Influence of cytokinins on the expression of phosphate starvation responsive genes in Arabidopsis

The increase in the ratio of root growth to shoot growth that occurs in res ponse to phosphate (Pi) deprivation is paralleled by a decrease in cytokini n levels under the same conditions. However, the role of cytokinin in the r escue system for Pi starvation remains largely unknown. We have isolated a gene from Arabidopsis thaliana (AtlPS1) that is induced by Pi starvation, a nd studied the effect of cytokinin on its expression in response to Pi depr ivation. AtlPS1 belongs to the TPSI1/Mt4 family, the members of which are s pecifically induced by Pi starvation, and the RNAs of which contain only sh ort, non-conserved open reading frames. Pi deprivation induces AtlPS1 expre ssion in all cells of wild-type plants, whereas in the pho1 mutant grown on Pi-rich soils, AtlPS1 expression in the root was delimited by the endoderm is. This supports the view that pho1 is impaired in xylem loading of Pi, an d that long-distance signals controlling the Pi starvation responses act vi a negative control. Exogenous cytokinins repress the expression of AtlPS1 a nd other Pi starvation-responsive genes in response to Pi deprivation. Howe ver, cytokinins did not repress the increase in root-hair number and length induced by Pi starvation, a response dependent on local Pi concentration r ather than on whole-plant Pi status. Our results raise the possibility that cytokinins may be involved in the negative modulation of long-distance, sy stemically controlled Pi starvation responses, which are dependent on whole -plant Pi status.