The chloroplast-localized SIB1 protein was previously identified by its interaction with SIGMA FACTOR 1 (SIG1), a component of the RNA polymerase machinery responsible for transcription of plastid genes. The physiological function of SIB1 is little known. We found that expression of SIB1 is induced by infection with Pseudomonas syringae, suggesting its possible involvement in the defence response. The sib1 loss-of-function mutation compromises induction of some defence-related genes triggered by pathogen infection and the treatments with salicylic acid (SA) and jasmonic acid (JA), two key signalling molecules in the defence response. Conversely, constitutive over-expression of SIB1 causes the plants to hyper-activate defence-related genes following pathogen infection or the SA and JA treatments, leading to enhanced resistance to infection by P. syringae. SIB1 is a member of the large plant-specific VQ motif-containing protein family, and might act as a link to connect defence signalling with chloroplast function.