Morphological and Molecular Analyses of the Interaction between Rosa multiflora and Podosphaera pannosa

Powdery mildew disease caused by Podosphaera pannosa is the most widespread disease in global cut-rose production, as well as a major disease in garden and pot roses. In this study, the powdery mildew resistance of different wild rose varieties was evaluated. Rose varieties with high resistance and high sensitivity were used for cytological observation and transcriptome and expression profile analyses to study changes at the morphological and molecular levels during the interaction between Rosa multiflora and P. pannosa. There were significant differences in powdery mildew resistance among three R. multiflora plants; R. multiflora '13' had high resistance, while R. multiflora '4' and '1' had high susceptibility. Cytological observations showed that in susceptible plants, 96 and 144 h after inoculation, hyphae were observed in infected leaves; hyphae infected the leaf tissue through the stoma of the lower epidermis, while papillae were formed on the upper epidermis of susceptible leaf tissue. Gene ontology enrichment analysis showed that the differentially expressed genes that were significantly enriched in biological process functions were related to the secondary metabolic process, the most significantly enriched cellular component function was cell wall, and the most significantly enriched molecular function was chitin binding. Changes in the transcript levels of important defense-related genes were analyzed. The results showed that chitinase may have played an important role in the interactions between resistant R. multiflora and P. pannosa. Jasmonic acid and ethylene (JA/ET) signaling pathways might be triggered in the interaction between susceptible R. multiflora and P. pannosa. In the resistant R. multiflora, the salicylic acid (SA) signaling pathway was induced earlier. Between susceptible plants and resistant plants, key phenylpropanoid pathway genes were induced and upregulated after P. pannosa inoculation, demonstrating that the phenylpropanoid pathway and secondary metabolites may play important and active roles in R. multiflora defense against powdery mildew infection.