Various fungi are significant threats to vegetable yield via causing destructive diseases. Rhizoctonia solani is a soilborne fungal pathogen affecting tomato with considerable damage in crop production, worldwide. Understanding plant defense mechanisms against different Rhizoctonia spp. isolates can be useful in designing novel and effective disease control approaches. Therefore, we performed cellular, molecular and biochemical assays to investigate the interaction of various taxonomic groups of multinucleate R. solani and a binucleate isolate of Rhizoctonia sp. (BNR) with tomato as a susceptible host. Histochemical and enzymatic experiments showed that the highest levels of O2???? , H2O2, callose accumulation and superoxide dismutase activity were observed for the isolate of R. solani AG 3, which caused lower disease severity compared to the highly pathogenic R. solani isolates belonging to AG 4 HG-I and AG 4 HG-II. Gene expression investigations via quantitative reverse transcription polymerase chain reaction revealed the highest levels of the SOD gene upregulation in the plants with R. solani AG 3 and the BNR inoculations, which showed the lowest levels of pathogenicity on tomato plants. Whereas, the highest upregulation of the POX expression was detected in tomato plant infected with the highly pathogenic R. solani isolates belonging to AG 4 HG-I and AG 4 HG-II at most of the time points investigated. Xanthine/xanthine oxidase treatment, which induced O2???? production, increased the disease symptoms. But, glucose/glucose oxidase, which increased H2O2, decreased the disease symptoms. Ascorbate, as an inhibitor of H2O2 accumulation, reduced callose deposition and increased the disease severity. In overall, upregulation of the SOD gene expression and activation of the corresponding enzyme, which are linked to H2O2 and callose accumulation, might be related to lower level of the disease progress on the plants inoculated with R. solani AG 3 and the BNR isolates. So, priming these defense mechanisms via different environmentally safe treatments could be helpful in disease management, which needs further investigations in future.

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