The monokaryotic filamentous fungus Ustilago sp. HFJ311 promotes plant growth and reduces Cd accumulation by enhancing Fe transportation and auxin biosynthesis.

Infection with smut fungus like Ustilago maydis decreases crop yield via inducing gall formation. However, the in vitro impact of Ustilago spp. on plant growth and stress tolerance remains elusive. This study investigated the plant growth promotion and cadmium stress mitigation mechanisms of a filamentous fungus discovered on a cultural medium containing 25 μM CdCl₂. ITS sequence alignment revealed 98.7 % similarity with Ustilago bromivora, naming the strain Ustilago sp. HFJ311 (HFJ311). Co-cultivation with HFJ311 significantly enhanced the growth of various plants, including Arabidopsis, tobacco, cabbage, carrot, rice, and maize, and improved Arabidopsis tolerance to abiotic stresses like salt and metal ions. HFJ311 increased chlorophyll and Fe contents in Arabidopsis shoots and enhanced root-to-shoot Fe translocation while decreasing root Fe concentration by approximately 70 %. Concurrently, HFJ311 reduced Cd accumulation in Arabidopsis by about 60 %, indicating its potential for bioremediation in Cd-contaminated soils. Additionally, HFJ311 stimulated IAA concentration by upregulating auxin biosynthesis genes. Overexpression of the Fe transporter IRT1 negated HFJ311's growth-promotion effects under Cd stress. These results suggest that HFJ311 stimulates plant growth and inhibits Cd uptake by enhancing Fe translocation and auxin biosynthesis while disrupting Fe absorption. Our findings offer a promising bioremediation strategy for sustainable agriculture and food security.