Treatment effects on the two-step selection process
We show a decrease in bacterial diversity in the rhizosphere and
endosphere versus bulk soils, which is consistent with previous findings
expected under the two-step selection process (Bulgarelli et al. 2012; Lundberg et al. 2012; Urbina et al. 2018). T.
triandra plants recruited different communities of bacteria from the
soil into their rhizospheres and endospheres depending on whether they
underwent soil sterilisation or water-stress treatments. These findings
show that the plant’s growth environment alters the recruitment dynamics
of soil bacteria. It also shows that T. triandra plants under
stress appear to alter their entry screening strategies of soil bacteria
when growing under drought-like conditions.
Endosphere recruitment dynamics were most sensitive to the long-term
effects of soil sterilisation, compared to soil aridity or water-stress
treatments. In all sterilised treatments, endosphere diversity was lower
and bacterial communities were differently structured to the
unsterilised soils. However, it remains unclear how bacteria from
sterilised soils were selectively recruited into the endospheres –
whether they originated from the seed microbiome or were microbiota that
were not entirely removed from the soils during sterilisation (Kimet al. 2022; Ling et al. 2022; He et al. 2024).
Given the reduced T. triandra growth rates (biomass) in the low
aridity sterilised soils (which was much lower than in high aridity
sterilised soil treatments), we suspect that this grass may also be more
susceptible to colonisation by microbial pathogens that possible thrive
under the low competition environment created by soil sterilisation
(Mallon et al. 2015; Mawarda et al. 2022). Shotgun
metagenomic analysis could help identify properties of microbial
endosphere colonisation, such as the acquisition of growth-promoting
functions. Alternatively, it could reveal whether colonisation dynamics
are being hijacked by pathogenic or opportunistic microbes (i.e.,
‘cheater’ organisms) that do not provide the same host plant services,
despite other shared traits (Kiers et al. 2002; Kiers et
al. 2011). The consequences of these interactions could help inform the
vulnerability of T. triandra to soil degradation, making this an
important avenue for future research.