INTRODUCTION
Grasslands are in significant decline globally (Bardgett et al. 2021). The productivity, diversity, and resilience of these ecosystems
is heavily shaped by their soil microbiota (Koziol & Bever 2017; Wanget al. 2019; Liu et al. 2022). Despite strong plant-soil
interactions in grasslands (i.e., plant-soil feedbacks), these
interactions are under acknowledged and underutilised in conservation
and restoration efforts (Robinson et al. 2023; Peddle et
al. 2024). As climate change and land-use pressures intensify,
understanding how soil microbiota support grassland productivity and
stress tolerance is increasingly important to aid conservation and
restoration efforts (Trivedi et al. 2022; Fadiji et al. 2023).
Carbon and nutrient cycling are among the many microbial-driven
processes in soil that can shape plant communities (Bever et al. 2010; Wagg et al. 2014). Plants also form direct symbioses with
soil microbiota in their rhizospheres (areas around plant roots) and
endospheres (inside plant roots) (Bulgarelli et al. 2013). The
colonisation of these plant compartments by soil microbiota is described
by the two-step selection process (Bulgarelli et al. 2012;
Lundberg et al. 2012; Bulgarelli et al. 2013). This
process involves initial resource provision through plant roots which
support microbial assemblages from the bulk soil to colonise host
rhizospheres (step 1). Microbiota are then filtered into the endosphere
with plant immune system regulation (step 2) (Bulgarelli et al. 2013). These rhizosphere and endosphere microbiota aid in plant nutrient
acquisition and metabolic processes, but we currently lack a clear
understanding of how recruitment is affected by plants growing under
stressful conditions, such as drought. We also lack knowledge of how
plant recruitment of these microbiota is affected by ecological contexts
(e.g., high vs low aridity) (Ling et al. 2022; Santoyo 2022).
Harnessing soil biodiversity is increasingly recognised for its
potential to enhance plant growth in applied ecology contexts (Mariotteet al. 2018; Porter & Sachs 2020; Peddle et al. 2024).
One promising method to do this is through whole soil inoculations via
the translocation of soil, including their microbiota, into new areas
(Gebhardt et al. 2017; Wolfsdorf et al. 2021; Han et
al. 2022). This approach leverages positive soil legacies where plant
populations naturally cultivate soil microbiota that support the
offspring of these plants (Kaisermann et al. 2017; Pinedaet al. 2017; Buchenau et al. 2022). Positive soil legacies
can improve plant tolerance to water stress and herbivory (Kaisermannet al. 2017; Hannula et al. 2021), but we lack theoretical
understanding of the colonisation mechanisms within soil and plant
compartments. Experimental testing of how different soils and their
microbiota influence plant growth along with comprehensive
characterisation of bacterial colonisation patterns can address these
knowledge gaps, especially when accounting for stress scenarios.
Themeda triandra (Forssk.) is a globally important keystone C4
grass species with a pan-palaeotropical distribution (Snyman et
al. 2013; Dunning et al. 2017; Pascoe 2018). Currently, the
processes by which microbiota colonise and influence the growth ofT. triandra remain poorly understood. To address this, we
conducted a greenhouse experiment on how soil microbiota from high and
low aridity regions affected the germination and growth of T.
triandra under both water-available and drought-like (i.e., water
stress) conditions. We used 16S rRNA amplicon sequencing to characterise
the T. triandra -associated microbiota of high and low aridity
soils under live versus sterilised, and water stress treatment
conditions, plus the recruitment patterns of these microbiota from the
bulk soils into T. triandra rhizospheres and endospheres. We
hypothesised that: (1) soil microbiota sourced from arid sites would
enhance T. triandra growth under stress conditions by providing
mutualistic microbiota that support growth under drought-like
conditions; (2) distinct microbial communities would be recruited into
the rhizosphere and endosphere under each water treatment, reflecting
shifts in host plant requirements; and (3) the presence of T.
triandra plants would alter the bacterial community in soil due to a
cumulative influence of microbe-root interactions. By assessing how
microbiota impact the drought responses of this important grass, and
monitoring their recruitment across root compartments, we can better
understand the value of soil biodiversity as a tool for improving the
resilience of grassland ecosystems.
MATERIALS AND METHODS