Cytokinin–auxin crosstalk Laila Moubayidin, Riccardo Di Mambro and Sabrina Sabatini Dipartimento di Genetica e Biologia Molecolare, Laboratory of Functional Genomics and Proteomics of Model Systems, Universita ` La Sapienza - P.le Aldo Moro, 5 - 00185 Rome, Italy Post-embryonic plant growth and development are sustained by meristems, a source of undifferentiated cells that give rise to the adult plant structures. Two hormones, cytokinin and auxin, are known to act antagonistically in controlling meristem activities. Here, we review recent significant progress in elucidating the molecular mechanisms through which these hormones interact to control specific aspects of plant development. For example, in the root meristem of Arabidopsis thaliana, cytokinin promotes cell differentiation by repressing both auxin signalling and transport, whereas auxin sustains root meristem activity by promoting cell division. The coordinated action of these two hormones is essential for maintaining root meristem size and for ensuring root growth. Cytokinin and auxin: master regulators of plant development Cytokinin and auxin have long been recognized as crucial signalling molecules controlling plant growth and devel- opment. In 1957, it was shown that root and shoot de- velopment in tobacco pith tissue cultures depends on the cytokinin:auxin ratio, and that organ differentiation can be regulated by changing the relative concentrations of these two growth factors in the culture medium: high levels of cytokinin supported shoot formation and high concen- trations of auxin promoted rooting, whereas, at equal concentrations of cytokinin and auxin, the tissue tended to grow in an unorganized fashion [1]. In this classic paper, the concept of hormonal control of organ formation was suggested; however, little is still understood about the in vivo significance of these tissue culture experiments and of the molecular mechanisms through which the two hor- mones act in concert to exert these effects. In addition, the mode of interaction between cytokinin and auxin often depends upon the plant species and organ being studied. This has hampered the elaboration of a general model for the control of plant growth and development by these hormones. Here, we summarize the state-of-the-art concerning cytokinin and auxin biosynthesis, transport and signalling. We also focus on recent progress in understanding the molecular mechanisms through which these hormones interact to control plant organ development. Cytokinin biosynthesis, transport and signalling Cytokinins came to light because of their ability to promote cell division in tobacco tissue culture [2] and, since their discovery, these N 6 -substituted adenine-based molecules have been associated with important developmental roles, including shoot and root development [3,4]. Isopentenyla- denine (iP), trans-zeatin (tZ) and dihydrozeatin (dZ) are the predominant cytokinins found in higher plants [5,6] and their activity in planta is thought to be controlled by a fine balance between synthesis and catabolism. The rate-limiting step of cytokinin biosynthesis in Ara- bidopsis thaliana is catalyzed by the ATP/ADP-isopente- nyltransferase (AtIPT) gene family [6]. Expression patterns of IPT genes indicate that cytokinin is produced at various sites in the plant, including roots, shoots and immature seeds [7]. Most metabolic cytokinin inactivation depends on the activity of the cytokinin oxidase/dehydro- genase (CKX) genes, which catalyze the irreversible degra- dation of cytokinins [6]. These genes, as well as IPT genes, show a spatially and temporally regulated pattern of expression during Arabidopsis development [7,8], confirm- ing a fine regulation of cytokinin turnover. By contrast the molecular basis of cytokinin transport is still unknown. The spatial expression patterns of cytokinin metabolic genes and the unequal distribution of tZ- and iP- type cytokinins in vascular transporting systems suggest that cytokinins act as both local and long-distance signals. Indeed, evidence for cytokinin acting as long distance signal [9,10] and local signal mediators [4,11] is available. Plants respond to cytokinin via a two-component signalling pathway. According to the current model in Arabidopsis, three ARABIDOPSIS HIS KINASE, AHK2, AHK3 and AHK4/WOL1/CRE1 [originally independently isolated as WOODENLEG 1 (WOL1) and CYTOKININ RESPONSE 1 (CRE1)] act as transmembrane cytokinin receptors [12–14] (Figure 1). These receptors transfer the signal via phos- phorelay to the nucleus, activating two classes of primary ARABIDOPSIS RESPONSE REGULATORS (ARRs) denominated type-A and type-B response regulators [15,16] (Figure 1). Type-B ARRs act as transcription factors [14,17] and induce the transcription of cytokinin primary response genes, including type-A ARRs [14,16]. Whereas type-B ARRs are positive regulators of the cytokinin response [14,17], type-A ARRs are negative regulators of cytokinin signalling [14,16] (Figure 1). Thus, as in the case of auxin signal transduction, the cytokinin effectors net- work is regulated by a negative feedback loop to control the magnitude of subsequent responses (Figure 1). Recently, early components of the cytokinin response pathway have been discovered: the CYTOKININ RESPONSE FACTORS (CRFs), which belong to the AP2 Arabidopsis gene family and are transcriptionally induced through the cytokinin two-component signalling pathway, in tandem with the type-B ARRs. It has been proposed that the activated CRFs, together with the activated type-B ARRs, mediate Review Corresponding author: Sabatini, S. (sabrina.sabatini@uniroma1.it). 1360-1385/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tplants.2009.06.010 Available online 4 September 2009 557