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The Histidine Kinases CYTOKININ-INDEPENDENT1 and ARABIDOPSIS HISTIDINE KINASE2 and 3 Regulate Vascular Tissue Development in Arabidopsis Shoots^[W]

Jan Hejátko^a,1,2, Hojin Ryu^b,1, Gyung-Tae Kim^c,d, Romana Dobeová^a, Sunhwa Choi^b, Sang Mi Choi^b, Pemysl Souek^e,f, Jakub Horák^a, Blanka Pekárová^a, Klaus Palme^g, Betislav Brzobohat^e,f and Ildoo Hwang^b,2,3

^a Department of Functional Genomics and Proteomics, Institute of Experimental Biology, Faculty of Science, Masaryk University, CZ-61137, Brno, Czech Republic
^b Department of Life Sciences and Functional Genomics Center, Pohang University of Science and Technology, Pohang 790-784, Korea
^c Department of Molecular Biotechnology, Dong-A University, Busan 604-714, Korea
^d Environmental Biotechnology National Core Research Center, Gyeongsang National University, Jinju 660-701, Korea
^e Institute of Biophysics AS CR, CZ-612 65, Brno, Czech Republic
^f Mendel University of Agriculture and Forestry in Brno, CZ-613 00, Brno, Czech Republic
^g Institut für Biologie II/Botany, Freiburg Institute of Advances Studies, Centre of Biological Signaling Studies, Faculty of Biology, University of Freiburg, D-79104 Freiburg, Germany

³ Address correspondence to ihwang{at}postech.ac.kr.

The development and activity of the procambium and cambium, which ensure vascular tissue formation, is critical for overall plant architecture and growth. However, little is known about the molecular factors affecting the activity of vascular meristems and vascular tissue formation. Here, we show that the His kinase CYTOKININ-INDEPENDENT1 (CKI1) and the cytokinin receptors ARABIOPSIS HISTIDINE KINASE2 (AHK2) and AHK3 are important regulators of vascular tissue development in Arabidopsis thaliana shoots. Genetic modifications of CKI1 activity in Arabidopsis cause dysfunction of the two-component signaling pathway and defects in procambial cell maintenance. CKI1 overexpression in protoplasts leads to cytokinin-independent activation of the two-component phosphorelay, and intracellular domains are responsible for the cytokinin-independent activity of CKI1. CKI1 expression is observed in vascular tissues of inflorescence stems, and CKI1 forms homodimers both in vitro and in planta. Loss-of-function ahk2 and ahk3 mutants and plants with reduced levels of endogenous cytokinins show defects in procambium proliferation and an absence of secondary growth. CKI1 overexpression partially rescues ahk2 ahk3 phenotypes in vascular tissue, while the negative mutation CKI1^H405Q further accentuates mutant phenotypes. These results indicate that the cytokinin-independent activity of CKI1 and cytokinin-induced AHK2 and AHK3 are important for vascular bundle formation in Arabidopsis.

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