The Use of Fluorescent Tracers to Characterize the Post-Phloem Transport Pathway in Maternal Tissues of Developing...

Various polar fluorescent tracers were used to characterize the pathways for apoplastic and symplastic transport in the "crease tissues" (i.e. the vascular strand, chalaza, nucellus, and adjacent pericarp) of developing wheat (Triticum aestivum L.) grains. With mostly minor exceptions, the results strongly support existing views of phloem unloading and post-phloem transport pathways in the crease. Apoplastic movement of Lucifer yellow CH (LYCH) from the endosperm cavity into the crease was virtually blocked in the chalazal cell walls before reaching the vascular tissue. However, LYCH could move slowly along the cell wall pathway from the chalaza into the vascular parenchyma. Slow uptake of LYCH into nucellar cell cytoplasm was observed, but no subsequent symplastic movement occurred. Carboxyfluorescein (CF) imported into attached grains moved symplastically from the phloem across the chalaza and into the nucellus, but was not released from the nucellus. In addition, CF moved in the opposite direction (nucellus to vascular parenchyma) in attached grains. Thus, the post-phloem symplastic pathway can accommodate bidirectional transport even when there is an intense net assimilate flux in one direction. When fresh sections of the crease were placed in fluorochrome solutions (e.g. LYCH or pyrene trisulfonate), dye was rapidly absorbed into intact cells, apparently via unsealed plasmodesmata. Uptake was not visibly reduced by cold or by respiratory inhibitors, but was greatly reduced by plasmolysis. Once absorbed, the dye moved intercellularly via the symplast. Based on this finding, a size-graded series of fluorescein-labeled dextrans was used to estimate the size-exclusion limits (SEL) for the post-phloem symplastic pathway. In most, and perhaps all, cells of the crease tissues except for the pericarp, the molecular diameter for the SEL was about 6.2 nm. The SEL in much of the vascular parenchyma may be smaller, but it is still at least 3.6 nm. Channel diameters would likely be about 1 nm larger, or about 4.5 to 7.0 nm in the vascular parenchyma and 7.0 nm elsewhere. These dimensions are substantially larger than those for "conventional" symplastic connections (about 3 nm), and would have a greater than proportionate effect on the per channel diffusive and hydraulic conductivities of the pathway. Thus, relatively small and probably ultrastructurally undetectable adjustments in plasmodesmatal structure may be sufficient to account for assimilate flux through the crease symplast.