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Abstract
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 an Mr up to 50 kDa could move
freely through plasmodesmata. During the sink-source transition, the capacity to traffic
proteins decreased substantially and was accompanied by a developmental switch from
simple to branched forms of plasmodesmata. Inoculation of sink leaves with a movement
protein-defective virus showed that virally expressed GFP, but not viral RNA, was
capable of trafficking between sink cells during infection. Contrary to dogma that
plasmodesmata have a size exclusion limit below 1 kDa, the data demonstrate that nonspecific
"macromolecular trafficking" is a general feature of simple plasmodesmata in sink
leaves.