| Cytokinins are a family of substituted adenine derivatives, which were discovered at the University of Wisconsin-Madison in 1956. Initially identified by their ability to promote cell division in plant cell cultures, this class of plant hormones was subsequently linked to shoot organogenesis, chloroplast development, vascular development, nutrient mobility, and release from apical dominance. Since the discovery of cytokinin, much research has revolved around the search for mechanisms of cytokinin synthesis, transport, perception, and signaling. Great strides have been made toward each of these aims in the last eight years, beginning with the identification of the first putative cytokinin receptor: the CKI1 histidine kinase. The discovery of gene families in Arabidopsis encoding all proteins characteristic of a prokaryotic histidine to aspartate (His-Asp) signal transduction pathway soon followed. In 2001, nearly fifty years after the discovery of cytokinin, the first true cytokinin receptor was identified: the CRE1 histidine kinase, leading to a model for His-Asp-mediated cytokinin signal transduction.;The current study employed reverse genetics to phenotypically characterize putative cytokinin receptors in Arabidopsis: CYTOKININ INDEPENDENT 1 (CKI1), CYTOKININ RESPONSE 1 (CRE1), ARABIDOPSIS HISTIDINE KINASE 2 ( AHK2), and ARABIDOPSIS HISTIDINE KINASE 3 ( AHK3). First, an essential role for CKI1 in female gametophyte development was identified. Specifically, our work with CKI1 mutants revealed that the CKI1 gene product is required for progression of megagametogenesis beyond stage FG5. Second, the presence of a cytokinin receptor family in Arabidopsis (CRE1, AHK2, and AHK3) was confirmed by characterizing the cytokinin-responses of single, double, and triple mutants within this gene family. Strikingly, the cre1 ahk2 ahk3 triple mutant was unable to respond to cytokinins in all assays tested.;Additionally, a role for CRE1 in the habituation was discovered. Habituation is a natural process whereby plant cell cultures lose their dependence on cytokinin for continued growth. Transcriptome analyses revealed an 18-fold up-regulation of the CRE1 transcript in habituated tissues, which was subsequently confirmed at the protein level.;This work identified important roles for histidine kinase proteins in plant growth and development. Future work will fine-tune our understanding of the roles histidine kinases play in cytokinin perception and signaling. |
