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ARR5 and ARR6 Mediate Tissue Specific Cross-talk between Auxin and Cytokinin in Arabidopsis  [PDF]
Aparna Kakani, Zhaohua Peng
American Journal of Plant Sciences (AJPS) , 2011, DOI: 10.4236/ajps.2011.24065
Abstract: Auxin and cytokinin interaction plays an essential role in a wide range of plant growth and developmental processes. The interaction consequence of the two hormones is highly tissue specific. The molecular mechanisms underlying the tissue specificity are largely unknown. Here we show that the cytokinin signaling pathway key components ARR5 and ARR6 respond to auxin with a highly tissue specific and contrasted pattern in Arabidopsis seedlings and calli in the presence of cytokinin. Our results suggest that the two highly homologous but functionally distinct genes, ARR5 and ARR6, play a critical role in mediating tissue specific interaction between auxin and cytokinin.
Cytokinin and auxin intersection in root meristems
Elisabeth J Chapman, Mark Estelle
Genome Biology , 2009, DOI: 10.1186/gb-2009-10-2-210
Abstract: In plants, populations of undifferentiated cells called meristems serve as the source of all new growth throughout post-embryonic development. Meristem size, or the number of undifferentiated cells in the meristem, is maintained by balancing stem-cell replenishment (through cell division) with cell differentiation and elongation to form new tissues and organs. In the Arabidopsis root, meristem maintenance is controlled by the small-molecule hormones auxin and cytokinin, which affect cell division and cell elongation. Although auxin and cytokinin have long been known to interact during development, the molecular mechanisms of these interactions have been elusive. In a recent issue of Science, Sabrina Sabatini and colleagues (Dello Ioio et al. [1]) have elucidated a simple molecular mechanism by which cytokinin regulates meristem size in the Arabidopsis root by antagonizing auxin signaling in the transition zone, the region where cells leave the meristem to differentiate and elongate.Auxin and cytokinin have contrasting roles in root meristems. Auxin is required for meristem cell division: application of exogenous auxin increases root meristem size, for example, whereas cytokinin reduces it [2,3]. Basipetal transport and lateral distribution of auxin are required for stem-cell replenishment, as plants carrying mutations in three members of the PIN-FORMED (PIN) family of auxin-efflux carrier proteins have reduced meristem size [4]. In previous work, Sabatini and colleagues [3] had shown that endogenous cytokinin is required to control stem-cell division, as plants defective in cytokinin biosynthesis had expanded meristems. This phenotype is also observed in plants with mutations in ARABIDOPSIS HISTIDINE KINASE3 (AHK3), which encodes a cytokinin receptor, or ARABIDOPSIS RESPONSE REGULATOR1 (ARR1) or ARABIDOPSIS RESPONSE REGULATOR12 (ARR12), B-type response regulators that encode transcription factors that specifically activate 'cytokinin-responsive' genes [5] (Figure 1a
AHP6 Inhibits Cytokinin Signaling to Regulate the Orientation of Pericycle Cell Division during Lateral Root Initiation  [PDF]
Sofia Moreira, Anthony Bishopp, Helena Carvalho, Ana Campilho
PLOS ONE , 2013, DOI: 10.1371/journal.pone.0056370
Abstract: In Arabidopsis thaliana, lateral roots (LRs) initiate from anticlinal cell divisions of pericycle founder cells. The formation of LR primordia is regulated antagonistically by the phytohormones cytokinin and auxin. It has previously been shown that cytokinin has an inhibitory effect on the patterning events occurring during LR formation. However, the molecular players involved in cytokinin repression are still unknown. In a similar manner to protoxylem formation in Arabidopsis roots, in which AHP6 (ARABIDOPSIS HISTIDINE PHOSPHOTRANSFER PROTEIN 6) acts as a cytokinin inhibitor, we reveal that AHP6 also functions as a cytokinin repressor during early stages of LR development. We show that AHP6 is expressed at different developmental stages during LR formation and is required for the correct orientation of cell divisions at the onset of LR development. Moreover, we demonstrate that AHP6 influences the localization of the auxin efflux carrier PIN1, which is necessary for patterning the LR primordia. In summary, we show that the inhibition of cytokinin signaling through AHP6 is required to establish the correct pattern during LR initiation.
Effects of Combination of Different Levels of Auxin (NAA) and Cytokinin (BAP) on in vitro Propagation of Dioscorea rotundata L. (White Yam)
I.O. Ezeibekwe,C.L. Ezenwaka,F.N. Mbagwu,C.I.N. Unamba
Journal of Molecular Genetics , 2013,
Abstract: Studies were carried out with the aim of evaluating in vitro the effects of growth regulators-auxin (NAA) and cytokinin (BAP) combined at different levels on Dioscorea rotundata regeneration potentials on modified Murashige and Skoog media. Concentrations of 0, 0.25, 0.5, 0.75, 1.00 mg L-1 and 0, 0.1, 0.2, 0.3 and 0.4 mg L-1 of NAA and BAP, respectively were used to subculture healthy white yam plantlets. The plant height, number of leaves, nodes, vines, roots and fresh weights were evaluated. Results obtained when analyzed at 5% level of significance showed that the concentration of both hormones (auxin and cytokinin) had significant effects on plant regeneration. BAP (0.2 mg L-1) in combination with NAA (0.5 mg L-1) showed more increase in almost all the parameters measured when compared to other concentrations combined.
Two-Component Elements Mediate Interactions between Cytokinin and Salicylic Acid in Plant Immunity  [PDF]
Cristiana T. Argueso,Fernando J. Ferreira,Petra Epple,Jennifer P. C. To,Claire E. Hutchison,G. Eric Schaller,Jeffery L. Dangl,Joseph J. Kieber
PLOS Genetics , 2012, DOI: 10.1371/journal.pgen.1002448
Abstract: Recent studies have revealed an important role for hormones in plant immunity. We are now beginning to understand the contribution of crosstalk among different hormone signaling networks to the outcome of plant–pathogen interactions. Cytokinins are plant hormones that regulate development and responses to the environment. Cytokinin signaling involves a phosphorelay circuitry similar to two-component systems used by bacteria and fungi to perceive and react to various environmental stimuli. In this study, we asked whether cytokinin and components of cytokinin signaling contribute to plant immunity. We demonstrate that cytokinin levels in Arabidopsis are important in determining the amplitude of immune responses, ultimately influencing the outcome of plant–pathogen interactions. We show that high concentrations of cytokinin lead to increased defense responses to a virulent oomycete pathogen, through a process that is dependent on salicylic acid (SA) accumulation and activation of defense gene expression. Surprisingly, treatment with lower concentrations of cytokinin results in increased susceptibility. These functions for cytokinin in plant immunity require a host phosphorelay system and are mediated in part by type-A response regulators, which act as negative regulators of basal and pathogen-induced SA–dependent gene expression. Our results support a model in which cytokinin up-regulates plant immunity via an elevation of SA–dependent defense responses and in which SA in turn feedback-inhibits cytokinin signaling. The crosstalk between cytokinin and SA signaling networks may help plants fine-tune defense responses against pathogens.
The Clubroot Pathogen (Plasmodiophora brassicae) Influences Auxin Signaling to Regulate Auxin Homeostasis in Arabidopsis  [PDF]
Linda Jahn,Stefanie Mucha,Sabine Bergmann,Cornelia Horn,Paul Staswick,Bianka Steffens,Johannes Siemens,Jutta Ludwig-Müller
Plants , 2013, DOI: 10.3390/plants2040726
Abstract: The clubroot disease, caused by the obligate biotrophic protist Plasmodiophora brassicae, affects cruciferous crops worldwide. It is characterized by root swellings as symptoms, which are dependent on the alteration of auxin and cytokinin metabolism. Here, we describe that two different classes of auxin receptors, the TIR family and the auxin binding protein 1 (ABP1) in Arabidopsis thaliana are transcriptionally upregulated upon gall formation. Mutations in the TIR family resulted in more susceptible reactions to the root pathogen. As target genes for the different pathways we have investigated the transcriptional regulation of selected transcriptional repressors ( Aux/IAA) and transcription factors ( ARF). As the TIR pathway controls auxin homeostasis via the upregulation of some auxin conjugate synthetases (GH3), the expression of selected GH3 genes was also investigated, showing in most cases upregulation. A double gh3 mutant showed also slightly higher susceptibility to P. brassicae infection, while all tested single mutants did not show any alteration in the clubroot phenotype. As targets for the ABP1-induced cell elongation the effect of potassium channel blockers on clubroot formation was investigated. Treatment with tetraethylammonium (TEA) resulted in less severe clubroot symptoms. This research provides evidence for the involvement of two auxin signaling pathways in Arabidopsis needed for the establishment of the root galls by P. brassicae.
RcRR1, a Rosa canina Type-A Response Regulator Gene, Is Involved in Cytokinin-Modulated Rhizoid Organogenesis  [PDF]
Bin Gao, Lusheng Fan, Xingxing Li, Huifang Yang, Fengluan Liu, Ling Wang, Lin Xi, Nan Ma, Liangjun Zhao
PLOS ONE , 2013, DOI: 10.1371/journal.pone.0072914
Abstract: In vitro, a new protocol of plant regeneration in rose was achieved via protocorm-like bodies (PLBs) induced from the root-like organs named rhizoids that developed from leaf explants. The development of rhizoids is a critical stage for efficient regeneration, which is triggered by exogenous auxin. However, the role of cytokinin in the control of organogenesis in rose is as yet uncharacterized. The aim of this study was to elucidate the molecular mechanism of cytokinin-modulated rhizoid formation in Rosa canina. Here, we found that cytokinin is a key regulator in the formation of rhizoids. Treatment with cytokinin reduced callus activity and significantly inhibited rhizoid formation in Rosa canina. We further isolated the full-length cDNA of a type-A response regulator gene of cytokinin signaling, RcRR1, from which the deduced amino acid sequence contained the conserved DDK motif. Gene expression analysis revealed that RcRR1 was differentially expressed during rhizoid formation and its expression level was rapidly up-regulated by cytokinin. In addition, the functionality of RcRR1 was tested in Arabidopsis. RcRR1 was found to be localized to the nucleus in GFP-RcRR1 transgenic plants and overexpression of RcRR1 resulted in increased primary root length and lateral root density. More importantly, RcRR1 overexpression transgenic plants also showed reduced sensitivity to cytokinin during root growth; auxin distribution and the expression of auxin efflux carriers PIN genes were altered in RcRR1 overexpression plants. Taken together, these results demonstrate that RcRR1 is a functional type-A response regulator which is involved in cytokinin-regulated rhizoid formation in Rosa canina.
Interactions between Auxin, Microtubules and XTHs Mediate Green Shade- Induced Petiole Elongation in Arabidopsis  [PDF]
Rashmi Sasidharan, Diederik H. Keuskamp, Rik Kooke, Laurentius A. C. J. Voesenek, Ronald Pierik
PLOS ONE , 2014, DOI: 10.1371/journal.pone.0090587
Abstract: Plants are highly attuned to translating environmental changes to appropriate modifications in growth. Such phenotypic plasticity is observed in dense vegetations, where shading by neighboring plants, triggers rapid unidirectional shoot growth (shade avoidance), such as petiole elongation, which is partly under the control of auxin. This growth is fuelled by cellular expansion requiring cell-wall modification by proteins such as xyloglucan endotransglucosylase/hydrolases (XTHs). Cortical microtubules (cMTs) are highly dynamic cytoskeletal structures that are also implicated in growth regulation. The objective of this study was to investigate the tripartite interaction between auxin, cMTs and XTHs in shade avoidance. Our results indicate a role for cMTs to control rapid petiole elongation in Arabidopsis during shade avoidance. Genetic and pharmacological perturbation of cMTs obliterated shade-induced growth and led to a reduction in XTH activity as well. Furthermore, the cMT disruption repressed the shade-induced expression of a specific set of XTHs. These XTHs were also regulated by the hormone auxin, an important regulator of plant developmental plasticity and also of several shade avoidance responses. Accordingly, the effect of cMT disruption on the shade enhanced XTH expression could be rescued by auxin application. Based on the results we hypothesize that cMTs can mediate petiole elongation during shade avoidance by regulating the expression of cell wall modifying proteins via control of auxin distribution.
Glucose and Auxin Signaling Interaction in Controlling Arabidopsis thaliana Seedlings Root Growth and Development  [PDF]
Bhuwaneshwar S. Mishra, Manjul Singh, Priyanka Aggrawal, Ashverya Laxmi
PLOS ONE , 2009, DOI: 10.1371/journal.pone.0004502
Abstract: Background Plant root growth and development is highly plastic and can adapt to many environmental conditions. Sugar signaling has been shown to affect root growth and development by interacting with phytohormones such as gibberellins, cytokinin and abscisic acid. Auxin signaling and transport has been earlier shown to be controlling plant root length, number of lateral roots, root hair and root growth direction. Principal Findings Increasing concentration of glucose not only controls root length, root hair and number of lateral roots but can also modulate root growth direction. Since root growth and development is also controlled by auxin, whole genome transcript profiling was done to find out the extent of interaction between glucose and auxin response pathways. Glucose alone could transcriptionally regulate 376 (62%) genes out of 604 genes affected by IAA. Presence of glucose could also modulate the extent of regulation 2 fold or more of almost 63% genes induced or repressed by IAA. Interestingly, glucose could affect induction or repression of IAA affected genes (35%) even if glucose alone had no significant effect on the transcription of these genes itself. Glucose could affect auxin biosynthetic YUCCA genes family members, auxin transporter PIN proteins, receptor TIR1 and members of a number of gene families including AUX/IAA, GH3 and SAUR involved in auxin signaling. Arabidopsis auxin receptor tir1 and response mutants, axr2, axr3 and slr1 not only display a defect in glucose induced change in root length, root hair elongation and lateral root production but also accentuate glucose induced increase in root growth randomization from vertical suggesting glucose effects on plant root growth and development are mediated by auxin signaling components. Conclusion Our findings implicate an important role of the glucose interacting with auxin signaling and transport machinery to control seedling root growth and development in changing nutrient conditions.
Endogenous auxin and cytokinin contents associated with shoot formation in leaves of pineapple cultured in vitro
Mercier, Helenice;Souza, Beatriz Maia;Kraus, Jane Elizabeth;Hamasaki, Regina Mayumi;Sotta, Bruno;
Brazilian Journal of Plant Physiology , 2003, DOI: 10.1590/S1677-04202003000200006
Abstract: the in vitro culture of pineapple leaves on a shoot induction medium (sim) results in the formation of protuberances and further development in shoots, and plantlets. the contents of endogenous indoleacetic acid (iaa) and five cytokinins (cks), n6(2-isopentenyl)adenine (ip), n6(2-isopentenyl)adenosine (ipr), zeatin (z), zeatin riboside (zr) and n6-benzyladenine (ba), present in the basal portion of those leaves, were correlated to the organogenic response that occurs over 15 days of culture. the endogenous auxin/cytokinins ratio was lowest on the 3rd day, mainly due to a strong increase in the ip level. it seems that endogenous ip concentration triggered the induction signal for an organogenic response in pineapple leaf bases. the rise in ip content required the presence of ba and a-naphthaleneacetic acid (naa) in the medium, suggesting that endogenous ip production is regulated in response to these growth regulator uptakes.
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