Role of salicylic acid in low CO2 response in Arabidopsis

Plants use CO2 to produce carbohydrates via photosynthesis. A number of studies have focused on the high CO2 response of plants, whereas little is known about the low CO2 response of plants. Considering that phytohormones regulate responses to various environmental stresses, we hypothesized that they might also regulate the low CO2 response. In this study, we show that Arabidopsis plants adjust the low CO2 response by inducing salicylic acid (SA) and abscisic acid (ABA). Phytohormone analysis revealed a dramatic increase in SA under low CO2 conditions. Furthermore, the increase in SA was completely abolished in the SA induction-deficient2 mutant, indicating that SA is primarily synthesized via isochorismate synthase under low CO2 conditions. The level of ABA also increased under low CO2 conditions. Consistent with these observations, the expression of genes involved in SA and ABA accumulation was altered in response to low CO2 concentrations. Gene expression analysis indicated that the expression of SA- and ABA-responsive genes increased under low CO2 conditions, although the SA response of Pathogenesis-Related genes was compromised. Under low CO2 conditions, it became evident that the expression of programmed cell death-associated genes was altered, and that of photosynthesis-associated genes was suppressed. The responses of these genes were further manifested when plants accumulated high levels of SA. Taken together, we suggest that plants increase their levels of SA and ABA in response to low CO2 environments. Furthermore, the increase in SA appears to be associated with the suppression of non-essential genes and the alteration of cell death-associated genes.