Developmental ecology of pre-infection responses of Pythium species to plants
Pythium ultimum and Pythium aphanidermatum are among the most widespread and destructive oomycete plant pathogens. The emergence from dormancy in these species is particularly intriguing since persistent asexual sporangia and sexually-derived oospores will only break dormancy upon sensing molecules in plant exudates that trigger germination and initiate pathogenic development. The main goal of our project is to understand the mechanisms by which molecules released from seeds break sporangial dormancy and how seed-associated microbes interfere with these responses.
- Differential interference with Pythium ultimum sporangium activation and germination in the spermosphere by Enterobacter cloacae.
- Interference of Pythium aphanidermatum zoospore chemotaxis by spermosphere microbes.
- Fatty acid-induced transcriptional networks in germinating sporangia of the oomycete, Pythium ultimum.
The nature of compost-induced suppression of Pythium damping-off
Disease suppressive properties of compost amendments are well known and the spectrum of pathogens and diseases that can be effectively managed by compost amendments has been well documented. Many studies of compost-induced disease suppression have focused on diseases caused by Pythium species where suppressiveness has been linked directly to microbial populations and activities. However, little is known of the specific microbes involved in suppressiveness and the processes by which suppression is achieved.
We are particularly interested in discovering the microbial populations that contribute to the suppression of Pythium damping-off. Our approach is to look directly at the Pythium infection court for clues to the microbes involved in disease suppression. Since seeds represent the primary infection court for Pythium species, we believe that spermosphere microbial communities are likely to have strong effects on disease suppression. We study two very different species of Pythium: P. ultimum and P. aphanidermatum. With each pathosystem, we are interested in understanding how spermosphere microbial communities interact with these pathogens to affect disease suppression.
- Identification and characterization of seed-colonizing microbial communities from municipal biosolids compost that suppress Pythium ultimum damping-off.
- Mechanisms of Pythium aphanidermatum suppression in vericomposted dairy manure.
Soil-plant feedback mechanisms regulating oomycete community structure and plant species invasions
Wetlands represent one of the richest sources of biodiversity on earth. Yet, wetland habitats are extremely vulnerable to invasions by introduced plant species. In fact, nearly a quarter of the world’s most invasive plants are wetland species. Such invasions destroy biodiversity and alter ecosystem functioning. One of the more important invasive wetland species is the common reed, Phragmites australis. Although native to the US, an introduced European genotype is considerably more aggressive, excluding native species and forming monoculture stands. Established stands expand rapidly through rhizome growth whereas seeds are believed to be responsible for initial establishment. Invasive plants such as Phragmites australis appear to hijack many of the stabilizing mechanisms that regulate plant communities. Our research seeks to learn how this uncoupling occurs in order to understand the influence of natural or human-induced changes on these regulatory mechanisms and how they might be manipulated for plant management.
A broad aim this newly-initiated research with Phragmites is to understand the invasive success of introduced plants and the invasibility of native plant habitats. Currently, mechanisms that facilitate invasion success are not known. One widely tested hypothesis predicts that plant species introduced to an exotic habitat are less affected by natural enemies such as herbivores, parasites, and pathogens than in their native habitat. This reduced regulation allows for new habitat exploitation by invaders. There have been few tests of this hypothesis with natural enemies other than herbivorous insects. However, there is increasing evidence that plant pathogens, provide important regulatory feedbacks that influence plant communities. This will be a main focus of our proposed research.
As with plants and animals, wetlands are rich in microbial diversity, including fungal and oomycete pathogen diversity. Oomycetes, in particular, are known seed and seedling pathogens that thrive in wetland sites. It is known that fungi and oomycetes regulate native plant communities. Yet, the regulatory impact of pathogens on invasion success in wetlands remains obscure. The focus of our study therefore, is on the fungal and oomycete pathogens of Phragmites as a means determining their role in Phragmites invasions. An alternative explanation for invasion success is that invasive and native species are equally susceptible to pathogen regulation, but invaders selectively attract rhizosphere microbes that are natural enemies of pathogens. This phenomenon has been exploited for biological control efforts in agriculture and will be another major focus of our work.
We plan to generate important new knowledge essential for developing new invasive plant management programs and directing conservation efforts for wetland habitats. We believe that more detailed knowledge of what makes plants invasive and what makes native species invasible will aid in our overall mitigation efforts. An outcome of our work is to examine the possibility of shifting the focus of management efforts away from eradicating the invader to protecting the native species. We believe the results of this study will bring us one step closer to this goal.
- Pathogen regulation of invasive plant species: Mechanisms and potential for the management of Phragmites australis