Figure 8

Proposed location in the root and genetic interactions of AtPHT1;8 and AtPHT1;9. (A) Proposed root-to-shoot translocation pathway for Pi through the sequential action of AtPHT1;9 and AtPHT1;8. The endodermis and/or pericycle are the most likely locations for AtPHT1;9. AtPHT1;8 appears to act in cell layers deeper in the root than AtPHT1;9, perhaps in the pericycle or xylem. Two potential translocation pathways are envisaged. The location of AtPHT1;9 and/or AtPHT1;8 in the xylem pole pericycle would facilitate the transfer of Pi into the xylem for translocation to the shoot. (Image adapted from http://www.ens-lyon.fr/RDP/SiCE/Resources.html). (B) Summary of the observed genetic interactions of AtPHT1;8 and AtPHT1;9 with other Pi-responsive genes. The transcripts from the indicated genes were either more abundant (black lines) or less abundant (red lines) in the respective PHT1 knock-out lines than in the wild type. The genetic interactions are superimposed onto the relative position proposed for AtPHT1;8 and AtPHT1;9 in the root. The diagram takes into consideration that the P concentration in each mutant is likely to be higher on the exterior side than on the interior side of the missing protein than in the wild-type. For example, since all the genes tested for interactions were repressed by Pi supply, gene transcripts that were expressed at a lower level in a mutant were positioned toward the exterior of the root, where the P concentration was presumed to be higher in the mutant than in the wild type. Such an assignment could not be made for pri-miR399d (red dotted lines) in roots of Pi-deprived seedlings, indicating our imperfect understanding of the regulation of pri-miR399 expression. The repression of AtPHO2 in leaves of Pi-sufficient Atpht1;9-1 may be in response to changes in the expression of the regulatory RNAs mir399d and AT4 (green dotted arrow).