Research HighlightsBasic Research into the Pathogenicity of Sclerotinia Sclerotiorum
by Mehdi Kabbage and Damon L. Smith, Soybean Plant Pathologists, University of Wisconsin-Madison
Sclerotinia stem rot (SSR), also known as white mold of soybean, can be a significant yield-limiting disease in the north-central United States. The fungal pathogen, Sclerotinia sclerotiorum is one of the most successful of all plant pathogens — with an ability to infect over 350 plant species.
With soybean checkoff funds provided by the Wisconsin Soybean Marketing Board, the North Central Soybean Research Program, and the United Soybean Board, as well as the National Science Foundation, we have been able to make significant progress in our understanding of how Sclerotinia sclerotiorum is able to hijack plant defenses and cause disease, and in this process have revealed promising genetic targets for durable host resistance.
Our study was prompted by previous observations that molecules called “reactive oxygen species” (ROS) play a key role in regulating pathogen-host interactions in plants under pathogen attack. We know that ROS molecules are regulated by a group of plasma membrane-bound proteins, called NADPH oxidases, so we conducted basic research into the underlying mechanisms of ROS generation in soybean in response to S. sclerotiorum.
Our results show that a group of genes controlling soybean NADPH oxidase production are specifically induced in the plant following S. sclerotiorum infection, with peak expression at the later stages of the infection process. Thus, it appears that S. sclerotiorum may be co-opting the soybean ROS machinery to its benefit, by modulating the expression of host NADPH oxidases.
We used Virus Induced Gene Silencing (VIGS) to turn off, or “silence”, this group of genes to see if they are required for disease development. The VIGS system is an established technique in soybean molecular genetics used to study the function of genes, particularly the function of disease resistance genes and defense genes involved in plant-microbe interactions.
Using the VIGS system, we were able to achieve a 45 to 65% reduction in transcript levels in silenced plants compared to the control. The silenced soybean plants were then evaluated for their response to S. sclerotiorumchallenge. Five days following inoculation with the pathogen, the control plants showed typical SSR symptoms and began to wilt. In contrast, the silenced plants did not show any wilting symptoms and developed normally.
This is a remarkable result that shows that silencing of a specific group of soybean NADPH oxidase genes leads to markedly decreased ROS production and enhanced resistance in soybean against infection by S. sclerotiorum. These genes provide a potential target for the generation of SSR-resistant soybean lines.
We expanded our screen of silenced plants to include other pathogens and abiotic stress. Surprisingly, we found that these plants were also drought tolerant — possibly because decreased ROS production would limit the oxidative damage and ultimately death of the plant imposed by excessive ROS levels during drought stress.
Because of the significance of the traits associated with the silencing of these genes, we plan to generate stable transgenic plants that will be tested in the field against a broad range of abiotic and biotic factors.