Plasmodesmal closure elicits stress responses

Plant cells are connected to their neighbors via plasmodesmata facilitating the exchange of nutrients and signaling molecules. During immune responses, plasmodesmata close, but how this contributes towards a full immune response is unknown. To investigate this, we developed two transgenic lines with which we could induce plasmodesmal closure independently of immune elicitors, using the over-active CALLOSE SYNTHASE3 alleleicals3mand the C-terminus of PDLP1 to drive callose deposition at plasmodesmata. Induction of plasmodesmal closure increased the expression of stress responsive genes, salicylic acid accumulation and resistance toPseudomonas syringaeDC3000. More homogeneous plasmodesmal closure usingicals3malso led to the accumulation of starch and sugars, decreased leaf growth, as well as hypersusceptibility toBotrytis cinerea. Based on the profile of responses, we conclude that plasmodesmal closure itself activates stress signaling, raising questions of what signals mediate this response and whether these responses occur in all circumstances when plasmodesmata close.