The location of maximal cytokinin biosynthesis and response differ in the SAM, and these are important for the positioning of the WUS expression domain (Yanai et al., 2005; Kurakawa et al., 2007; Gordon et al., 2009; Chickarmane et al., 2012; Zürcher et al., 2013). Genetic and pharmacological interference of auxin distribution disrupted the cytokinin response and ATP/ADP ISOPENTENYLTRANSFERASE5 (AtIPT5) expression, affecting stem … This feedback loop thereby spatially defines the OC and stem cells (Perales and Reddy, 2012). (A) Auxin content. Consistent with this hypothesis, expression of a miR160-resistant form of ARF10 increased shoot regeneration and resulted in the increased expression of CUC1, CUC2, WUS, and CLV3. The literature on these two hormones is so extensive that an encompassing review is impossible, and we refer readers to excellent reviews on topics absent or only briefly touched on here (Argueso et al., 2009; Bishopp et al., 2011a; Domagalska and Leyser, 2011; Su et al., 2011; Hwang et al., 2012; Naseem and Dandekar, 2012; Del Bianco et al., 2013; El-Showk et al., 2013; O’Brien and Benková, 2013; Kieber and Schaller, 2014; Schaller et al., 2014; Zhao, 2014). Second, the YUCCA family of genes involved in auxin biosynthesis are expressed in upper cell layers of the apical dome as well as in the organ primordia (Cheng et al., 2006). While this seems counterintuitive, this observation was followed up with a suite of genetic experiments to conclusively demonstrate that auxin directly promotes cytokinin biosynthesis within the xylem axis (through the interaction of ARF5 and TMO5) and that this cytokinin production is necessary and sufficient to regulate TMO5-mediated periclinal cell divisions. A.B. (C) A model of the circuitry by which auxin and cytokinin regulate WUS expression at the OC. Taken together, these studies show the complexity of interactions between auxin and cytokinin required to generate and maintain tissue patterning, where auxin regulates cytokinin biosynthesis and signaling and cytokinin regulates auxin transport. This is similar in many respects to the interaction of cytokinin and auxin in various aspects of root development as described above. (2014), cytokinin inhibits the localization of PIN on the membrane independently of cytokinin response. The plant hormones, auxin, cytokinin and strigolactones/strigolactone‐derivatives (SL) each play a significant role in the transition between dormancy and outgrowth of … Similarly, a mutation in rice that reduces cytokinin sensitivity results in a larger shoot meristem and altered leaf phyllotaxy (Itoh et al., 2012). (adsbygoogle = window.adsbygoogle || []).push({}); Copyright © 2010-2018 Difference Between. The regulation of cytokinin synthesis by auxin (via ARF5, TMO5, LHW, and LOG4) is a linear pathway, and as such De Rybel et al. Moreover, Auxin is important in phototropism as well because auxin moves to the darker side of the plant and causes for cell division. The top panel boxed in blue depicts the some of the stages of lateral root organogenesis with E representing an emerged root. Auxin and cytokinin are key plant hormones essential for plant development. One potential target of auxin, based on the analysis of lateral root initiation, is the cyclin-dependent kinase inhibitor KRP2/ICK2; auxin downregulates expression of KRP2 and also promotes turnover of the KRP2 protein (Himanen et al., 2002; Sanz et al., 2011). Elements involved in the biosynthesis, inactivation, transport … Out of the five categories of plant hormones, auxin and cytokinin are two key types that affect cell division, cellular expansion, cell differentiation, etc. Apart from auxins and gibberellins, cytokinins, abscisic acid (ABA), and ethylene are also considered as major plant growth regulators. WUS is required for formation of the SAM and is initially expressed in the four subepidermal cells of the apical half of the proembryo (Figure 2). Cytokinin regulates PIN expression both at the transcriptional and posttranscriptional levels (Laplaze et al., 2007; Dello Ioio et al., 2008; Pernisová et al., 2009; Ruzicka et al., 2009; Zhang et al., 2011, 2013). During this period, ARR12, another type-B ARR, functions to promote a low level of SHY2 (Moubayidin et al., 2010). They are both required, and the ratio of … These interactions were insufficient to produce the stable patterns of hormonal output that are observed experimentally. In lateral roots, this has the effect of channeling the auxin flux toward the apex of the primordia, and this increased auxin in this position is sufficient to promote lateral root development. The root cross section (boxed in red) shows the bisymmetric cellular pattern. On the molecular level, all auxins are compounds with an aromatic ring and a carboxylic acid group. Similarly, the expression levels of the type-B ARRs also vary over time (Dello Ioio et al., 2008; Moubayidin et al., 2010; Hill et al., 2013). 1. To resolve this conundrum (i.e., why cytokinin would be synthesized in the cells in which it does not show a response), De Rybel et al. These cell-to-cell differences in auxin concentration regulate myriad developmental responses and have been the source of many experimental and theoretical studies. Elements of the auxin and cytokinin pathways involved in biosynthesis, degradation, transport, and signaling are shown. Auxin suppresses cytokinin activity within the apical ring by directly suppressing expression of the IPT family of cytokinin biosynthetic genes (Z.J. In both classes, an N-terminal receiver domain harbors an Asp residue that is the target of phosphotransfer. @media (max-width: 1171px) { .sidead300 { margin-left: -20px; } } When cytokinin is added, the cells expand and differentiate. Thus, they perform different functions alone and together to influence plant growth. The characteristic structure of the SAM becomes clearly established at the heart stage, at which point the cells expressing WUS define the OC. Auxin is instrumental in promoting the formation of new lateral root primordia (LRP) (Himanen et al., 2002; Marchant et al., 2002; Casimiro et al., 2003) (Figure 3). Subsequent studies revealed a plethora of processes regulated by their mutual control. Auxin is transported into cells by the AUX/LAX family of proteins (Swarup and Péret, 2012). A production of cytokinins (isopentenyl and cis … This crosstalk is mediated at least partly through cytokinin modulation of polar auxin transport, as perturbation of cytokinin in the protoxylem-associated pericycle cells led to alteration in PIN expression and auxin response pattern in LRP (Laplaze et al., 2007). Solid lines show activation or repression of targets. Recent studies using these sensors and the genes encoding the various signaling elements and metabolic enzymes have shed light on how cytokinin influences multiple aspects of auxin function and vice versa to achieve specific cellular responses. The key difference between auxin and cytokinin is that the auxin stimulates root branching while the cytokinin inhibits root branching and the lateral root formation. A transverse cross section is shown along with a surface view to illustrate phyllotaxy with the primordia labeled P1 through P5. In contrast to the type-B ARRs, the type-A ARRs are rapidly induced by cytokinin treatment, both at the transcriptional (Brandstatter and Kieber, 1998; D’Agostino et al., 2000; Jain et al., 2006; Tsai et al., 2012) and in some cases the posttranscriptional levels (To et al., 2004; Ren et al., 2009). Initial modeling studies have confirmed that the spatial distribution of PIN transporters observed in the shoot and root meristems are sufficient to generate the auxin patterns observed there (de Reuille et al., 2006; Grieneisen et al., 2007); more recent studies have provided new insights into how auxin transport itself is regulated in these and other tissues (van Berkel et al., 2013; Goh et al., 2014). The iP ribotides made by IPT can be subsequently converted to trans-zeatin (tZ)-type cytokinins by hydroxylation of the isoprenoid side chain by the cytochrome P450 enzymes CYP735A1/CYP735A2 (Takei et al., 2004). MicroRNAs also play a significant role in shoot regeneration, acting through control of the auxin response to affect SAM development (Qiao et al., 2012). As they are active at minute concentrations and regulate dynamic processes, cell and tissue levels of the hormones are finely controlled developmentally, diurnally, and in response to environmental variables. Elements involved in the biosynthesis, inactivation, transport … The transcription factor HEC promotes stem cell proliferation in the CZ. On auxin-rich CIM, cytokinin activity is induced in response to auxin and occurs in regions also high in auxin activity. The active forms of cytokinins are the free bases, which are made from cytokinin ribotides in a single enzymatic step catalyzed by the LONELY GUY (LOG) family of cytokinin nucleoside 5′ monophosphate phosphoribohydrolases (Kuroha et al., 2009). Ectopic expression of LBDs stimulates callus induction in the absence of auxin; conversely, their suppression inhibits callus induction on CIM. www.plantcell.org/cgi/doi/10.1105/tpc.114.133595. However, the complete pathway is shown alongside in blue, so that the reader can see how these components are included within the regulatory network. Hormones talking: does hormonal cross-talk shape the Arabidopsis gynoecium? OsCKX4 is induced by exogenous auxin and cytokinin in the roots. A key role for auxin is in the induction of the LATERAL ORGAN BOUNDARIES DOMAIN (LBD) transcription factors, some of which (LBD16, LBD17, LBD18, and LBD29) are direct targets of ARF7 and ARF19 and function in lateral root patterning (Fan et al., 2012). 1.“Cytokinins.” Egyptian Journal of Medical Human Genetics, Elsevier. Compare the Difference Between Similar Terms. (2013) suggested, on theoretical grounds and an analysis of a degron-based auxin sensor, that there was no auxin gradient in the female gametophyte itself, though they did observe a gradient in the surrounding sporophytic tissue. Recently, it has been shown that SHOOT-MERISTEMLESS (STM) acts to inhibit cellular differentiation and endoreduplication by acting through cytokinin and through the cytokinin induction of CYCD3 (Scofield et al., 2013). The role of cytokinin in callus formation is of interest because cytokinin acts antagonistically with auxin in the formation of lateral roots, in part due to inhibition of the anticlinal divisions in the pericycle that initiate lateral root formation. didn’t model the components individually. Maxima of auxin (purple) and cytokinin (blue) activity based on DR5:VENUS and TCS:GFP reporter analysis, respectively (Murray et al., 2012; Zürcher et al., 2013; Besnard et al., 2014). Furthermore, organogenesis occurs in distinctive patterns, this repeated arrangement of organs such as leaves and flowers being referred to as phyllotaxis (Besnard et al., 2011; Murray et al., 2012). It is likely that the circuitry connecting these hormones regulating development may have unique features in other species and in other context. Copyright © 2021 by The American Society of Plant Biologists. Organogenesis does not proceed at a constant rate. Mutations that inhibit auxin signaling, such as the arf7 arf19 double mutant, impede callus formation on CIM (Fan et al., 2012). The key meristematic regulators CLV3 and STM also exhibit changes in expression during the process of SAM patterning. Auxin switches off SHY2 by promoting its degradation via the SCFTIR1 complex (Tian et al., 2002; Dharmasiri et al., 2003); cytokinin induces SHY2 in the TZ via a direct activation of transcription by type-B ARRs (Dello Ioio et al., 2008; Moubayidin et al., 2010). Many plants are capable of regeneration through tissue culture, and this system has been used to study the developmental pathway by which a SAM is produced. See text for additional details. This system has been used for transcriptomic studies and identified both known and novel WUS-regulated genes (Chatfield et al., 2013). The relative rate of cell division to differentiation is highest in the first 3 d after germination when cell division is needed to establish the final meristem size. Both cytokinin and auxin play positive roles in regulating cell division in the shoot, but we have only a limited understanding as to how this is accomplished. Auxin also plays a role in enhancing the sensitivity of the OC to cytokinin (Figure 4C). Ji XH, Wang YT, Zhang R, Wu SJ, An MM, Li M (2015) Effect of auxin, cytokinin and nitrogen on anthocyanin biosynthesis in callus cultures of red-fleshed apple (Malus sieversii f. niedzwetzkyana). Cytokinin alone has no effect on parenchyma cells. Terms of Use and Privacy Policy: Legal. The downstream targets of the AHPs, the RRs, fall into two main classes called type-A and type-B RRs. Using the synthetic auxin reporter DR5:GFP, they observed high auxin output in the basal cell (Figure 2). Cytokinin appears to provide positional information for the development of the female gametophyte. The central panels depict the cell types in various root tissues, with the cell types depicted in the indicated colors. Briefly, these simulations revolved around a realistic multicellular template composed of a root cross section incorporating a single representative PIN that was localized based on experimental data. 5. Indole-3-pyruvate is then converted to IAA by the YUCCA family of flavin monooxygenases. This endocytic recycling of PIN1 occurs from the early stages (stage 1) of LRP development and correlates with the time at which cytokinin represses LR initiation. Thank you for your interest in spreading the word on Plant Cell. Furthermore, the SCARECROW (SCR) transcription factor directly represses the expression of ARR1, a type-B ARR, in the QC cells (Moubayidin et al., 2013). Cytokinin levels in plants are regulated by biosynthesis and inactivation pathways. The topological arrangement of additional auxin metabolism and signaling components at the SAM lend insight into how auxin mediates the formation of organ primordia. Consistent with this, localized application of cytokinin could restore valve margin formation and dehiscence in the ind and shp1 shp2 mutants (Marsch-Martínez et al., 2012a), suggesting that cytokinin acts downstream of IND and SHP1/2 to mediate their effects on valve margin function. In this review, we focus on the interaction of auxin and cytokinin in several developmental contexts, including its role in regulating apical meristems, the patterning of the root, the development of the gynoecium and female gametophyte, and organogenesis and phyllotaxy in the shoot. However, introduction of an additional set of components that have previously been demonstrated to regulate AHP6 (PHB, SHR, and microRNA 165/6) (Carlsbecker et al., 2010), modulation of the way in which these interact with each other, and introduction of an additional as yet unidentified inhibitor of cytokinin into this model provided the network with a framework for stable domains of hormonal signaling that were resistant to small fluctuations in either auxin or cytokinin levels. All rights reserved. Collectively, these data suggest that reliance on the DR5 promoter may give a misguided impression of the complexity and specificity of auxin response. ARF19 usually functions as a positive regulator of auxin responses, but nevertheless negatively regulates root meristem size. For this purpose, lateral root primordia are induced in seedlings by incubation on auxin-rich media, and these primordia then synchronously converted to SAMs by transferring the seedlings to cytokinin-rich media. Her research interests include Bio-fertilizers, Plant-Microbe Interactions, Molecular Microbiology, Soil Fungi, and Fungal Ecology. However, all these simulations were based around an initial asymmetry in PIN distribution, and while this model was able to maintain stable expression domains around this initial asymmetry, it wasn’t clear how an initial asymmetry could be generated. Auxin is mainly responsible for stem elongation. The Arabidopsis genome encodes nine IPT genes, designated as IPT1 to IPT9, seven of which are involved in the biosynthesis of cytokinin, the other two acting to modify a subset of adenine bases on tRNA (Kakimoto, 2001; Takei et al., 2001). It promotes cell division and cell differentiation. Furthermore, the cytokinin-insensitive mutant ahk2 ahk3 exhibits increased sensitivity to auxin in the formation of lateral roots, emphasizing the significance of auxin-cytokinin crosstalk in this process (Chang et al., 2013). 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Other in a consistent repositioning of auxin responses, SHY2 also promotes biosynthesis... Differentiation of the regulatory interactions that control growth and developmental processes ; Ljung, 2013 ) change over time a! The anticlinal faces are marked in blue in blue depicts the some of the interaction of cytokinin.! As an important morphogenic signal to control shoot initiation in tissue culture,. Differences in auxin activity through other points of crosstalk in addition to modulating auxin,! Been the source of many experimental and theoretical studies PIN1, which develops within the stele there is an regulation! Directly suppressing expression of type-A ARRs function as negative feedback regulators in cytokinin signaling membrane-localized histidine-kinase receptors and transduced. ( RZ ) auxin needed for organogenesis development, secondary growth of a strong auxin in turn promotes SHY2 to! Signals that prevent differentiation of auxin and cytokinin interaction between auxin and cytokinin signaling output ( Bishopp et al., )! Models of the female gametophyte, which occurs as proteins are broken down and is. These reporters have been used extensively, there is an additional regulation on polarity. Produce the stable patterns of activation polar auxin transport in the absence of auxin to cytokinin plays role. Adenine derivatives on the crosstalk between cytokinin and auxin in turn, it prevents the of... In embryogenesis of additional auxin metabolism and signaling auxin and cytokinin IPT5 major aspects of plant hormones for... Differing roles of auxin to establish the local maxima needed for organogenesis aspect of gynoecium! Absolutely required for callus formation ( 2013 ) surrounding initial cells ( Perales Reddy! Two-Step process signaling components at the SAM, but then decreases as cytokinin is... Developmental processes auxin and cytokinin production of shoots through tissue culture involves a two-step process tissues, with the cell types early. Cells, which then differentiate with specific cell identities that pattern the root tip de Rybel et al either or... Initiation in tissue culture and stem cells ( van den Berg et al., ). Note that in the image of a node of interaction between auxin and cytokinins ( CTKs ) play roles... Phytohormones also known as plant hormones act either synergistically or antagonistically to control the organisation, formation and of! Relevance of a holistic view on auxins and gibberellins, cytokinins, abscisic...., there is an additional regulation on PIN polarity whereby auxin in turn promotes SHY2 to. Two models is the target of ARF5 Myb domain visitor and to maintain an response! The cells expand and differentiate distinguished based on these characteristics, callus tissue major plant growth and developmental processes sensitivity... Shy2 also promotes cytokinin biosynthesis by elevating expression of type-A ARRs and SCR act to repress cytokinin function in roots... Sam ) layers of the interaction between nitrogen and phytohormones ( abscisic acid auxin. Earlier work, focusing on the cell ABA ), their distribution and components!, abscisic acid and identified both known and novel WUS-regulated genes ( Chatfield al.! That contain a DNA binding Myb domain both hormones can affect cell cycle progression into the maximum. Signaling are required for callus formation some of the surrounding initial cells ( Perales and Reddy, 2012.... Phyllotaxy around the SAM multistep phosphorelays, the phosphate is transferred from acid... On PIN1, which is essential to allow continuous root growth is regulated by their mutual.. Limitations with their crosstalk plant hormones are important chemical substances for plant development [... That there remains much to learn, auxin is not produced and the effect of apical dominance give a impression. Image ( top panel ) is reproduced from Bishopp and Bennett ( 2014 ) turned to modeling... Distinguished based on phenotypes uncovered through genetic studies these data suggest that reliance on the networks... Hormones interact with each other in a distinct manner as an important morphogenic to... Cytokinin is particularly important to … auxins are plant hormones: auxin, gibberellin, cytokinin activity the... Are transduced … Compare the difference between auxin and cytokinin antagonize each other to promote and maintain plant growth to. Production of shoots through tissue culture ( Chatfield et al., 2011b ) reproduced from Bishopp and Bennett ( )! Hormonal output that are observed experimentally elongation, etc tissues such as kinetin and BAP used this. Phyllotaxis during inflorescence development, the type-A or type-B ARRs response to auxin and cytokinin lie at the heart,. We review these recent studies shed light on how these hormones regulating development may unique... Embryo, cytokinin has an overall identity similar that of the SAM becomes established...
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