Cytokinins

Abstract

Cytokinins are N (6) substituted adenine derivatives that affect many aspects of plant growth and development, including cell division, shoot initiation and growth, leaf senescence, apical dominance, sink/source relationships, nutrient uptake, phyllotaxis, and vascular, gametophyte, and embryonic development, as well as the response to biotic and abiotic factors. Molecular genetic studies in Arabidopsis have helped elucidate the mechanisms underlying the function of this phytohormone in plants. Here, we review our current understanding of cytokinin biosynthesis and signaling in Arabidopsis, the latter of which is similar to bacterial two-component phosphorelays. We discuss the perception of cytokinin by the ER-localized histidine kinase receptors, the role of the AHPs in mediating the transfer of the phosphoryl group from the receptors to the response regulators (ARRs), and finally the role of the large ARR family in cytokinin function. The identification and genetic manipulation of the genes involved in cytokinin metabolism and signaling have helped illuminate the roles of cytokinins in Arabidopsis. We discuss these diverse roles, and how other signaling pathways influence cytokinin levels and sensitivity though modulation of the expression of cytokinin signaling and metabolic genes.

Figure 1.

Structures of cytokinins. Adenine is the parent compound of naturally occurring cytokinins, though it does not activate cytokinin responses; the N⁶ position is indicated with an arrow. trans-zeatin is the most abundant cytokinin in Arabidopsis, and the free base as well as the riboside and ribotide forms are shown. Kinetin is an artificial, aromatic cytokinin and benzyladenine is an example of a naturally occurring aromatic cytokinin. Thidiazuron is a diphenylurea-type cytokinin.

Figure 2.

Proposed biosynthetic and metabolic pathway for cytokinins. The proposed biosynthesis of trans-zeatin tri-/diphosphate in Arabidopsis is shown. Both ADP and ATP are likely substrates for the plant IPT enzyme, and these and their di- and tri-phosphate derivatives are indicted together (e. g. ATP/ADP). See text for more details.

Figure 3.

Metabolism of cytokinins. (A) Metabolic fates of trans-zeatin. The enzymes catalyzing each reaction are shown in red. (B) Reaction catalyzed by cytokinin oxidase enzymes.

Figure 4.

Cartoon representations of two-component phosphotransfer schemes. (A) A basic prokaryotic two-component system with a sensor histidine kinase and a response regulator. H and D represent the conserved phosphoaccepting histidine and aspartate residues involved in phosphorelay signaling. (B) A multistep phosphorelay system involving a hybrid sensor kinase, with input, transmitter and receiver domains, a histidine-containing phosphotransfer protein and a response regulator.

Figure 5.

Histidine kinases in Arabidopsis. (A) An unrooted phylogenetic tree of histidine kinase-related proteins derived using the amino acid sequences of the histidine kinase-like domains of these proteins (adapted from Schaller 2001). Phytochrome, ethylene receptor and cytokinin receptor families are indicated. (B) Cartoon diagram of the domain structure of the AHK cytokinin receptors. TM1 and TM2 refer to the transmembrane domains.

Figure 6.

Type-A and type-B response regulators in Arabidopsis. (A) An unrooted phylogenetic tree made using receiver domain sequences of type-A and type-B ARRs by a heuristic method. Phylogenetic analysis was performed using the PAUP 4.0 program, with 10,000 bootstrap replicates to assess the reliability of the tree. The bootstrap values are indicated on the tree. (B) Cartoon of the domain structure of type-A and type-B ARRs. Both classes of ARRs contain receiver domains. Type-B ARRs have long C-terminal extensions that include a GARP domain and a glutamine- and proline-rich region.

Figure 7.

Proposed model for phosphorelay signal transduction in cytokinin signaling. Cytokinin binds to the CHASE domains of the AHK2/AHK3/AHK4 cytokinin receptors within the lumen of the ER. Binding of cytokinin activates the transmitter domain, which autophosphorylates on a His (indicated by an H). The phosphate is then transferred an Asp residue (indicated by a D) within the fused receiver domain. The phosphate is then transferred to an AHP protein, which shuttles back and forth between the cytoplasm and the nucleus. In the nucleus, the AHPs transfer the phosphate to type-B ARRs, which then regulate the expression of many target genes, including the type-A ARRs. The type-A ARRs, which are also phosphorylated by the AHPs, in turn feedback to inhibit cytokinin signaling (indicated by ⊥). The pseudo HPt protein AHP6 and nitric oxide (NO) also negatively regulate cytokinin signaling. See text for additional details.