Plants need to cope with biotic and abiotic stress through well-coordinated cell-to-cell communication to survive as sedentary organisms. Environmental challenges such as wounding, low temperature, oxidative states and pathogen infection are known to affect the symplasmic molecular exchange between plant cells determined by plasmodesmal permeability. However, the signalling pathways and mechanisms by which different environmental stressors affect plasmodesmal permeability are not well understood. Here we show that regulating callose accumulation at plasmodesmal channels is a common strategy to alter plasmodesmal permeability under both pathogen infection and mechanical wounding stress. We have identified Arabidopsis callose synthase 1 (CalS1) and CalS8 as key genes involved in this process, and have integrated these new players into both known and novel signalling pathways that control responses to biotic and abiotic stress. Our studies provide experimental data that indicate the presence of specialized pathways tuned to respond to particular stressors, and new insights into how plants regulate plasmodesmata in response to environmental assaults.