Fig. 8.

Model of radial transport of amino acids in the mature root. Amino acids coming from the shoots are exported from the sieve elements and companion cells (SE/CC) of the phloem to the stele apoplasm by an exporter (i), possibly UMAMIT14. In the apoplasm, amino acids are taken up by pericycle cells by an importer or by the import ability of UMAMIT14 and UMAMIT18 (ii). Alternatively, amino acids can be taken up from the stele apoplasm by endodermis cells, (iii), by-passing the pericycle uptake. Amino acids can be further exported to the rhizosphere from the endodermis, cortex and/or epidermis cells by presently uncharacterized exporters. Symplasmic and apoplasmic routes are indicated by solid and dotted lines, respectively. PC, parenchyma cells.