Author: Riley Hodgson
Hodgson, Riley, 2025 Effects of aridity on the plant-soil interactions and microbial colonisation of the keystone grass, Themeda triandra, Flinders University, College of Science and Engineering
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In this thesis, I show that the pan-palaeotropical keystone grass, Themeda triandra forms important beneficial relationships with soil microbial communities across an aridity gradient. I also show that aridity alters microbial soil-to-endosphere colonisation dynamics. Plant-soil interactions are increasingly recognised as important to shaping ecosystem function and the health of host plants, particularly in grassland ecosystems. As such, understanding how these interactions influence the growth, fitness, and stress responses of ecologically significant species, such as, T. triandra, is key to advancing our knowledge of these processes. However, the mechanisms by which microbiota colonise different root compartments – across bulk soils, rhizospheres (microbiota around root surfaces), and root endospheres (microbiota within roots) – have been poorly characterised outside of model plant species. By investigating non-model species from wild populations under changing aridity conditions, and their microbiota, we can show how these microbial recruitment processes change across broad geographical distances. Furthermore, we can investigate changes in the functional properties of microbiomes from different plant compartments (i.e., soils, rhizospheres, and endospheres) to understand the extent to which plants can moderate the colonisation of microbiota into their roots.
Here, I address these knowledge gaps to produce a body of knowledge around a globally important grass. This research will contribute to our understanding of how host plants can respond to environmental stress, which represents essential knowledge for landscape management process faced with changing aridity due to the impending effects of climate change.
By using DNA-based approaches across natural field and greenhouse experiments, I make an original contribution to knowledge through exploring geographical patterns of T. triandra microbial community interactions over four research chapters. Specifically, I show that T. triandra enriches the abundance of key, host-associated, bacterial taxa with increasing aridity. I provide evidence that soil microbiota are progressively selected by T. triandra plants as they colonise host rhizospheres and endospheres, despite strong influences of local conditions within each population. Moreover, I provide evidence of T. triandra-microbial interactions by identifying changes in key functional gene profiles that promote growth and stress responses across T. triandra microbiomes. Furthermore, I enhance the current model of soil-to-endosphere colonisation, the ‘two-step selection process’, by showing that functional alpha diversity increases from bulk soils into rhizospheres, and endospheres – directly contrary to established theory on bacterial taxonomic diversity. Finally, I show how high and low aridity soil legacies significantly impact T. triandra growth under both stress and non-stress conditions. I identify that increasing bacterial diversity across soils, rhizospheres, and endospheres directly correlates with T. triandra growth, and that aridity is a key factor in determining the availability of soil microbiota for recruitment across broad geographic ranges.
Ultimately, my research makes new contributions to our understanding of the formation, distribution, and impacts of plant-microbe interactions in wild populations of a globally distributed keystone grass. These findings have important implications for conservation, ecosystem restoration, and agricultural practices related to T. triandra and other C4 grasses, and how these species might tolerate stress conditions across these different landscapes.
Keywords: aridity, plant-soil interactions, themeda triandra, two-step selection process, microbiome
Subject: Environmental Science thesis
Thesis type: Doctor of Philosophy
Completed: 2025
School: College of Science and Engineering
Supervisor: Martin Breed