Cell-to-cell and long-distance trafficking of the green fluorescent protein in the phloem and symplastic unloading of the protein into sink tissues

Macromolecular trafficking within the sieve element-companion cell complex, phloem unloading, and post-phloem transport were studied using the jellyfi sh green fluorescent protein (GFP). The GFP gene was expressed in Arabidops is and tobacco under the control of the AtSUC2 promoter. In wild-type Arabi dopsis plants, this promoter regulates expression of the companion cell-spe cific AtSUC2 sucrose-H+ symporter gene. Analyses of the AtSUC2 promoter-GFP plants demonstrated that the 27-kD GFP protein can traffic through plasmod esmata from companion cells into sieve elements and migrate within the phlo em. With the stream of assimilates, the GFP is partitioned between differen t sinks, such as petals, root tips, anthers, funiculi, or young rosette lea ves. Eventually, the GFP can be unloaded symplastically from the phloem int o sink tissues, such as the seed coat, the anther connective tissue, cells of the root tip, and sink leaf mesophyll cells. In all of these tissues, th e GFP can traffic cell to cell by symplastic post-phloem transport. The pre sented data show that plasmodesmata of the sieve element-companion cell com plex, as well as plasmodesmata into and within the analyzed sinks, allow tr afficking of the 27-kD nonphloem GFP protein. The data also show that the s ize exclusion limit of plasmodesmata can change during organ development. T he results are also discussed in terms of the phloem mobility of assimilate s and of small, low molecular weight companion cell proteins.