Cytokinin regulation of plant growth and stress responses

Abstract

Plant hormones cytokinins stimulate cell division; regulate shoot and root development; promote leaf growth and flower, fruit, and seed formation; stabilize photosynthetic machinery; suppress senescence; and enhance sink strength and nitrogen acquisition. Cytokinin signaling is mediated by multistep phosphorelay. Binding of the cytokinin molecule to CHASE domain of the histidine kinase receptors triggers an autophosphorylation of the histidine domain and subsequent intramolecular transfer to receiver domain. Phosphoryl group is then transmitted to histidine phosphotransfer proteins and subsequently to type B response regulators (transcription factors) in the nucleus. Phosphotransfer proteins interact also with transcription factors CRFs (cytokinin response factors) that represent signaling side branch. The signal strength is regulated by cytokinin metabolism, which controls levels of active cytokinins, through feedback inhibition of signal transduction via type A response regulators (primary response genes), S-nitrosylation of phosphotransfer proteins, and/or proteasome degradation of type B and type A response regulators. Practical applications of cytokinins include their use in in vitro micropropagation, stimulation of flower branching, crop tillering or berry formation, and prolongation of fruit or tuber shelf life. Targeted elevation of cytokinin levels was found to increase the tolerance of plants to abiotic stresses, at least partially by diminishing the negative stress effects on photosynthesis. Recently, function of cytokinins in biotic stress responses has been also recognized. Full utilization of cytokinin potential to improve plant productivity by regulation of plant development has been until now limited by the necessity of targeting modulation of their levels or signal transduction in a time- and tissue-specific manner.