Response of phytohormones and correlation of SAR signal pathway genes to the different resistance levels of grapevine against Plasmopara viticola infection

Phytohormones play an important role in the process of disease resistance in plants. Here, we investigated which among salicylic acid, jasmonic acid, and abscisic acid performs a key role in plant defense after Plasmopara viticola infection in grapevine. We used grapevines possessing different resistance levels against P. viticola infection to study the relationship between the expression of key genes in the related resistance signaling pathways and the level of resistance. We performed high-performance liquid chromatography-mass spectrometry to estimate the phytohormone contents in grape leaves at different time points after the infection. Furthermore, we performed quantitative analyses of key genes such as EDS1, PAD4, ICS2, PAL, NPR1, TGA1, and PR1 in the systemic acquired resistance pathway by quantitative reverse transcription-polymerase chain reaction. The results showed an increased variation in the SA content, which was maintained at high levels, after P. viticola infection in plant species exhibiting stronger resistance to the pathogen; this finding highlights the importance of SA in plant defense mechanisms. Moreover, EDS1 and PAD4 expression did not show a positive correlation with disease resistance in grape; however, higher expression of other genes that were analyzed was observed in highly resistant grape varieties. Our results provide insights into the role of phytohormone regulation in the induction and maintenance of plant defense response to pathogens.