The Characterisation of root exudation and colonisation in the rhizosphere of land plants

Thesis

The rhizosphere of land plants is a diverse and complex environment that is critical to the life cycles of plants. Plants release photosynthetically fixed carbon into the rhizosphere that can act to solubilise nutrients, detoxify metals, and modify soil microbial communities. Within these soil microbial communities a diverse range of microbes exist, ranging from the beneficial plant growth promoting rhizobacteria to plant pathogens. These microbes must be able to effectively colonise the rhizosphere and plant rooting systems for their survival. To study root exudation and the bacterial colonisation of the roots two separate techniques were used.

To study the spatial and temporal release of exudation in Oryza sativa, Zea mays, Arabidopsis thaliana, and Marchantia polymorpha, luciferase based bioreporters, which respond to plant derived carbon were used. These bioreporters based in Rhizobium leguminosarum allowed for the production of maps showing the spatial nature of exudation from plant roots, alongside changes in exudation temporally. In this work I showed that the release of the C4-Dicarboxylic acids was conserved across all plant species tested. Moreover, the release of these exudates occurred on specific rooting structures, such as lateral roots, and not across the whole rooting system. This work has shown that R. leguminosarum based bioreporters can be flexibly used across a range of different plant species.

For studying the genetic basis of Azorhizobium caulinodans colonisation of the O. sativa rhizosphere, a technique known as insertion sequencing was used. This technique highlighted bacterial genes involved in rhizosphere and root colonising growth. Using these data, several interesting gene operons were identified that played a crucial role in rhizosphere growth. One example was a putative propionate catabolism operon, indicating that A. caulinodans is using plant-derived propionate as a carbon source for growth. Insertion sequencing was also used to probe the genetic basis of nitrogen fixation in A. caulinodans, here I showed that insertion sequencing was applicable to this study and could identify a number of genes involved in low nitrogen growth.

Authors: Roworth, J

• DOI: 10.5287/ora-kom2zgdrj
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English