AtIPT3 is a key determinant of nitrate-dependent cytokinin biosynthesis in Arabidopsis

We analyzed the spatial expression pattern of Arabidopsis thaliana adenosine phosphates-isopentenyltransferase genes (AtIPT1, AtIPT3 to AtIPT8) and the effect of inorganic nitrogen sources on their regulation. In mature plants, the AtIPTs were differentially expressed in various tissues including the roots, leaves, stems, flowers and siliques. In transgenic seedlings expressing a gene for green fluorescent protein (GFP) driven by the AtIPT promoters, AtIPT1::GFP was predominantly expressed in the vascular stele of the roots, AtIPT3::GFP was in the phloem companion cells, AtIPT5::GFP was in the lateral root primordium and pericycle, and AtIPT7::GFP was in both the vascular stele and the phloem companion cells of the roots. In a long-term treatment, the accumulation level of AtIPT5 transcript was correlated with the concentrations of NO₃^- and NH₄⁺ in the growth medium. However, under nitrogen-limited conditions, AtIPT3 expression was rapidly induced by NO₃^- in the seedlings accompanying the accumulation of cytokinins, whereas AtIPT5 expression was little affected. The NO3^--dependent accumulation of both the AtIPT3 transcript and the cytokinins was markedly reduced in a Ds transposon-insertion mutant of AtIPT3. These results suggest that nitrogen availability differentially regulates expression of AtIPT3 and AtIPT5, and that AtIPT3 is a key determinant of cytokinin biosynthesis in response to rapid changes in the availability of NO₃^-.