Cytokinins
Authors: Lindner Ann‐Cathrin , von Schwartzenberg Klaus 
Published: November 2016

Cytokinins are phytohormones involved in the regulation of many physiological and developmental processes in plants. Generally, they stimulate cell division and help to control root and shoot meristem activity, but they further exert control on processes related to leaf senescence, nutrient uptake and pathogen resistance. In flowering plants, the cytokinin system has evolved to a high degree of complexity and the key elements of cytokinin biosynthesis, activation, inactivation, degradation and perception are well understood on a molecular level. Cytokinin activity is interconnected with many other signalling pathways including those of other hormones. Cytokinin signalling already appears at the evolutionary level of early divergent land plants. There is evidence that in future crop plant properties and productivity improvements could be achieved by making use of the increasing knowledge of the cytokinin regulatory network.

Key Concepts

Cytokinins are

N
6

derivatives of adenine with aromatic or isoprenoid side chains which vary in abundance and biological activity.

Cytokinin biosynthesis is mediated by two types of isopentenyl transferases (IPTs), directly via isoprenylation of ATP or indirectly via modification of tRNA and their subsequent breakdown.

Homeostasis of cytokinin activity can be achieved by degradation involving cytokinin oxidase/dehydrogenases (CKX), conjugation by glycosyltransferases or by transport.

Cytokinin perception is mediated by sensor histidine kinases (CHK) which initiate a multistep phosphorelay to histidine phosphotransfer proteins (HPT) and from there to type‐B response regulators (RRBs). These act as transcription factors and modulate the expression rate of cytokinin responsive genes. Type‐A response regulators (RAA) are strongly upregulated and play a role as negative regulators of cytokinin action.

Cytokinin responsive genes were detected by means of microarray and RNAseq‐based methodologies and address areas of stress and defence (drought, pathogens), redox status (oxidative burst), protein degradation and modification, signalling (including light, nutrients and hormonal interactions), cell wall, development (cell proliferation and differentiation) and secondary metabolism (flavonoids, lignin, anthocyanin and alkaloids). Cytokinin homeostasis genes can also be subject of cytokinin action.

Cytokinin signalling is interconnected to other signalling pathways on the level of biosynthesis, metabolism and signalling forming complex regulatory networks. Many crosstalk connections, antagonistic and synergistic ones, have been described for auxin, but important interactions exist as well with respect to signalling by other hormones such as gibberellin, ethylene, abscisic acid and strigolactone.

The cytokinin system has been described in Arabidopsis and a few other flowering plants in detail. However, there is increasing evidence that cytokinin‐based regulation is already functional on the evolutionary level of bryophytes.

Increasing experimental evidence makes it likely that the cytokinin regulatory system can be subject in targeted breeding approaches on crop plants in order to improve productivity‐related plant properties.