Simple, but not branched, plasmodesmata allow the nonspecific trafficking of proteins in developing tobacco leaves

Leaves undergo a sink-source transition during which a physiological change occurs from carbon import to export. In sink leaves, biolistic bombardment of plasmids encoding GFP-fusion proteins demonstrated that proteins with a n M-r up to 50 kDa could move freely through plasmodesmata. During the sink -source transition, the capacity to traffic proteins decreased substantiall y and was accompanied by a developmental switch from simple to branched for ms of plasmodesmata. inoculation of sink leaves with a movement protein-def ective virus showed that virally expressed GFP, but not viral RNA, was capa ble of trafficking between sink cells during infection. Contrary to dogma t hat plasmodesmata have a size exclusion limit below 1 kDa, the data demonst rate that nonspecific "macromolecular trafficking" is a general feature of simple plasmodesmata in sink leaves.