nature of the resistance promises to increase the longevity of all the resistance genes in the stack. This was challenging because combining all these large genes together results in a very large molecule to manipulate and transform. To achieve this, we had to develop a unique sequential cloning system and screen a large number of transgenic wheat lines to find ones that had all the genes present. These lines, however, showed very high levels of field resistance to wheat stem rust and were resistant to a diverse global panel of wheat stem rust isolates. More gene stacks to stem rust and stripe rust are now being developed.
This work was undertaken in collaboration with the University of Minnesota (USA), Aarhus University (Denmark), John Innes Centre (UK), United States Department of Agriculture and Xianjing University (China) and Funded by the 2Blades Foundation.