Physiological mechanism of the response to Cr(VI) in the aerobic denitrifying ectomycorrhizal fungus Pisolithus sp.1

Pisolithus sp. 1 (P sp. 1) is an ectomycorrhizal fungus (EMF) with a strong Cr(VI) tolerance and reduction ability. The noninvasive microttest technique (NMT), real-time quantitative PCR (qRT–PCR), and the three-dimensional excitation-emission matrix (3D-EEM) were used to deeply explore the physiological mechanism of the P sp. 1 response to Cr(VI) and investigate the relationship between Cr(VI) reduction and denitrification in P sp. Cr(VI) induced the strongest elevations in nitrate reductase (NR) activity and NO production in the mycelia after treatment with Cr(VI) for 48 h under aerobic conditions. The NR inhibitor tungstate significantly inhibited Cr(VI) reduction, proton efflux and the expression of the NR gene (niaD) and NiR gene (niiA). In addition, NO was generated via NR-regulated denitrification. Combined treatments with Cr(VI) and the NO scavenger carboxy-PTIO (cPTIO) significantly increased O2-, H2O2 and MDA contents and reduced SDH, CAT, GSH, GR and GSNOR activity. Therefore, the NR-driven aerobic denitrifying process requires protons, and the generated NO reduces the oxidative stress effect of Cr(VI) on mycelia by reducing ROS accumulation and lipid peroxidation, enhancing mycelial and CAT activity, and promoting GSH recycling and regeneration. Psp.1 can also secrete humic acid-like and protein-like substances to combine with Cr(III) in a culture system.