TMEM37 and infection: The significantly lower expression of all PR genes tested in this study under elevated [CO2] conditions compared to ambient [CO2] conditions could have resulted from the lower DIs of plants infected by ToMV under elevated [CO2], leading to the reduced activation of these genes, which are considered markers of defense phytohormones (e.g., salicylic acid) upon infection by biotrophic pathogens [35].