A Systems View on Microbial Establishment on Growing Root Surfaces

The rhizosphere is a region of soil holding incredible biological complexity and diversity. Microbial activity in the rhizosphere contributes to processes such as plant pathogenesis and nutrient cycling. Previous studies have viewed the rhizoplane as static, failing to explain the large shifts in microbial composition taking place in the rhizosphere during root expansion. The objective of this project is to identify and quantify factors contributing to early stage root colonisation, when dynamic processes such as root elongation and bacterial motility are essential to the establishment of the rhizosphere community. Using Lettuce (Lactuca sativa) and Pseudomonas fluorescens as a model system, assays have been developed measuring factors of early colonisation such as bacterial motility, chemotaxis and adherence to roots. Data from these assays will be supplemented with live imaging of fluorescently marked bacteria in transparent soil, providing information on bacterial localisation on roots. The Preliminary experiments have characterised bacterial adherence during the early stages of root development and the effect of the presence of root exudate on bacterial growth and movement. Transformation of Pseudomonas fluorescens with fluorescently labelled plasmids has prepared the model system for live imaging. The challenge now is to determine which plant and bacterial factors controls establishment of the rhizosphere community and to relate quantitative data from microbiological assays with that seen during live imaging. Going forward, this data will feed into model showing how plant and microbial traits explain observed shifts in the rhizosphere community, potentially enabling more accurate predictions about root colonisation, by beneficial bacteria and pathogens, to be made.