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

TLDR: In this paper, the authors focused on cytokinin signaling in various plant developmental processes, including responses to biotic and abiotic stresses, and the two-component signaling pathway, operating through the His-Asp phosphorelay, controls many physiological and developmental processes throughout the plant lifecycle.

Abstract: 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.