Location: Mycology and Nematology Genetic Diversity and Biology LaboratoryTitle: The diversification of downy mildew species was not driven by the loss of mycorrhizal associations or the evolution of C4 photosynthesis
|DAVIS, WILLIAM - Oak Ridge Institute For Science And Education (ORISE)|
|Crouch, Jo Anne|
Submitted to: PhytoFrontiers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/30/2021
Publication Date: 9/2/2021
Citation: Davis, W., Crouch, J.A. 2021. The diversification of downy mildew species was not driven by the loss of mycorrhizal associations or the evolution of C4 photosynthesis. Phytofrontiers. 2:60-65. https://doi.org/10.1094/PHYTOFR-04-21-0027-R.
Interpretive Summary: Downy mildew diseases cause significant problems for many different plants in agriculture and the environment. For two decades scientists have theorized that downy mildew pathogens have evolved to take advantage of chemicals called secondary metabolites produced by plants. To do so, it was thought that downy mildews would primarily infect plants that produce more carbon than the plants can use such as those whose roots are not colonized by fungi (non-mycorrhizal) or that make use of a particular photosynthetic pathway (C4). We tested this prediction and found it was not supported by the overall host range of downy mildews. Instead, the majority of downy mildew pathogens infect plants that are mycorrhizal and use the C3 photosynthetic pathway. This information will be useful for mycologists and plant pathologists working to understand the factors that influence the association between plants and downy mildews.
Technical Abstract: There are approximately 700 obligate biotrophic species grouped into 20 genera (Oomycota, Peronosporaceae) that cause downy mildew diseases. Dick hypothesized in 2001 that diversification of downy mildew species was driven, in part, by host plant secondary metabolites. Dick further speculated that this was driven by the transition of host plants away from mycorrhizal associations or the evolution of C4 photosynthesis. Specifically, loss of mycorrhizal associations or the use of C4 photosynthesis would result in more free carbon that the plants could then use to produce more secondary metabolites. If true, then there should be more downy mildew species that infect hosts from plant lineages that lack mycorrhizal associations or use C4 photosynthesis. However, analysis of 677 downy mildew species for mycorrhizal associations and photosynthetic pathway type shows that this is not what occurred. Seventy percent of downy mildew species parasitize hosts that form mycorrhizal associations and 94% of downy mildew species parasitize hosts that use C3 photosynthesis. From this it is concluded that the diversification of downy mildew species was not driven by the loss of mycorrhizal associations or the evolution of C4 photosynthesis. However, 85% of downy mildew species that parasitize Poaceae (grasses) parasitize C4 hosts. Thus, it is possible that C4 photosynthesis plays a role in the diversification of these genera.