Q&A: How do plants respond to cytokinins and what is their importance?

Abstract

Cytokinins comprise a family of signaling molecules essential for regulating the growth and development of plants, acting both locally and at a distance. Although much is known about their biosynthesis and transport, important open questions remain.

Fig. 1

Structure and composition of cytokinins. a Structures of various cytokinins (CKs). N ⁶-(Δ²-isopentenyl)adenine (iP), trans-zeatin (tZ), cis-zeatin (cZ), dihydrozeatin (DZ), and ortho-topolin (oT) are shown as representative natural CKs. Kinetin and thidiazuron (TDZ) may activate cytokinin receptors when administered, but are not physiological regulators of plant growth. b Pie charts showing relative abundance of cytokinin species in shoots of Arabidopsis thaliana (left) and Oryza sativa (right). The upper charts show side chain-variant breakdown: tZ and its conjugates (tZ-type), iP and its conjugates (iP-type), and cZ and its conjugates (cZ-type) cytokinins. The lower charts show conjugate-variant breakdown: active form (CK), ribosides (CK-ribosides), ribotides (CK-ribotides), and glucosides (CK-glucosides). Calculations are based on typical quantification data from [31] and [28] for Arabidopsis and rice, respectively. FW fresh weight

Fig. 2

Diagram of the cytokinin two component system (TCS). AHKs (AHK2, AHK3 and AHK4/WOL1/CRE1) are autophosphorylated in response to cytokinins. The phosphoryl group is transferred to type-B ARRs through AHPs. Phosphorylated type-B ARRs bind to target DNA and induce the expression of a set of genes involved in cytokinin primary response. The stability of type-A ARRs, which repress cytokinin TCS signaling, is controlled through proteolysis by the 26S proteasome in a feedback loop. Expression of AHP6, which inhibits phosphotransfer between AHKs and canonical AHPs, is repressed by cytokinin. Red arrows indicate phosphotransfer. Blue solid arrows and T-end lines represent positive and negative regulation, respectively

Fig. 3

Current model of iP and tZ biosynthesis and metabolic pathways in Arabidopsis. In Arabidopsis, IPT preferentially utilizes ATP and ADP, and CYP735A preferentially utilizes iPRMP and iPRDP, as substrates. LOG exclusively reacts with their monophosphate forms. Active cytokinins are degraded by CKX, glucosylated by UGT, or reverted to their precursors by the purine salvage pathway

Fig. 4

Spatial expression patterns of IPT3 and CYP735A2 in Arabidopsis. Spatial expression patterns of IPT3 and CYP735A2 are indicated in red and blue, respectively. IPT3 is predominantly expressed in phloem, and CYP735A2 in root vasculature. tZR is the major form of xylem cytokinins, and iPR and cZR are found in phloem [40], suggesting that iP-type and tZ-type cytokinins are directionally translocated between organs. The Arabidopsis picture is modified from Sowerby et al. [53]

Fig. 5

Local function of cytokinins in shoot apical meristem and root tip. a Schematic diagram of cytokinin production and response in shoot apical meristem. Active cytokinins are produced in the epidermal L1 layer and central zone. They diffuse basipetally and function in the AHK4-expressing organizing center. b Schematic diagram of cytokinin production and response in root tip. Active cytokinins are produced in AHP6-expressing protoxylem. They diffuse and function in adjacent procambium cells while repressing AHP6 expression