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 develop two transgenic lines which allow to induce plasmodesmal closure independently of immune elicitors, using the over-active CALLOSE SYNTHASE3 allele icals3m and the C-terminus of PDLP1 to drive callose deposition at plasmodesmata. Induction of plasmodesmal closure increases the expression of stress responsive genes, salicylic acid accumulation and resistance to Pseudomonas syringae DC3000. More homogeneous plasmodesmal closure using icals3m also leads to the accumulation of starch and sugars, decreases leaf growth, as well as hypersusceptibility to Botrytis cinerea. Based on the profile of responses, we conclude that plasmodesmal closure activates stress signaling, raising questions about the signals mediating this response and whether these responses occur in all circumstances when plasmodesmata close.