Main conclusion Carbonic anhydrases CA1 and CA4 attenuate plant immunity and can contribute to altered disease resistance levels in response to changing atmospheric CO 2 conditions. Abstract β-Carbonic anhydrases (CAs) play an important role in CO 2 metabolism and plant development, but have also been implicated in plant immunity. Here we show that the bacterial pathogen Pseudomonas syringae and application of the microbe-associated molecular pattern (MAMP) flg22 repress CA1 and CA4 gene expression in Arabidopsis thaliana. Using the CA double-mutant ca1ca4, we provide evidence that CA1 and CA4 play an attenuating role in pathogen-and flg22triggered immune responses. In line with this, ca1ca4 plants exhibited enhanced resistance against P. syringae, which was accompanied by an increased expression of the defense-related genes FRK1 and ICS1. Under low atmospheric CO 2 conditions (150 ppm), when CA activity is typically low, the levels of CA1 transcription and resistance to P. syringae in wild-type Col-0 were similar to those observed in ca1ca4. However, under ambient (400 ppm) and elevated (800 ppm) atmospheric CO 2 conditions, CA1 transcription was enhanced and resistance to P. syringae reduced. Together, these results suggest that CA1 and CA4 attenuate plant immunity and that differential CA gene expression in response to changing atmospheric CO 2 conditions contribute to altered disease resistance levels. Keywords Arabidopsis • CO 2 metabolism • Defense signaling • Plant immunity • Pseudomonas syringae Abbreviations CA Carbonic anhydrase ET Ethylene JA Jasmonic acid MAMP Microbe-associated molecular pattern Pst Pseudomonas syringae pv. tomato DC3000 Psm Pseudomonas syringae pv. maculicola 4326 PTI Pattern-triggered immunity SA Salicylic acid Electronic supplementary material The online version of this article (