REVIEW PAPER
New insights into the signaling and function of cytokinins in higher plants

Phytohormones (plant hormones) play a role in the regulation of cellular activities including cell division, elongation

and differentiation, pattern formation, organogenesis, reproduction, sex determination, and responses to

abiotic and biotic stress. The phytohormonal signal transduction pathways operate via cytokinin perception by

transmembrane receptors consisting of two domains: an extracellular domain responsible for hormone binding

and an intracellular domain, sensory His / Asp kinase (HK). Upon phytohormone binding, the receptor undergoes

a conformational change which activates its protein kinase activity. The phosphorylated intracellular domain transfers

the phosphoryl group to the His residue of the histidine phosphotransfer protein (HPt) which in turn phosphorylates

Asp residue in the Response Regulator (RR). The phosphorylated RR protein acts as either a positive

or negative transcription factor that interacts with the gene promoter or other target protein. The two-component

signaling pathway, operating through the His-Asp phosphorelay, controls many physiological and developmental

processes throughout the plant lifecycle (from seed-to-seed). Downstream plant hormone signaling also includes

proteolysis of transcriptional regulators that block the activity of transcription factors. Two-component signaling

operates more frequently in higher plants. Five classic plant hormones had been discovered by the middle of the

twentieth century: auxins, cytokinins, gibberellins, ethylene and abscisic acid. The more recently characterized

hormones include brassinosteroids, strigolactones, jasmonates and salicylates. Considering the tremendous work

that has been undertaken during the last decades, in this review we decided to concentrate on cytokinin signaling

in various plant developmental processes, including responses to biotic and abiotic stresses.