Root Fungal Endophytes Enhance Heavy-Metal Stress Tolerance of <i>Clethra barbinervis</i> Growing Naturally at Mining Sites via Growth Enhancement, Promotion of Nutrient Uptake and Decrease of Heavy-Metal Concentration

<div><p><i>Clethra barbinervis</i> Sieb. et Zucc. is a tree species that grows naturally at several mine sites and seems to be tolerant of high concentrations of heavy metals, such as Cu, Zn, and Pb. The purpose of this study is to clarify the mechanism(s) underlying this species’ ability to tolerate the sites’ severe heavy-metal pollution by considering <i>C</i>. <i>barbinervis</i> interaction with root fungal endophytes. We measured the heavy metal concentrations of root-zone soil, leaves, branches, and fine roots collected from mature <i>C</i>. <i>barbinervis</i> at Hitachi mine. We isolated fungal endophytes from surface-sterilized root segments, and we examined the growth, and heavy metal and nutrient absorption of <i>C</i>. <i>barbinervis</i> seedlings growing in sterilized mine soil with or without root fungal endophytes. Field analyses showed that <i>C</i>. <i>barbinervis</i> contained considerably high amounts of Cu, Zn, and Pb in fine roots and Zn in leaves. The fungi, <i>Phialocephala fortinii</i>, <i>Rhizodermea veluwensis</i>, and <i>Rhizoscyphus</i> sp. were frequently isolated as dominant fungal endophyte species. Inoculation of these root fungal endophytes to <i>C</i>. <i>barbinervis</i> seedlings growing in sterilized mine soil indicated that these fungi significantly enhanced the growth of <i>C</i>. <i>barbinervis</i> seedlings, increased K uptake in shoots and reduced the concentrations of Cu, Ni, Zn, Cd, and Pb in roots. Without root fungal endophytes, <i>C</i>. <i>barbinervis</i> could hardly grow under the heavy-metal contaminated condition, showing chlorosis, a symptom of heavy-metal toxicity. Our results indicate that the tree <i>C</i>. <i>barbinervis</i> can tolerate high heavy-metal concentrations due to the support of root fungal endophytes including <i>P</i>. <i>fortinii</i>, <i>R</i>. <i>veluwensis</i>, and <i>Rhizoscyphus</i> sp. via growth enhancement, K uptake promotion and decrease of heavy metal concentrations.</p>