Characterization of a wheat histidine-containing phosphotransfer protein (HP) that is regulated by cytokinin-mediated inhibition of leaf senescence

Abstract

A wheat histidine-containing phosphotransfer (HP) cDNA ( TaHP1) was isolated from a wheat seedling cDNA library using a RT-PCR product as a probe. The predicted amino acid sequence of TaHP1 is homologous to the histidine-containing phosphotransfer module of the multistep His-Asp phosphorelay from maize and Arabidopsis. The occurrence of HP in wheat was extensively inspected, two homologues were retrieved from wheat EST database and were named as TaHP2 and TaHP3, respectively. TaHP1, 2 and 3 are high homology and form as TaHP1 gene family. Phylogenetic analysis suggests that two classes of HP existed in plants. The secondary and three-dimensional structure analysis by molecular modeling revealed that the basic structure of plant HP proteins is similar with that of YDP1, a yeast HP protein. However, a slight difference in helix arrangement was found between TaHP1 and AHP1, the representative of class I and class II HP proteins, respectively. TaHP1 is present as a single copy gene in the wheat genome as demonstrated by DNA gel blot analysis. RNA gel blot hybridization demonstrated that TaHP1 family was actively expressed in seedlings, but could not be detected in root tissues. Treatment of leaf segments with BA induced TaHP1 family transcript accumulation in a dose-dependent manner, while treatment with trans-zeatin did not. These effects paralleled BA-mediated retardation leaf senescence. These results suggest that HP, like response regulator (RR), is also involved in the resetting or fine-tuning of cytokinin signal transduction and participation in cytokinin-mediated signaling during specific physiological processes, such as leaf senescence.