Elucidating the Cytokinin Pathway

Abstract

Histidine kinase two-component systems are involved in sensing and responding to cytokinins. Hwang and Sheen established an assay that involved transfection of mesophyll protoplasts of Arabidopsis leaves to monitor the activation of a cytokinin response gene. They expressed proteins belonging to the putative receptor proteins (histidine kinase sensors), the candidate histidine phosphotransmitter proteins (AHPs), and the candidate response regulators (ARRs) with and without added cytokinin and measured the expression of a cytokinin-responsive reporter gene. The CKI1 sensor is constitutively active in this system, but the CRE1 sensor requires added cytokinins for activity. Expression of AHP1 and AHP2 activated the reporter gene, and these two proteins exhibited cytokinin-dependent nuclear translocation. Expression of the A-type ARR proteins resulted in inhibition of the cytokinin-stimulated reporter gene activity, and expression of A-type ARR proteins activated the reporter gene even in the absence of added cytokinin (ARR2 being the strongest activator). The ability of CKI1 and ARR2 to promote cytokinin-mediated phenotypes was confirmed in transfected plants. The cytokinin pathway appears to involve receptor sensing and to require histidine kinase activity based on the analysis of mutants of the CKI1 and CRE1. Unlike other two-component pathways that converge on mitogen-activated protein kinase pathways (see Santos and Shiozaki ), cytokinin signaling stimulates the nuclear translocation of a histidine phosphotransmitter protein (AHP), which leads to the activation of the response regulators, which are themselves transcription factors. I. Hwang, J. Sheen, Two-component circuitry in Arabidopsis cytokinin signal transduction. Nature 413 , 383-389 (2001). [Online Journal] J. L. Santos, K. Shiozaki, Fungal histidine kinases. Science's STKE (2001), http://stke.sciencemag.org/cgi/content/full/OC_sigtrans;2001/98/re1 [Abstract] [Full Text]