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

BACKGROUND

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.

RESULTS

RNA was sequenced from M. phaseolina-resistant and -susceptible soybean genotypes following M. phaseolina inoculation, LAA, and hydrogen peroxide (HO)-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. HO pre-treatment following inoculation up-regulated many stress-response genes, while hormone signal transduction and photosynthesis-related genes were down-regulated in both genotypes.

CONCLUSIONS

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 HO 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.