Evidence for symplastic phloem unloading in sink leaves of barley

The pathway of phloem unloading in sink barley (Hordeum vulgare) leaves was studied using a combination of electron microscopy, carboxyfluorescein tra nsport, and systemic movement of barley stripe mosaic virus expressing the green fluorescent protein. Studies of plasmodesmatal frequencies between th e phloem and mesophyll indicated a symplastic sieve element- (SE) unloading pathway involving thick-walled and thin-walled SEs. Phloem-translocated ca rboxyfluorescein was unloaded rapidly from major longitudinal veins and ent ered the mesophyll cells of sink leaves. Unloading was "patchy" along the l ength of a vein, indicating that sieve element unloading may be discontinuo us along a single vascular bundle. This pattern was mirrored precisely by t he unloading of barley stripe mosaic virus expressing the green fluorescent protein. Transverse veins were not utilized in the unloading process. The data collectively indicate a symplastic mechanism of SE unloading in the si nk barley leaf.