Plant growth: the translational connection.

The TOR (target of rapamycin) pathway is a phylogenetically conserved transduction system in eukaryotes linking the energy status of the cell to the protein synthesis apparatus and to cell growth. The TOR protein is specifically inhibited by a rapamycin-FKBP12 complex (where FKBP stands for FK506-binding protein) in yeast and animal cells. Whereas plants appear insensitive to rapamycin, Arabidopsis thaliana harbours a single TOR gene, which is essential for embryonic development. It was found that the product of this gene was capable of binding to rapamycin and yeast FKBP12. In-frame fusion with a GUS reporter gene shows that the TOR protein is produced essentially in proliferating zones, whereas the TOR mRNA can be detected in all organs suggesting a translational regulation of TOR. Phenotypic analysis of Arabidopsis TOR mutants indicates that the plant TOR pathway fulfils the same role in controlling cell growth as its other eukaryotic counterparts.