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.