Cytokinins (CKs) are well-established as important phytohormonal regulators of plant growth and development. An increasing number of studies have also revealed the function of these hormones in plant responses to biotic and abiotic stresses. While the function of certain CK classes, including trans-zeatin and isopentenyladenine-type CKs, have been studied in detail, the role of cis-zeatin-type CKs (cZs) in plant development and in mediating environmental interactions is less well defined. Here we provide a comprehensive summary of the current knowledge about abundance, metabolism and activities of cZs in plants. We outline the history of their analysis and the metabolic routes comprising cZ biosynthesis and degradation. Further we provide an overview of changes in the pools of cZs during plant development and environmental interactions. We summarize studies that investigate the role of cZs in regulating plant development and defence responses to pathogen and herbivore attack and highlight their potential role as 'novel' stress-response markers. Since the functional roles of cZs remain largely based on correlative data and genetic manipulations of their biosynthesis, inactivation and degradation are few, we suggest experimental approaches using transgenic plants altered in cZ levels to further uncover their roles in plant growth and environmental interactions and their potential for crop improvement.

Department of Molecular Ecology Max Planck Institute for Chemical Ecology Hans Knöll Str 8 07745 Jena Germany

Department of Plant and Environmental Sciences Copenhagen Plant Science Centre University of Copenhagen Højbakkegård Allé 13 2630 Taastrup Denmark

KWS SAAT AG Molecular Physiology Grimsehlstrasse 31 37555 Einbeck Germany

Laboratory of Hormonal Regulations in Plants Institute of Experimental Botany AS CR v v i Rozvojová 263 165 02 Prague 6 Czech Republic