Cytokinin: perception, signal transduction, and role in plant growth and development

Abstract

Cytokinins are essential hormones for the proper growth and development of plants. They exert their actions through the phosphorylation of two-component signaling factors. The two-component elements in cytokinin signaling display not only overlapping, but also specific functions throughout a life cycle. These elements regulate the development of shoots, roots, and inflorescence meristems in Arabidopsis; shoot meristems in rice; and nodule formation in the lotus. They are also involved in interactions between plants and pathogens. In this review, we examine the mechanism for signaling events initiated by cytokinins in Arabidopsis.Keywords: cytokinin, development, perception, signal transduction SUMMARY OF CYTOKININ SIGNALING AND ITS FUNCTIONCytokinins play a role in many aspects of plant growth and development, including cell division, apical dominance, organ formation and regeneration, vascular development, nutrient mobilization, and senescence (Mok and Mok, 2001;Sa et al., 2001 ;Kurakawa et al., 2007).Pathways have been elucidated for cytokinin biosynthesis, metabolism, and signal transduction. The action of cytokinins is mediated through a non-canonical, two-component signaling mechanism (Kakimoto, 1996;Hwang and Sheen, 2001) (Fig. 1). Three histidine protein kinase cytokinin receptors --AHK2, AHK3, and AHK4 --bind cytokinins at the plasma membrane. These receptors initiate the Asp-HisAsp phosphorelay via histidine phosphotransfer proteins (AHPs) to response regulator proteins (ARR). The translocation of AHP proteins into the nucleus is thought to cause the transfer of a phosphoryl group to type-B ARRs. AHPs activate type-B ARR proteins, which function as transcriptional activators in cytokinin signaling. Individual activation of type-B ARRs induces the expression of a subset of primary cytokinin-response genes, including type-A ARP, s. These appear to be negative regulators in the cytokinin signal transduction pathway, and provide a feedback mechanism that is thought to fine-tune cytokinin signaling (Hwang and Sheen, 2001;Kiba et al., 2004;.Recently, novel and unexpected roles have been proposed for cytokinins in the de-differentiation process that occurs during nodule formation (Murray et al., 2007;Tirichine et al., 2007). Furthermore, cytokinins may be directly involved in defense responses (Felix and Meins, 1985; Shinshiet al., 1987). Nitrogen assimilation and resistance to disease pathogens are very important issues in agricultural science. The possible involvement of cytokinins in those processes highlights the overall importance of these phytohormones to plant health.In this review, we focus on the recent progress made in *Corresponding author; fax +82-54-279-2291 e-mail ihwang@postech.ac.kr understanding cytokinin signal transduction mechanisms, their multi-phosphorylation systems, and potential applications that exploit their novel roles in plant growth and development. THE CYTOKININ RECEPTORSSix histidine kinases are now known, in addition to a number of ethylene receptors. These are thought to be hyb...

Interaction of cytokinins with UV-B (280 -315nm) on the expansion growth of cucumber cotyledons

“…In plants cytokinins (CKs) are a class of growth-promoting phytohormone regulating a wide range of growth and developmental processes. [2][3][4] CKs are a family of ligands synthesized enzymatically from adenine, 5,6 which play a important role in various physiological events such as cell expansion and differentiation, seed germination, leaf and chloroplast senescence. 7,8 Numerous evidences indicate that CKs have both positive and negative effects on stress tolerance.…”

Effects of the coordination mechanism between roots and leaves induced by root-breaking and exogenous cytokinin spraying on the grazing tolerance of ryegrass

“…ZR is the main active form of cytokinin; thus, cytokinin is a key factor influencing the grazing tolerance of ryegrass. Several researchers have also reported that as a plant hormone, cytokinin promotes plant growth (Choi and Hwang 2007;Xu et al 2008;San-oh et al 2006;Madhaiyan et al 2006).…”

A Rapid, Simple Method for the Determination of 6-Benzyladenine by Pyrolysis Mass Spectrometry

“…BA is a synthetic cytokinin affecting plant growth and development by stimulating cell division. The role of cytokinin in plant growth and development has recently been reviewed by (Choi and Hwang 2007).…”