PHLOEM UNLOADING IN SINK LEAVES OF NICOTIANA-BENTHAMIANA - COMPARISONOF A FLUORESCENT SOLUTE WITH A FLUORESCENT VIRUS

Using noninvasive imaging techniques, we compared phloem unloading of the membrane-impermeant, fluorescent solute carboxyfluorescein (CF) wi th that of potato virus X expressing the gene for the green fluorescen t protein. Although systemic virus transport took considerably longer to occur than did CF transport, unloading of both solute and virus occ urred predominantly from the class III vein network, a highly branched veinal system found between class II veins. The minor veins (classes IV and V) played no role in solute or virus import but were shown to b e functional in xylem transport at the time of import by labeling with Texas Red dextran. After virus exit from the class III phloem, the mi nor veins eventually became infected by cell-to-cell virus movement fr om the mesophyll, During the sink/source transition, phloem unloading of CF was inhibited from class III veins before the cessation of phloe m import through them, suggesting a symplastic isolation of the phloem in class III veins before its involvement in export. The progression of the sink/source transition for carbon was unaffected by the presenc e of the virus in the sink leaf. However, the virus was unable to cros s the sink/source boundary for carbon that was present at the time of viral entry, suggesting a limited capacity for cell-to-cell virus move ment into the apical (source) region of the leaf. A functional model o f the sink/source transition in Nicotiana benthamiana is presented. Th is model provides a framework for the analysis of solute and virus mov ement in leaves.