Cytokinin Regulation of Gene Expression in the AHP Gene Family in Arabidopsis thaliana

In higher plants histidine-aspartate phosphorelays are involved in hormone and stress signaling via a two-component system of signal transduction. In this system a histidine-containing phosphotransmitter (HPt) mediates signal transmission from a sensory histidine kinase to a response regulator, providing integration and/or branching of several different signaling pathways. Five genes encoding HPts, AHP1-5, have been identified in Arabidopsis. Histidine-aspartate phosphorelays involving HPts have been at least partly implicated in cytokinin signaling. We analyzed the regulation by cytokinins of AHP gene expression. We compared the effects on steady-state levels of AHP transcripts of a short-term treatment with an aromatic cytokinin and increase in endogenous isoprenoid cytokinin levels using an activable ipt system in 8-day-old Arabidopsis seedlings. Following ipt activation, a rapid and highly preferential increase in trans-zeatin-type cytokinins was observed, whereas other isoprenoid-type cytokinins showed no or only marginal increases. The levels of cytokinin metabolites under long-term ipt activation suggest that the seedlings may have difficulties in efficiently downregulating active forms of the hormone. Using real-time RT-PCR, transient increases in steady-state levels of AHP1-4 transcripts in response to both the short-term N ⁶-benzyladenine treatment and the increase in endogenous trans-zeatin-type cytokinin levels were observed. In contrast, both the full and the alternatively spliced AHP5 transcripts remained unaltered. On the other hand, increases in steady-state levels of AHP1-4 transcripts observed in seedlings cultivated continuously in the presence of exogenous N ⁶-benzyladenine were not paralleled in seedlings with constitutively increased endogenous trans-zeatin-type cytokinins, providing further indirect evidence for distinct functions of aromatic and isoprenoid cytokinins.