Gatekeepers and Gatecrashers of the Symplasm: Cross-Kingdom Effector Manipulation of Plasmodesmata in Plants

Plasmodesmata (PD) are dynamic nanochannels interconnecting plant cells and coordinating development, nutrient distribution, and systemic defense. Their permeability is tightly regulated by callose turnover, PD-localized proteins, lipid microdomains, and endoplasmic reticulum (ER)-plasma membrane (PM) tethers, which together form regulatory nodes that gate symplastic exchange. Increasing evidence demonstrates that effectors from diverse kingdoms-fungi, oomycetes, bacteria, viruses, viroids, phytoplasmas, nematodes, insects, parasitic plants, and symbiotic microbes-converge on these same nodes to modulate PD gating. Pathogens typically suppress callose deposition or destabilize PD regulators to keep channels open, whereas mutualists fine-tune PD conductivity to balance resource exchange with host immunity. This review synthesizes current knowledge of effector strategies that remodel PD architecture or exploit PD for intercellular movement, highlighting novel cross-kingdom commonalities-callose manipulation, reprogramming of PD proteins, lipid rewiring, and co-option of ER-PM tethers. We outline unresolved questions on effector-PD target specificity and dynamics, and identify prospects in imaging, proteomics, and synthetic control of PD. Understanding how effectors reprogram PD connectivity can enable engineering of crops that block pathogenic trafficking while safeguarding beneficial symbioses.