In the present study, the protective effects of thiamine and Piriformospora indica against the pathogenic fungus Rhizoctonia solani AG-4 HGII, which causes root rot, have been investigated in bean plants. Our results revealed that thiamine and P. indica significantly reduced the disease progress in the leaf discs. Disease suppression via application of thiamine and the endophytic fungus, P. indica, was linked to biochemical and metabolic changes in bean plants. Accumalation of hydrogen peroxide (H2O2), superoxide anion (O2– ), and iron ions decreased in the leaf discs pretreated with thiamine and/or P. indica, while the formation of polyamines (PAs) such as putrescine (Put), spermidine (Spd), and spermine (Spm) dramatically increased. We observed that occurrence of cell death, the levels of ferric (Fe3+) and ferrous (Fe2+) ions were positively correlated with H2O2 accumulation. Simultaneous increase in the generation of H2O2, Fe3+ and Fe2+ may play a role in induction of cell death. A positive correlation was also observed between PAs and H2O2 levels. Decreased accumulation of H2O2 in different treatments may be a result of an increase in the PAs contents. These findings suggest that formation of PAs may decrease progress of the disease via decreasing H2O2 accumulation in the plant cells. This is the first report on the crucial role of PA contents and their interplay with iron ions and reactive oxygen species in thiamine and P. indica mediated induction of bean defense responses against a necrotrophic fungal pathogen.

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