1 Introduction
Endophytes are symbiotic microbes that live within a plant and are important components of the plant microbiome (Porras-Alfaro & Bayman, 2011). These microbes are found distributed in terrestrial and marine plants and have high phylogenetic diversity and ecological function. Endophytes and their host plants have coevolved, and both benefit from this mutual symbiosis. Studies have shown that endophytes improves host plant physiology to adapt to stressed environments; endophytes reprogrammed the host response to pathogen invasion and increased toxic chemical production to provide protection against herbivores (Arnold et al., 2003; Gange, Eschen, Wearn, Thawer, & Sutton, 2012; Rodriguez et al., 2008). In the other side, endophytes live inside plant tissues, which protect them from dehydration, poor nutrition, ultraviolet radiation, and competition (Saikkonen, Faeth, Helander, & Sullivan, 1998; Yao et al., 2020). The mutual interaction profoundly shaped community and changed the biodiversity of plant and microbes, however, the related knowledge remains limited. Therefore, the biology and ecology of endophytic microbiota and their hosts have gained attention and become important research topics.
Endophytic microbial community structure is influenced by various biotic and abiotic factors, such as host identity, climate, biological or circumstantial stresses, and geochemical factors (U’Ren, Lutzoni, Miadlikowska, Laetsch, & Arnold, 2012). Host identity is the major factor that determines endophytic community composition (X. Sun, Ding, Hyde, & Guo, 2012; U’Ren et al., 2012), whereas environmental factors have a complex influence on the endophytic community. Biotic and abiotic stress alters structure of endophytic communities. Studies have shown that endophytic population is affected by climate changes such as increased carbon dioxide emission, global warming, and drought (Compant, Heijden, & Sessitsch, 2010). Rodriguez et al. (2008) demonstrated habitat-specific, symbiotically-conferred stress tolerance in plants under high-stress environment. Therefore, research on how endophytic community shift under stressed conditions will shed light into fundamental issues in how the mutual interaction between plants and endophytes promotes the adaption of the symbiotic entity
Ionizing radiation is a type of radiation, in the form of electromagnetic waves or particles, with sufficient energy to ionize an atom or a molecule. Overdosed exposure to ionizing radiation causes harmful effects in living organisms. Ionizing radiation is generally detected in areas exposed to radioactive minerals, in high altitude environments, or in aerospace. Nuclear power plants, nuclear weapon tests, nuclear accidents, and mining produced new radioactive habitats. However, certain plants, animals, and microbes survive under radioactive environment. Studies have reported the existence of shrubs, rodents, terrestrial algae, and fungi in Nevada Test Site (NTS) few years after the nuclear tests (Allred & Beck, 1963; Beatley, 1964; Rickard & Beatley, 1965; Shields & Drouet, 1962; Shields, Durrell, & Sparrow, 1961). Durrell and Shields (1960) isolated 41 fungal taxa from soil within a mile radius of ground zero sites two years after nuclear tests in NTS. Thirty-seven culturable fungal species of 19 genera were detected on the walls and other building structures in the inner parts of the shelter of the damaged fourth unit of the Chernobyl Nuclear Power Plant (Zhdanova, Zakharchenko, Vember, & Nakonechnaya, 2000). Radiation has also resulted in shifts in the local microbial communities. For example, soil fungal community structure appeared to shift toward species that may be more radiation resistant, and melanin-containing fungi ascended to dominate the soil fungal communities with increase in radionuclide pollution (Zhdanova, Zakharchenko, & Haselwandter, 2005). Lavrinienko, Tukalenko, Mappes, and Watts (2018) correlated the gut microbiomes of bank vole Myodes glareolus inhabiting Chernobyl zone with the radioactivity level. Wehrden et al. observed physiological and morphological changes in the organisms of radioactive environments and other negative effects of radiation on the ecosystem (Wehrden et al., 2012). The biological and ecological traits of ecosystem under radiation stress is a great concern in policy making on nuclear power utilization and in designing aerospace sustainable life support system. However, there are no studies on the symbiotic microbes of plants exposed to ionizing radiation.
Endophytic microbes symbiotically associate with host plants, and the symbiotic entity of inter-kingdom jointly challenge harsh circumstances. However, the relationship between endophytes and the host plant under radioactive environment is unknown. High-radiation habitats because of Caesium-137 (137Cs) accumulation from historic nuclear test exist in arid, saline semi-arid desert of northwest China. This paper investigates the endophytic microbes (fungi and bacteria) in the roots and aerial parts of Amaranthaceae halophyte Kalidium schrenkianum (Pall.) Moq., a dominant population of local flora. Here, we collected plant samples and soil samples from sites representing different levels of radiations. We aim to understand the diversity and community structure of endophytic bacteria and fungi in a radioactive habitat; determine the main environmental factor (geochemical characteristics of soil and local radiation level) shaping the endophytic communities; and elucidate the shift in pattern of endophytic microbes with radiation level.