Below ground crop/weed interaction

Reducing herbicide use is an important social and environmental goal, as concern is growing about the development of herbicide-resistant weeds and the ecological consequences of herbicide application. Several cover crops are known to successfully suppress weeds, providing a pertinent answer to this problem. However, in order to use cover crops adequately, the mechanisms of weed suppression need to be elucidated and understood. Besides direct resource competition, growth repression through root interactions can play a decisive role. However, so far rhizosphere interactions of two neighboring plants have received little attention in the scientific community. In previous experiments we could demonstrate that below ground interactions between the cover crops Fagopyrum esculentum (buckwheat) and Avena strigosa (black oat) led to growth suppression of Amaranthus retroflexus (redroot pigweed), presumably induced by specific cover crop root exudates. Based on these findings, we aim to further investigate cover crop root exudates and to identify putative growth suppressive compounds. We will test six research hypotheses: (H1) The selected cover crops can recognize the presence of heterospecific neighbors via interacting root systems, which leads to a systemic modification of cover crop root exudation. (H2) Certain compound groups and/or specific molecules respond to a species-specific recognition, while others respond more generally to the presence of another plant. (H3) Certain compound groups and/or specific molecules from cover crop root exudates are responsible for growth suppressive effects. (H4) Growth suppressive effects are reflected by transcriptome changes of Arabidopsis thaliana and Brachypodium distachyum. (H5) Arabidopsis thaliana and Brachypodium distachyum root exudation is altered by the presence of different cover crops. (H6) Putative compounds responsible for growth repression can be detected in agricultural soil. Our methodological approach employs a validated split-root set-up enabling differential root exudate collection and analysis. First experiments will be performed in undisturbed soil-free glass bead cultures. Differential chemical analysis of the collected exudates will follow an already implemented workflow utilizing accurate mass spectrometry in combination with fit-for- purpose separation methods. Identity confirmation of significant compounds will make use of dedicated accurate mass databases including information on fragment spectra. Subsequently, root exudates will be collected from soil grown roots and rhizosphere soil solution to confirm their presence in soil. Moreover, phenotypic and transcriptomic changes induced by direct interaction of roots and the impact of selected compounds will be studied in Arabidopsis thaliana and Brachypodium distachyum. The results of the studies will provide novel insights in belowground plant-plant interactions and provide information for the development and use of weed suppressive cover crops, as a step towards new cultural control strategies for integrated weed management.

The major research question of the project conducted as a collaboration between the Institute of Analytical Chemistry at BOKU University in Vienna, and AGROSCOPE in Changins, Switzerland was if selected cover crops can recognize the presence of heterospecific neighbours via interacting root systems, which leads to a systemic modification of cover crop root exudation. Moreover, we have investigated if certain compound groups and/or specific molecules are exuded in respond to a species-specific recognition, while others are exuded more generally in respond to the presence of another plant. A novel plant growth setup allowed for a cover crop to be grown with its roots split between two compartments. The first compartment contains only roots isolated from the cover crop while the second contains roots from both the cover crop and a neighboring species. This allows the measurement of exudates isolated from a cover crop which has also been grown in the presence of a neighboring plant. Root exudate collection and analysis was performed with newly developed sample extraction protocols as well as high resolution mass spectrometry based non targeted analytical workflows. The major biological conclusions from this research show that root exudate metabolite profile of buckwheat and black oat do change in response to neighboring plants in general and that the root system and morphology of both buckwheat, black oat and the weed neighbours pigweed and black grass are impacted by their interaction. A further conclusion from this research is that a set of compounds have been observed which have been upregulated by the different cover crops in the presence of different weeds. While absolute structure elucidation for these compounds has not yet been performed, a database match has been successful for a small portion of the upregulated compounds and compound classes have been predicted for most of them. The finding that cover crops appear to recognize weeds and modify their exudation has important implications on the ability to use cover crops to control weed species. Isolating the metabolites which have been highlighted in the work performed would allow for the testing of these compounds in future research to see how they directly impact weeds. At the same time our project also has generated new research questions. First, what is the absolute structure of these compounds? What is the stability of these exudates and what is their persistence in the soil? What is their impact on the environment? What is the best method for weed control using cover crops i.e., is it better to use cover crops as a mulch or is their active exudation in the field more effective?