RNA-seq analysis reveals genes associated with Macrophomina phaseolina-induced host senescence in soybean

Charcoal rot of soybean is caused by the hemibiotrophic fungus Macrophomina phaseolina, a global crop destroyer and an important pathogen in the midwestern USA. The quantitative nature of host resistance and the complexity of the soybean-M. phaseolina interaction at the molecular level have hampered resistance breeding. A previous study showed that L-ascorbic acid (LAA) pre-treatment before M. phaseolina inoculation reduced charcoal rot lesion length in excised soybean stems. This study aimed to elucidate the genetic underpinnings of M. phaseolina-induced senescence and the mitigating effects of ascorbic acid on this physiological process within the same pathosystem.

RNA was sequenced from M. phaseolina-resistant and -susceptible soybean genotypes following M. phaseolina inoculation, LAA, and hydrogen peroxide (H₂O₂)-an oxidative stress inducer-application followed by inoculation. More genes were down-regulated in the resistant and susceptible genotypes than up-regulated when the M. phaseolina-inoculated treatments were compared to mock-inoculated control treatments. Gene ontology (GO) term and KEGG pathways analysis detected M. phaseolina-induced up-regulation of receptor-like kinase genes. In contrast, many genes related to antioxidants, defense, and hormonal pathways were down-regulated in both genotypes. LAA pre-treatment induced genes related to photosynthesis and reactive oxygen species responses in both genotypes. H₂O₂ pre-treatment following inoculation up-regulated many stress-response genes, while hormone signal transduction and photosynthesis-related genes were down-regulated in both genotypes.

Results revealed transcriptional variation and genes associated with M. phaseolina-induced senescence in soybean. Ascorbic acid induced many photosynthetic genes, suggesting a complex regulation of defense and immunity in the plant against the hemibiotroph. Soybean plants also exhibited enhanced stress responsiveness when treated with H₂O₂ followed by inoculation with M. phaseolina. This study will broaden more research avenues related to transcriptional regulation during the M. phaseolina-soybean interaction and the potential role of receptor-like kinases, oxidative stress-responsive genes, ethylene-mediated signaling and enhanced photosynthetic gene expression when mounting host resistance to this important soybean pathogen.