The molecular physiology of belowground interactions between plants

Plants have evolved for hundreds of millions of years to grow alongside neighbouring plants, and this is a natural state for them. Due to their sessile nature, neighbouring plants are also the most enduring associations a plant experiences throughout its lifetime. Evidence shows that plants can detect, respond to, and be influenced by their neighbours, often with fitness consequences. While there are several models of plant-plant interactions, the mechanisms driving plant strategies remain unclear. In this thesis, I have employed Arabidopsis thaliana to explore the molecular physiology behind neighbour responsiveness and tolerance. I have thoroughly documented above- and belowground growth and measured reproductive outcomes in various intraspecific interaction scenarios. I have shown that two of the most essential resources for roots—nutrients and space—though utilised by neighbouring plants, are relatively straightforward and not interchangeable with the more complex influence of neighbour presence. I have also explored the strategies used by different, yet closely related species within the Brassicae family, as well as ecotypes of A. thaliana, and found that instead of uniform reactions, distinct multitrait responses to neighbours can be identified. I have clarified previous misconceptions regarding the role of rhizodeposits in A. thaliana plant-plant communication showing that it rarely occurs in soils condition. I provide the extensive characterisation of proteins present in root exudates and emphasise its nutrient regulation, which has general implications for rhizosphere research, but also sheds light on the prevalence of proteins involved in biotic interactions in soil. My results highlight the rhizosphere role of VOC ethylene in neighbour detection and response and demonstrate its effect on plants fitness. This work also demonstrates the internal role of CEP-CEPR1 signalling pathway in neighbour response execution. Overall, this thesis provides a foundational basis on which to develop further research aimed at molecular mechanistic understanding of plant-plant interactions.