Charcoal rot, a disease of Sorghum bicolor caused by Macrophomina spp, is of particular concern under high temperatures and drought stress. The necrotrophic pathogen invades vascular tissues, disrupting water and nutrient transport, which leads to water stress, nutrient deficiencies, stalk rot, and yield loss. Under optimal conditions, yield losses can be as high as 70%. Compounding the challenge, Macrophomina persists in soil for up to 15 years as microsclerotia and has a broad host range, rendering most conventional management practices ineffective. Given these limitations, alternative approaches that can enhance crop resistance and mitigate disease severity are needed.
Beneficial microbes, such as arbuscular mycorrhizal (AM) fungi, offer a promising avenue for improving plant defence against Macrophomina spp. These mycorrhizal fungi occur as complex communities in soils where they form symbiotic associations with plant roots, enhancing nutrient and water uptake. Additionally, AM fungi can prime systemic plant defence responses, and influence key metabolites, such as flavonoids and terpenoids, which play crucial roles in pathogen resistance. Such defence-enhancing effects are influenced by the composition of the AM fungal communities that associate with the host plant. Yet we currently do not know how pathogens, such as Macrophomina, affect AM fungal diversity in crop roots, and the dynamics of AM-mediated resistance in sorghum are still elusive.
We investigated this using a factorial glasshouse experiment in which sorghum was grown with different AM fungal communities or without AM fungi, and inoculated M. phaseolina (the most studied species of Macrophomina), or left pathogen-free. Plant phenotypic responses were monitored throughout by image analysis, and plants were harvested at three timepoints to assess temporal responses to treatments. In addition to measuring plant biomass and nutrient concentrations, defence-associated metabolites such as jasmonic acid and salicylic acid were measuring at each timepoint using liquid chromatography-mass spectrometry (LC-MS). The composition of the AM fungal communities in sorghum roots was assessed through 18S (SSU) DNA metabarcoding using Illumina NextSeq sequencing.
Initial analyses reveal significant differences in growth and chlorophyll levels among Sorghum bicolor plants subjected to different AM fungal communities, including controls without AM fungi. Photographic evidence demonstrates marked phenotypic variations, affirming the role of AM fungi in enhancing plant vigor. These promising preliminary findings pave the way for elucidating the mechanisms of AM fungal-mediated disease resistance in sorghum. The comprehensive data anticipated from this study are expected to significantly advance our understanding of the interactions between AM fungi, S. bicolor, and M. phaseolina, offering new strategies for managing soil-borne diseases and enhancing crop resilience.