4 Discussion
Studies have reported differences in endophytic communities between root
and aerial tissues in various plants (Ma, Lv, Warren, & Gong, 2013; Su,
Guo, & Hyde, 2010). In addition to the difference in biochemical
environments between roots and aerial tissues, distinct environmental
propagule pools above ground and underground, colonizing and coexisting
mechanisms with roots and aerial tissues, and inter-tissue transport
limitation contribute to the differences in communities (Fang et al.,
2019; Su et al., 2010; X. Sun et al., 2012). Community composition inK. schrenkianum was different between aerial tissues and roots
for both bacteria and fungi. Interestingly, the diversity of endophytic
bacteria in aerial tissues was less than that in roots, whereas the
diversity of endophytic fungi showed an opposite pattern. Microbial
species in soil is generally more than that in air. Therefore, the
bacterial communities might have largely assembled by immigrating from
propagules from the environmental source. However, a differential
selection by the tissue significantly shaped the fungal communities in
present study. The community composition of endophytic fungi determined
by the host is restricted by the environmental propagule pool. Soil
related taxa, including Saitozyma podzolica and
Sporormiaceae_F_OTU_583, showed significant preference for the root
in K. schrenkianum . Researchers first identified S .podzolica (previously Candida podzolica ) as a soil yeast
(Babjeva & Reshetova, 1975; Liu et al., 2015); Sporormiaceae members
are cosmopolitan saprobes (Cannon & Kirk, 2007). These microbes might
be soil inhabitants and be able to colonized K .schrenkianum and switched between endophytism and saprophytism
(Xiang Sun, Guo, & Hyde, 2011). In summary, our research suggested that
the assembly of both bacterial and fungal endophytic microbiota in roots
of K. schrenkianum were affected by soil microbial propagule
pools, while roots have stronger selection to fungal colonizer.
In addition to the differences in community composition, our study
demonstrates differences between aerial tissues and roots in the
responses of endophytic microbiota to environmental factors. The
diversity of endophytic microbiota in roots correlated with soil
chemical characteristics while the microbiota in aerial tissues showed
no response to varied environmental factors. We observed increased
diversity in endophytes associated with root from control treatment to
radioactive environments and not in endophytes associated with aerial
tissues. This finding indicates larger effect of environment factors on
root endophytic microbiota.
Soil pH is a major determinant of microbial community structure and
assembly (Fan et al., 2018; Lauber, Hamady, Knight, & Fierer, 2009;
Rousk et al., 2010). Lauber et al. (2009) stated that soil pH predicted
the composition of soil bacterial communities, and phylogenetic
diversity attained a peak at near neutral pH. Diversity of endophytic
bacteria showed a significant negative correlation with soil pH of our
study sites that ranged from 8.8 to 9.6. This is consistent with the
findings of Lauber et al. (2009). Conclusively, the recruitment of root
bacterial microbiota is largely dependent on the diversity of soil
species. However, we found no correlation between pH and fungal
diversity. Rousk et al. had suggested that fungi prefer a wider pH range
for optimal growth compared with bacteria Rousk et al. (2010).
In a recent study, Bahram et al. (2018) reported strong antagonism
between fungi and bacteria globally. In K. schrenkianum , we found
limited interaction between bacteria and fungi; however, there was
bacterial-fungal competition at the niche level. The co-occurrence
correlations are prone to restrict to the intra-kingdom members of
bacteria and fungi. Meanwhile, inter-kingdom members were negatively
correlated. In the roots, bacteria and fungi were less correlated, which
indicates less competition in the rhizosphere.
Despite the presence of various chemical factors of soil, radiation was
the dominant factor that structured the endophytic communities. Higher
genetic diversity was found in radioactive environments for both
bacterial and fungal communities. However, mutations due to ionizing
radiation and its effect on genetic diversity need to be investigated.
Environmental stress resulted in increase in genetic diversity in the
community. Nevo (2001) discovered higher genetic diversity in several
tested model organisms under stressful environments with thermal,
chemical, climatic, and biotic stresses. Increase in genetic diversity
enhanced probability of population survival (Lande & Shannon, 1996),
and populations with low genetic diversity had reduced fitness and
increased extinction rates (Markert et al., 2010). Therefore, higher
genetic diversity acts as an inherent mechanism of community assembly to
maintain the community under moderately stressed environments.
Meanwhile, the current study also revealed increased community diversity
in roots at radiation stressed treatments apart from the increased
population genetic diversity. Diversity as an essential descriptive and
metrological feature, could predict the stability, function, and
productivity of an ecosystem (Allison & Martiny, 2008;
Delgado-Baquerizo et al., 2020; Tilman, 1996). High species and
phylogenetic diversity are related to high functional diversity and
redundancy; however, it is difficult to establish direct correspondence
yet, because studies that convincingly tested the linkage between
phylogeny and physiology in microbial communities are limited (Allison
& Martiny, 2008). High diversity of endophytic communities in K.
schrenkianum roots might imply that the radionuclide sediments in soil
severely stressed the root niche and consequently diverse microbes
assembled to maintain a stable endophytic microbial communities.
Environmental radiation evoked different responses in bacterial and
fungal communities. Vries et al. (2018) showed that bacterial networks
were less stable under drought stress than fungal networks in grassland
mesocosms set in UK. However, in the present study, the community
structure of endophytic fungi was sensitive to radiation more than that
of endophytic bacteria. Fungal co-occurrence networks were more
fragmented under radiation-stressed environments; connectance decreased
and modularity increased at all radiation levels. We observed
significant differences in the community composition of fungi in both
aerial tissues and roots and of bacteria in roots across all treatments.
In addition, the fungal community composition showed dramatic
differences at class level and at species level across treatments.
Current study is the first to investigate the community structure of
plant symbiotic microbiota under radioactive stress. Nevertheless,
present study gave additional questions when answering to mechanisms of
community assembly and maintenance under the extreme environments. We
did not investigate the correlation between ionizing radiation-induced
mutation and increase in genetic diversity and the inherent mechanism
that drives community assembly under radiation-stressed environment.
Besides, we did not analyse the OTUs identified as unassigned fungi,
which showed extraordinary abundance and dominance in roots. The OTUs
matched (low E-value and identity) the members of phylum Ascomycota
(data not shown); however, these fungi are still unknown (since no
strain was recovered from the plant). Further studies are necessary to
expand our knowledge to plant-microbe symbiosis under radioactive
environments.