A project undertaken at The University of Adelaide, and supervised by Dr Tatiana Soares da Costa.
Weeds represent one of the biggest threats to our environment and agricultural industry. They abolish native habitats, destroy the balance of ecological communities and aggressively compete with crops for resources, resulting in decreased harvest yields and quality. The most effective and economical strategy for controlling weeds is herbicide use. However, herbicide resistance is increasingly threatening the utility of our current herbicides.
Herbicide resistance was first recognised in Australia in 1981 and since then, we have accrued 89 unique resistant weeds (species × herbicide class). This figure places us second in the world for the highest number of resistant varieties. Of particular concern is annual ryegrass. In Australia alone, herbicide-resistant annual ryegrass invades over 8M hectares of land, resulting in decreased crop yields of >340K tonnes and revenue losses of >AUD$90M annually. Commercialising herbicides with new modes of action is notoriously difficult, which is reflected by the lack of new products brought to market over the last 40 years.
Preserving the efficacy of our current herbicides is crucial to conserve biodiversity and ensure food security. Understanding how resistance is generated and proliferated paves the way for the development of tools to combat it. Moreover, the monitoring and early detection of herbicide resistance is essential so that intervention strategies can be implemented to minimise the spread of resistance. Our team of early career researchers has identified a potential mechanism through which weeds become resistant to herbicides that remains completely unexplored to date. Specifically, this project aims to investigate for the first time the role extracellular vesicles play in resistance mechanisms that have evolved in weeds such as annual ryegrass.
Extracellular vesicles (EVs) are nano-sized capsules secreted by cells, that contain cargo for delivery to recipient cells. The cargo encapsulated in EVs includes genetic information in the form of DNA and RNA. Additionally, EVs contain proteins, which have myriad functions within an organism. The transport of these cargo in EVs between cells is therefore imperative for cell-to-cell communication.
EVs are ubiquitous across all kingdoms of life. The role of EVs in the emergence and spread of antibiotic and cancer drug resistance is well established. EVs can mediate drug resistance through a variety of mechanisms. The cargo of EVs includes proteins that can detoxify drugs, rendering them ineffective. Additionally, the efficacy of drugs can be reduced by EV cargo proteins that sequester drugs, resulting in the export of the drug from the cell along with the EV. EVs are also able to transfer drug resistance from one cell to another via the transmission of genetic and protein cargo. However, no work has been done to date exploring the potential parallel roles of EVs in herbicide resistance.
Understanding the importance of EVs in herbicide resistance mechanisms could lead to the development of much needed diagnostic, monitoring and circumvention tools. Indeed, knowledge of plant EVs is lagging behind the rapid growth in mammalian EV studies, with the existence of plant EVs only recently being confirmed. The opportunities for combatting herbicide resistance by understanding the involvement of EVs are evidenced by the tools that are beginning to be developed from a growing understanding of EV-associated drug resistance. For example, inhibitors of EV uptake and biogenesis have been used to increase drug sensitivity, and biomarkers of resistance have been used to inform strategies. Understanding the roles of EVs in herbicide resistance would pave the way for the development of similar intervention and monitoring tools, which is the focus of this project.
Over the course of this project, methods will be developed to isolate EVs from a weed species for the first time. Once this method is established, it will be applied to the isolation of EVs from herbicide-sensitive and herbicide-resistant annual ryegrass. The protein cargo of EVs from each of these ryegrass strains will be compared to determine whether EVs from the resistant ryegrass are enriched in herbicide detoxification and sequestration proteins. The RNA cargo will also be compared to identify RNA biomarkers which have the potential to be used as diagnostic tools for identifying resistance. We will also assess whether EVs from the resistant strain can confer resistance to the sensitive strain, therefore elucidating the role of EVs in the spread of resistance.