Stem rust resistance in wheat is suppressed by a subunit of the mediator complex

Stem rust is a disease of wheat that is caused by a fungus and is capable of devastating a crop. Stem rust can be controlled by using genes that make the wheat plant resistant to the fungus. A gene was previously identified that prevents some stem rust resistance genes from functioning. This type of gene is called a suppressor. When this gene is knocked out by mutation, plants are resistant to strains of the stem rust fungus that would normally cause disease. Fifty years since the discovery of the suppressor gene, we have determined the DNA sequence of this gene and the type of gene responsible for suppressing stem rust resistance. To achieve this, intact chromosomes were isolated from plants with the suppressor and also from plants that had mutated forms of the suppressor gene. Using a technique that sorts chromosomes, the chromosome carrying the suppressor gene was isolated from each plant type. Following chromosome sorting, the resulting DNA samples were sequenced and we were able to use the DNA sequence information to 1) map the position of the gene using a mapping population, 2) identify the gene responsible for suppressing stem rust resistance, and 3) explain what changes took place in each of the mutant plants that rendered the suppressor gene non-functional. The gene responsible for suppressing stem rust resistance codes for a subunit of a large complex (Mediator complex) that controls when genes are turned on or turned off. Experiments were we sequenced RNA in normal and mutant plants identified chromosome large regions that were co-regulated by this subunit of the Mediator complex. Our results demonstrated an unknown function of the Mediator complex which adds knowledge to fundamental biology. Also, we can use this knowledge to unlock stem rust resistance that may be present in wheat cultivars. Finally, studying Mediator subunits may allow new disease resistance to be unlocked in wheat and in other crop species.