1,8-dihydroxy naphthalene (DHN) - melanin confers tolerance to cadmium in isolates of melanised dark septate endophytes.

The contribution of 1,8-dihydroxy naphthalene (DHN) melanin to cadmium (Cd) tolerance in two dark septate endophytes (DSE) of the genus Cadophora with different melanin content was investigated in vitro. The DSE isolate Cad#148 with higher melanin content showed higher tolerance to Cd than the less melanised Cad#149. Melanin synthesis was significantly reduced by Cd in both isolates with uninhibited melanin synthesis, in a dose-dependent manner. Inhibition of melanin synthesis by tricyclazole reduced the relative growth of Cad#148 exposed to Cd and did not affect Cad#149. Cd accumulation was not altered by tricyclazole in the two isolates, but it increased catalase and reduced glutathione reductase activity in more melanised Cad#148, indicating higher stress levels. In contrast, in Cad#149 the enzyme activity was less affected by tricyclazole, indicating a more pronounced role of melanin-independent Cd tolerance mechanisms. Cd ligand environment in fungal mycelia was analysed by extended EXAFS (X-ray absorption fine structure). It revealed that Cd was mainly bound to O- and S-ligands, including hydroxyl, carboxyl, phosphate and thiol groups. A similar proportion of S- and O- ligands (~35% and ~65%) were found in both isolates with uninhibited melanin synthesis. Among O-ligands two types with Cd-O-C- and Cd-O-P- coordination were identified. Tricyclazole altered Cd-O- ligand environment in both fungal isolates by reducing the proportion of Cd-O-C- and increasing the proportion of Cd-O-P coordination. DHN-melanin, among other tolerance mechanisms, significantly contributes to Cd tolerance in more melanised DSE fungi by immobilising Cd to hydroxyl groups and maintaining the integrity of the fungal cell wall.

The effects of hydrothermal processing and germination on Fe speciation and Fe bioaccessibility to human intestinal Caco-2 cells in Tartary buckwheat.

Tartary buckwheat is a gluten-free crop with great potential as a wheat substitute. Iron (Fe) is an important mineral element in staple foods which is required in sufficient bioaccessible quantities. The aim of the study was to investigate how processing of grains into groats (hydrothermal processing to remove the husk) and sprouts (7-day-old seedlings) affected Fe speciation (Fe(2+) or Fe(3+)), Fe ligand composition and Fe bioaccessibility to human Caco-2 cells. Groats contained the least Fe (23.8 ± 1.65 mg kg(-1)) and the lowest amounts of Fe(2+) (8%). Grains and sprouts had comparable Fe concentrations (78.2 ± 2.65 and 68.9 ± 2.73 mg kg(-1)) and similar proportions of Fe(2+) (15% and 18%). The main ligands for Fe in Tartary buckwheat material were phytate and citrate. Phytate was less abundant in sprouts, which did not correlate with greater Fe bioaccessibility. Iron bioaccessibility was 4.5-fold greater for grains than groats, suggesting that Fe is more bioaccessible in the husk than in the rest of the grain.