Cytokinin – A Developing Story

Abstract

In the past decade, tremendous advance has been made in understanding the biosynthesis, perception and signaling pathways of the plant hormone cytokinin. It also became clear that interfering with any of these steps greatly impacts all stages of growth and development. This has spurted renewed effort recently to understand how cytokinin signaling affects developmental processes. As a result, new insights on the role of cytokinin signaling and the downstream targets during e.g. shoot apical meristem, flower, female gametophyte, stomata and vascular development are being unraveled. In this review, we aim to give a comprehensive overview of recent findings on how cytokinin influences growth and development in plants and highlight areas for future research. 2 Cytokinins: from signaling to development Cytokinins, a class of phytohormones with diverse molecular structures [1], were first discovered in the late 1950's while looking for the molecule responsible for the growthpromoting activity of autoclaved herring sperm DNA [2]. Since then, the signaling pathway of cytokinins has been intensely studied. In our current understanding, cytokinins are synthesized by the ISOPENTENYL TRANSFERASE (IPT) and LONELY GUY (LOG) enzymes, whereas cytokinin conjugation act mainly through CYTOKININ OXIDASE (CKX) enzymes.Cytokinins are next perceived by the ARABIDOPSIS HISTIDINE KINASE2-4 (AHK2-4) receptors which initiates a phosphorylation signaling cascade. This phospho-relay starts with auto-phosphorylation of the receptors and will ultimately lead to phosphorylation and activation of B-type ARABIDOPSIS REPSONSE REGULATORS (ARRs) through the ARABIDOPSIS HISTIDINE PROTEINS (AHPs). The active ARRs will then induce cytokinin responsive genes, such as those encoding the cytokinin signaling repressors A-type ARRs or CYTOKININ RESPONSE FACTORS (CRFs). For detailed information on the biosynthesis, transport, perception and signaling cascades of cytokinins, we refer to several excellent recent reviews [1,[3][4][5]. With the now well studied aspect of cytokinin signaling, the next step is to understand how these processes impinge on plant growth and development. Over the past few years, an increasing number of studies highlight the importance of cytokinins during many developmental processes. In this review, we discuss how these insights aid in understanding how cytokinins controls development at a molecular level. Shoot developmentAlready in the early years of cytokinin research, it was clear that these molecules have a big influence on plant growth, especially together with auxin [6,7]. Plants grown on high levels of auxin and cytokinin massively proliferate and dedifferentiate leading to callus (see Glossary).Growing callus on high cytokinin levels induces shoot regeneration [6]. Based on these classical experiments, cytokinins are considered the main class of hormones involved in shoot development. Cytokinin controls shoot cell fateCytokinin signaling was shown to be vital in the context of shoot apical meristem (SAM; see Glossary) develo...

A general model to explain repeated turnovers of sex determination in the Salicaceae

“…3 ), the only Salicaceae species with SDR precisely assembled (Zhou et al , 2020; Xue et al , 2020). Recent progress has revealed that the genes involved in cytokinin signaling play important roles in the regulation of unisexual flower development in plants (Wybouw et al , 2019; Kieber et al , 2018; Feng et al , 2020). Specifically, a Y-specific type-C cytokinin response regulator ( Shy Girl , SyGI ) was recently identified as a suppressor of carpel development and therefore is a strong candidate of sex determination in kiwifruit (Akagi et al , 2018).…”

A Willow Sex Chromosome Reveals Convergent Evolution of Complex Palindromic Repeats

“…The cytokinin signaling pathway has emerged in recent years as a prominent candidate for regulating floral development and sex expression in plants [46, 47]. Cytokinin signaling in Arabidopsis functions as a His-Asp phosphorelay system that transduces the signal from dual function membrane-bound histidine kinases in the endoplasmic reticulum, through histidine phosphotransferases to the type B response regulators (ARR).…”

“…In Arabidopsis, disruption of this pathway at multiple stages has resulted in modified reproductive development, with cytokinin typically being implicated in enhancing development of female floral parts [46]. For example, it has recently been shown that the Type B response regulators ARR1 and ARR10 bind to the promoter region of the floral homeotic gene AGAMOUS , and down-regulation of these genes inhibits carpel regeneration in an in vitro assay [49].…”

Phytohormone profiles are strongly altered during induction and symptom development of the physiological ripening disorder berry shrivel in grapevine

“…The crucial cytokinin role is stimulation of cell division and differentiation, e.g. in shoot and flower development, female gametophyte development as well as vascular tissue and root meristem development and root nodule formation (Wybouw and De Rybel 2019). In plants, the active cytokinins are trans-zeatin (tZ), N6-(Δ 2 -isopentenyl)-adenine (iP), cis-zeatin (cZ), and dihydrozeatin (Haberer and Kieber 1 3 2002).…”