Root endophytic Aquapteridospora rhizicola enhances growth and heavy metal tolerance of Miscanthus sinensis Andersson at a sedimentary site via indole-3-acetic acid and melanin production enhanced by high Fe and high temperature

Aquapteridospora rhizicola, a dark septate endophyte, was isolated from the roots of Miscanthus sinensis Andersson, which grows naturally as pioneer grass at a sedimentary site in an old mine. In the early successional stage, M. sinensis seedlings begin to grow on bare ground under heavy metal stress (especially Fe) and sharp soil temperature fluctuations. This study aimed to clarify the effects of root-endophytic A. rhizicola on M. sinensis seedling growth and tolerance, in bare-ground soil, while considering the sharp alteration in soil temperature of the bare ground at the study site. We conducted inoculation tests using sterilized bare soil under normal and actual soil temperature conditions, thereby mimicking the bare ground at the study site. Although A. rhizicola did not promote seedling growth under normal temperature conditions, this fungus enhanced plant growth under actual soil temperature conditions. To elucidate the underlying mechanism of these results, we analyzed the production of indole-3-acetic acid (IAA) and melanin by A. rhizicola under high Fe and high temperature conditions. The root endophyte increased IAA production in response to the combined conditions of high Fe and high temperature, and this might have contributed to the growth of M. sinensis. Moreover, A. rhizicola might alleviate heavy metal toxicity in the host plant by suppressing the translocation of toxic Zn from roots to terrestrial parts under actual soil temperature conditions. This might be explained by the high production of melanin, which possesses heavy metal binding capacity and antioxidant activity, and whose effect was enhanced under conditions of high Fe and high temperature. Our results indicate that A. rhizicola might enhance growth and heavy metal tolerance of M. sinensis in bare ground of the study site, via increased IAA and melanin production by habitat environmental factors, such as high Fe and high temperature.