Plants use CO2 to produce carbohydrates via photosynthesis. Although previous studies have focused on how plants respond to high levels of CO2, little is known about their response to low CO2. In this issue, Yoneda et al. have investigated whether phytohormones might be potentially involved in regulating this response; they found that high levels of salicylic acid (SA) and abscisic acid were induced under low CO2 conditions. Moreover, they showed that the increase in SA appears to be associated with the suppression of non-essential genes and affects cell death-associated genes.