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.