Dual transcriptional analysis of Ocimum basilicum and Peronospora belbahrii in susceptible interactions

Authors: Johnson, Eric; Kim, Hye-Seon; TIAN, MIAOYING - University Of Hawaii; DUDAI, NATIV - Agricultural Research Organization Of Israel; TAL, OFIR - Agricultural Research Organization Of Israel; GONDA, ITAY - Agricultural Research Organization Of Israel

Submitted to: Plant Gene
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/7/2021
Publication Date: 12/11/2021
Citation: Johnson, E.T., Kim, H., Tian, M., Dudai, N., Tal, O., Gonda, I. 2021. Dual transcriptional analysis of Ocimum basilicum and Peronospora belbahrii in susceptible interactions. Plant Gene. 29. Article 100350. https://doi.org/10.1016/j.plgene.2021.100350.
DOI: https://doi.org/10.1016/j.plgene.2021.100350

Interpretive Summary: Downy mildew damages susceptible basil cultivars every year in the United States. The pathogen responsible for downy mildew disease on basil has not been well studied at the molecular level, primarily because the pathogen will not grow on artificial growth medium, but only on living plants. This report is the first to identify a large set of basil genes that are expressed in susceptible leaves in response to basil downy mildew three and six days after inoculation. A number of potential plant defense genes were upregulated at these time points in response to the pathogen. Many genes contributing to efficient photosynthesis were downregulated at these time points. The downy mildew pathogen infecting the leaves expressed high levels of genes coding for enzymes that likely degrade plant cell walls, which provides energy for the pathogen. These results improve our knowledge of the infection process of basil downy mildew and could aid the development of more effective measures for reducing the severity of the disease.

Technical Abstract: Basil downy mildew, caused by the pathogen Peronospora belbahrii, is a major problem for sweet basil growers worldwide. The genome sequences of both Ocimum basilicum and P. belbahrii were recently completed but extensive transcriptome analysis of this pathosystem has not been completed. RNA sequencing was performed using basil leaf samples (of a susceptible cultivar) collected three and six days after inoculation with sporangia from an Illinois isolate of P. belbahrii and 11,890 differentially expressed basil genes were identified (q value <0.05). Gene enrichment analysis identified several Gene Ontology (GO) terms that were upregulated in inoculated basil leaves compared to mock control leaves which included groups of putative defensive genes (False Discovery Rate (FDR)'<'0.05). In addition, there were several GO terms related to photosynthesis in the infected leaves that were downregulated in comparison to control leaves at statistically significant levels (FDR'<'0.05). The pathogen's transcriptome data also identified three highly expressed transcripts encoding secreted glycoside hydrolases (GH, each transcript level was greater than 70,000 transcripts per million, TPM), which likely release sugars from the plant cell walls needed for the growth of the pathogen. Moreover, two of these GH transcripts were also highly expressed (greater than 30,000 TPM) in susceptible basil leaves infected by a Hawaiian isolate of P. belbahrii in an independent experiment performed with a different basil cultivar. These results demonstrate a multilevel response of sweet basil to P. belbahrii that is not effective enough to block the pathogen.