The necrotrophic fungus Macrophomina phaseolina induces oxidative stress-associated genes and related biochemical responses in charcoal rot susceptible sorghum genotypes

Macrophomina phaseolina (MP) is a necrotrophic fungus that causes charcoal rot disease in sorghum [Sorghum bicolor (L.) Moench]. The host resistance and susceptibility mechanisms for this disease are poorly understood. Here, the transcriptional and biochemical aspects of the oxidative stress and antioxidant system of charcoal rot resistant and susceptible sorghum genotypes in response to MP inoculation were investigated. RNA sequencing revealed 96 differentially expressed genes between resistant (SC599) and susceptible (Tx7000) genotypes that are related to the host oxidative stress and antioxidant system. Follow-up functional experiments demonstrated MPs ability to significantly increase reactive oxygen (ROS) and nitrogen species (RNS) content in the susceptible genotypes. This was confirmed by increased malondialdehyde content, an indicator of ROS/RNS-mediated lipid peroxidation. The presence of nitric oxide (NO) in stalk tissues of susceptible genotypes was confirmed using a NO-specific fluorescent probe (DAF-FM DA) and visualized by confocal microscopy. Inoculation significantly increased peroxidase activity in susceptible genotypes while catalase activity was significantly higher in MP-inoculated resistant genotypes. MP inoculation significantly reduced superoxide dismutase activity in all genotypes. These findings suggested MPs ability to promote a host-derived oxidative stress response in susceptible sorghum genotypes, which contributes to induced cell death-associated disease susceptibility to this necrotrophic phytopathogen.