Strong Host Modulation of Rhizosphere-to-Endosphere Microbial Colonisation in Natural Populations of the Pan-Palaeotropical Keystone Grass Species, Themeda triandra

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Soil microbiota can colonise plant roots via a two-step selection process, which involves the recruitment of microbiota first from bulk soil into plant rhizospheres, then into root endospheres. This process is poorly understood in all but a few model species (i.e., Arabidopsis), which is surprising given its fundamental role in plant and soil ecology. Here we examined the microbial assembly processes across the rhizospheres and root endospheres in eight natural populations of the pan-palaeotropical C4 grass, Themeda triandra, in southern Australia. We assessed whether root endosphere colonisation patterns aligned with the two step-selection process using space-for-time substitutions to compare their bacterial communities across these root compartments, and test whether their assembly was dominated by deterministic versus stochastic processes. We show that the two-step selection was the dominant recruitment dynamic across these natural T. triandra populations, and present clear evidence that host plants influenced microbial assembly via deterministic pressures that produced strong convergence of endospheres. Furthermore, the T. triandra endospheres were strongly shaped by the host plant and displayed patterns consistent with the two-step selection process. These findings raise intriguing questions about the functions of this ‘core’ microbial endosphere, but our limited understanding of their ecology hinders our ability to harness these important relationships to, for example, improve plant propagation and revegetation practices.