Dark septate endophytic fungi (DSEF) can promote the growth response of host plants and can increase their resistance to heavy metals. In this study, six isolated DSEFs were proven to be able to form microsclerotia with plants. The objective of this study was to investigate the promoting and inoculation effects of these six DSEFs. Molecular analysis showed that the DSEFs may be new additions to the fungal flora of Taiwan and may be new species in the world. Screening and selection of Zn-resistant DSEFs showed that only two strains, CkDB5 and RrtHH10, were Zn-resistant. Evaluation of the Zn resistance range of CkDB5 and RrtHH10 revealed that the former presented lower EM50 values than the latter, and that both strains showed the same minimum inhibitory concentration values. As the Zn concentration increased, twisting and looping of the mycelial morphology of the strains was observed. Resynthesis experiments proved that CkDB5 and RrtHH10 could promote the growth response of tomato. The biomasses of CkDB5-inoculated plants was up to four times higher than that of the controls. This study showed that, of the six DSEFs, only CkDB5 and RrtHH10 showed Zn-resistance. CkDB5 is a fungus with particularly high Zn resistance, and it could facilitate tomato growth.
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