In eukaryotes, membrane contact sites ( MCS ) allow direct communication between organelles. Plants have evolved a unique type of MCS , inside intercellular pores, the plasmodesmata, where endoplasmic reticulum ( ER )–plasma membrane ( PM ) contacts coincide with regulation of cell‐to‐cell signalling. The molecular mechanism and function of membrane tethering within plasmodesmata remain unknown. Here, we show that the multiple C2 domains and transmembrane region protein ( MCTP ) family, key regulators of cell‐to‐cell signalling in plants, act as ER ‐ PM tethers specifically at plasmodesmata. We report that MCTP s are plasmodesmata proteins that insert into the ER via their transmembrane region while their C2 domains dock to the PM through interaction with anionic phospholipids. A Atmctp3/Atmctp4 loss of function mutant induces plant developmental defects, impaired plasmodesmata function and composition, while MCTP 4 expression in a yeast Δtether mutant partially restores ER ‐ PM tethering. Our data suggest that MCTP s are unique membrane tethers controlling both ER ‐ PM contacts and cell‐to‐cell signalling.

Plasmodesmata act as key elements in intercellular communication, coordinating processes related to plant growth, development, and responses to environmental stresses. While many of the developmental, biotic, and abiotic signals are primarily perceived at the plasma membrane (PM) by receptor proteins, plasmodesmata also cluster receptor-like activities; whether these two pathways interact is currently unknown. Here, we show that specific PM-located Leu-rich-repeat receptor-like-kinases, Qi an Sh ou kinase (QSK1) and inflorescence meristem kinase2, which under optimal growth conditions are absent from plasmodesmata, rapidly relocate and cluster to the pores in response to osmotic stress. This process is remarkably fast, is not a general feature of PM-associated proteins, and is independent of sterol and sphingolipid membrane composition. Focusing on QSK1, previously reported to be involved in stress responses, we show that relocalization in response to mannitol depends on QSK1 phosphorylation. Loss-of-function mutation in QSK1 results in delayed lateral root (LR) development, and the mutant is affected in the root response to mannitol stress. Callosemediated plasmodesmata regulation is known to regulate LR development. We found that callose levels are reduced in the qsk1 mutant background with a root phenotype resembling ectopic expression of PdBG1, an enzyme that degrades callose at the pores. Both the LR and callose phenotypes can be complemented by expression of wild-type and phosphomimic QSK1 variants, but not by phosphodead QSK1 mutant, which fails to relocalize at plasmodesmata. Together, the data indicate that reorganization of receptor-like-kinases to plasmodesmata is important for the regulation of callose and LR development as part of the plant response to osmotic stress.