Abstract
Cytokinin receptors (CRs) are hybrid-type histidine kinases, membrane proteins with a cytokinin-binding extracellular domain. CRs initiate and propagate cytokinin signaling by means of phosphorylation and phosphotransfer to downstream proteins. In legumes, some members of the CR multigenic family are essential for nodulation. In two recent works, we investigated the involvement of two new CRs, MsHK1 from Medicago sativa, and LaHK1 from Lupinus albus, in nodule morphogenesis, senescence and stress response. LaHK1 expression increased during the first stages of lupin nodule development, while MsHK1 expression was localized in the meristem and the invasion zone of alfalfa nodules pointing to a role for CRs in nodule cell proliferation and differentiation. Both CRs were also induced during nodule senescence. MsHK1 expression increased under osmotic stress and both genes were induced following dark stress, indicating that CRs are also likely to play a significant role in the response to stress. We propose multiple roles for CRs which, when analyzed jointly with recent results from other authors, suggest coordinated cross-talk of different signaling systems.
Addendum to: Coba de la Peña T, Cárcamo CB, Almonacid L, Zaballos A, Lucas MM, Balomenos D, Pueyo JJ. A salt stress-responsive cytokinin receptor homologue isolated from Medicago sativa nodules. Planta 2008; 227:769-79.
and
Coba de la Peña T, Cárcamo CB, Almonacid L, Zaballos A, Lucas MM, Balomenos D, Pueyo JJ. A cytokinin receptor homologue is induced during root nodule organogenesis and senescence in Lupinus albus L. Plant Physiol Biochem 2008; 46:219-25.
Cytokinins play a key role in different processes related to plant growth and development and adaptation to environmental conditions.Citation1 Elucidation of the involvement of transduction molecules in cytokinin-induced processes is a fascinating current topic of research. The first step in cytokinin perception and signal transduction is accomplished by cytokinin receptors (CRs), hybrid-type histidine kinases located at the plasma membrane. When a cytokinin molecule binds to the CR extracellular domain, phosphorylation of a conserved His residue and phosphotransfer to a conserved Asp residue take place. Additional phosphorelay involves His phosphotransfer proteins (HPTs) in the cytosol which are translocated to the nucleus where response regulators (RRs) are subsequently phosphorylated. Some phosphorylated RRs bind to the DNA and induce transcription of cytokinin-regulated genes.Citation2–Citation4 Cytokinin-responsive factors (CRFs) translocating to the nucleus and inducing gene expression have also been identified.Citation4,Citation5
CRs belong to a multigenic familiy, three different CRs having been identified in Arabidopsis, AHK2, AHK3 and AHK4 (also known as CRE1).Citation2–Citation4 Orthologues have been identified in maize,Citation6 rice,Citation7,Citation8, Catharantus roseus,Citation9 Medicago truncatulaCitation10 and Lotus japonicus,Citation11,Citation12 and several functional roles of CRs have been elucidated. summarizes the proposed roles for identified CRs. Functional analysis of single, double and triple loss-of-function AHK2, AHK3 and CRE1 mutants showed that these genes exhibit a degree of overlapping and some specific functions in Arabidopsis.Citation4,Citation5,Citation13–Citation16
Cytokinins play an important role in the nodulation of legumes by rhizobia. Rhizobia produce Nod factors, molecules that trigger specific signaling in the roots, inducing expression of plant genes involved in symbiosis (nodulins) and cell division in the root cortex giving rise to the nodule primordium. It is known that the addition of exogenous cytokinins induces root cortical cell division, nodulin expression and formation of nodule-like structures in different legume speciesCitation17 while lower cytokinin levels induce a reduction in the number of nodules.Citation18
Only recently have CRs been shown to have a crucial role in nodulation. The CR MtCRE1 (an orthologue of Arabidopsis AHK4) is fundamental for nodulation in Medicago truncatula. RNA interference in the expression of this gene induces a blockage of infection thread progression and a reduction in nodulation.Citation10 RR MtRR4 seems to be regulated by nodulation signals, while MtRR1 and nodulin MtNIN (Nodule INception, required for initiation of nodule primordia) are regulated by both nodulation and cytokinin signals. These genes seem to form part of a cross-regulation system between nodulation signals and cytokinin signaling.Citation10 Additionally, CR LHK1 (also an orthologue of AHK4 and MtCRE1) from Lotus japonicus has proven to be vital for nodulation: a loss-of-function mutant is not able to nodulate,Citation12 while a gain-of-function mutant induces nodule organogenesis even in the absence of bacteria.Citation11 A general nodulation model based on LHK1 signaling has been proposed where signals from symbiotic bacteria induce accumulation of cytokinins that bind to CR LHK1 in the root cortical cells triggering nodule organogenesis.Citation19
In order to investigate the role of other CRs in nodulation, we identified a CR gene in Medicago sativa nodules and another one in Lupinus albus, MsHK1 and LaHK1, respectively,Citation20,Citation21 both genes being AHK3 orthologues. Unlike other CRs, orthologues of AHK4,Citation10–Citation12 MsHK1 may not be directly involved in nodulation as expression levels did not vary throughout this process.Citation20 However, elevated expression of MsHK1 and/or other CR homologues was detected in the nodule persistent meristem and the rhizobial invasion zone,Citation20 where high levels of cytokinins have also been detected.Citation22 We conclude, therefore, that CRs are not only essential for nodulation, but are also involved in nodule cell proliferation and differentiation processes during nodule morphogenesis. In Medicago truncatula, RNA interference in the expression of CRs, MtHK2 and MtHK3 (an orthologue of MsHK1), did not apparently affect nodulation but a putative role attributable to nodulin MtNIN regulation was not excluded.Citation10 In contrast, the expression levels of LaHK1 (also an orthologue of MsHK1 and AHK3) seem to be regulated during nodule development in lupinCitation21 suggesting a role in nodule organogenesis.
MsHK1 was induced during salt stress in leaves, roots and nodulesCitation20 suggesting that cytokinin signaling plays a role in osmotic stress. In Medicago truncatula, CRs MtHK2 and MtCRE1, and RRs MtRR4 and MtRR5 were rapidly induced in roots in the presence of salt stress; both CRs were likewise induced in roots during recovery from salt stress.Citation23 Drought-induced rapid induction of AHK2, AHK3 and CRE1 was reported in Arabidopsis; AHK3 is also induced by cold stress and high salinity.Citation24 Loss-of-function analysis of these CRs showed that ahk2, ahk3 and cre1 mutants are more tolerant of drought and salt stress and highly sensitive to ABA (especially ahk3). Upregulation of many stress- and ABA-responsive genes was detected in the ahk2 ahk3 double mutant in the absence of stress. Downregulation of ARR4 and ARR7 was also observed in double mutants and these RRs may provide negative feed-back regulation of stress signaling.Citation24 CRs act as negative regulators during abiotic stress responses and in ABA signalingCitation24 and probably take part in a signaling cross-talk involving cytokinins, ABA and stress-signaling pathways.
An increase in endogenous cytokinin levels induces a delay in leaf senescence.Citation25 A gain-of-function mutant of AHK3 displayed delayed leaf senescence, and overexpression of RR ARR2 induced a delay in senescence.Citation16 Phosphorylation of ARR2 by AHK3 has a positive effect on leaf longevity.Citation16 We reported induction of LaHK1 expression during both age-dependent and dark-induced nodule senescence.Citation21 MsHK1 is also induced in leaves and nodules following dark stressCitation20 and some studies have revealed that leaf and nodule senescence are closely related processes.Citation26 It has been postulated that ABA may play an important role in nodule senescence, and cytokinin is a putative candidate for involvement in this process.Citation26,Citation27 Involvement of CRs in abiotic stress and ABA signalingCitation24 reinforces this possibility. LaHK1 is involved in signaling during natural lupin nodule senescence and in the response to dark stressCitation21 while MsHK1 is induced under both salt and dark stress.Citation20
In addition to the critical role played by CRs in nodulation,Citation10–Citation12 our results indicate that CRs are involved in cytokinin sensing and signal transduction throughout the entire process of nodule development from morphogenesis through senescence, including nodule cell proliferation and differentiation. CRs likewise appear to play a significant role in stress response including salinity and dark stress. Cross-talk of signaling pathways appears to play a crucial role in ensuring coordinated plant responses.
Abbreviations
| CR | = | cytokinin receptor |
| CRF | = | cytokinin-responsive factor |
| HPT | = | his phosphotransfer protein |
| RR | = | response regulator |
Figures and Tables
Table 1 Proposed roles for identified cytokinin receptors
Acknowledgements
This work was supported by the Comunidad de Madrid and the Spanish Ministry of Education and Science.
Addendum to:
Addendum to:
References
- Mok DW, Mok MC. Cytokinin metabolism and action. Annu Rev Plant Physiol Plant Mol Biol 2001; 52:89 - 118
- Kakimoto T. Perception and signal transduction of cytokinins. Annu Rev Plant Physiol Plant Mol Biol 2003; 54:605 - 627
- Ferreira FJ, Kieber JJ. Cytokinin signaling. Curr Opin Plant Biol 2005; 518 - 525
- To JPC, Kieber JJ. Cytokinin signaling: two components and more. Trends Plant Sci 2008; http://dx.doi.org/10.1016/j.tplants.2007.11.005
- Müller B, Sheen J. Advances in cytokinin signaling. Science 2007; 318:68 - 69
- Yonekura Sakakibara K, Kojima M, Yamaya T, Sakakibara H. Molecular characterization of cytokinin-responsive hisitidine kinases in maize: differential ligand preferences and response to cis-zeatin. Plant Physiol 2004; 134:1654 - 1661
- Ito Y, Kurata N. Identification and characterization of cytokinin-signalling gene family in rice. Gene 2006; 382:57 - 65
- Pareek K, Singh A, Kumar M, Kushwaha HR, Lynn AM, Singla Pareek SL. Whole-genome analysis of Oryza sativa reveals similar architecture of two-component signaling machinery with Arabidopsis. Plant Physiol 2006; 142:380 - 397
- Papon N, Clastre M, Andreu F, Gantet P, Rideau M, Créche J. Expression analysis in plant and cell suspensions of CrCKR1, a cDNA encoding a histidine kinase receptor homologue in Catharanthus roseus (L.) G. Don. J Exp Bot 2002; 53:1989 - 1990
- Gonzalez Rizzo S, Crespi M, Frugier F. The Medicago truncatula CRE1 cytokinin receptor regulates lateral root development and early symbiotic interaction with Sinorhizobium meliloti. Plant Cell 2006; 18:2680 - 2693
- Tirichine L, Sandal N, Madsen LH, Radutoiu S, Albreksten AS, Sato S, Asamizu E, Tabata S, Stougaard J. A gain-of-function mutation in a cytokinin receptor triggers spontaneous root nodule organogenesis. Science 2007; 315:104 - 107
- Murray JD, Karas BJ, Sato S, Tabata S, Amyot L, Szczyglowski K. A cytokinin perception mutant colonized by Rhizobium in the absence of nodule organogenesis. Science 2007; 315:101 - 104
- Higuchi M, Pischke MS, Mähönen AP, Miyawaki K, Hashimoto Y, Seki M, Kobayashi M, Shinozaki K, Kato T, Tabata S, Helariutta Y, Sussman MR, Kakimoto T. In planta functions of the Arabidopsis cytokinin receptor family. Proc Natl Acad Sci USA 2004; 101:8821 - 8826
- Nishimura C, Ohashi Y, Sato S, Kato T, Tabata S, Ueguchi C. Histidine kinase homologs that act as cytokinin receptors possess overlapping functions in the regulation of shoot and root growth in Arabidopsis. Plant Cell 2004; 16:1365 - 1377
- Riefler M, Novak O, Strnad M, Schmülling T. Arabidopsis cytokinin receptor mutants reveal functions in shoot growth, leaf senescence, seed size, germination, root development, and cytokinin metabolism. Plant Cell 2006; 18:40 - 54
- Kim HJ, Ryu H, Hong SH, Woo HR, Lim PO, Lee IC, Sheen J, Nam HG, Hwang I. Cytokinin-mediated control of leaf longevity by AHK3 through phosphorylation of ARR2 in Arabidopsis. Proc Natl Acad Sci USA 2006; 103:814 - 819
- Ferguson BJ, Mathesius U. Signaling interactions during nodule development. J Plant Growth Regul 2003; 47 - 72
- Lohar DP, Schaff JE, Laskey JG, Kieber JJ, Bilyeu KD, Bird DM. Cytokinins play opposite roles in lateral root formation, and nematode and rhizobial symbiosis. Plant J 2004; 38:203 - 214
- Oldroyd GE. Nodules and hormones. Science 2007; 315:52 - 53
- Coba de la Peña T, Cárcamo CB, Almonacid L, Zaballos A, Lucas MM, Balomenos D, Pueyo JJ. A salt stress-responsive cytokinin receptor homologue isolated from Medicago sativa nodules. Planta 2008; 227:769 - 779
- Coba de la Peña T, Cárcamo CB, Almonacid L, Zaballos A, Lucas MM, Balomenos D, Pueyo JJ. A cytokinin receptor homologue is induced during root nodule organogenesis and senescente in Lupinus albus L. Plant Physiol Biochem 2008; 46:219 - 225
- Syono K, Newcomb W, Torrey JG. Cytokinin production in relation to the development of pea root nodules. Can J Bot 1976; 54:2155 - 2162
- Merchan F, De Lorenzo L, González-Rizzo S, Niebel A, Manyani H, Frugier F, Sousa C, Crespi M. Identification of regulatory pathways involved in the reacquisition of root growth alter salt stress in Medicago truncatula. Plant J 2007; 51:1 - 17
- Tran LSPT, Urao T, Qin F, Maruyama K, Kakimoto T, Shinozaki K, Yamaguchi Shinozaki K. Functional analysis of AHK1/ATHK1 and cytokinin receptor histidine kinases in response to abscisic acid, drought, and salt stress in Arabidopsis. Proc Natl Acad Sci USA 2007; 104:20623 - 20628
- Gan S, Amasino RM. Inhibition of leaf senescence by autoregulated production of cytokinin. Science 1995; 270:1986 - 1988
- Van de Velde W, Pérez Guerra JC, De Keyser A, De Rycke R, Rombauts S, Maunoury N, Mergaert P, Kondorosi E, Holsters M, Goormachtig S. Aging in legume symbiosis. A molecular view of nodule senescente in Medicago truncatula. Plant Physiol 2006; 141:711 - 720
- Puppo A, Groten K, Bastian F, Carzaniga R, Soussi M, Lucas MM, De Felipe MR, Harrison J, Vanacker H, Foyer CH. Legume nodule senescence: roles for redox and hormone signalling in the orchestration of the natural aging process. New Phytol 2005; 165:683 - 701
- Inoue T, Higuchi M, Hashimoto Y, Seki M, Kobayashi M, Karo T, Tabata S, Shinozaki K, Kakimoto T. Identification of CRE1 as a cytokinin receptor from Arabidopsis. Nature 2001; 409:1060 - 1063
- Franco Zorrilla JM, Martin AC, Solano R, Rubio V, Leyva A, Paz Ares J. Mutations at CRE1 impair cytokinin-induced repression of phosphate starvation response in Arabidopsis. Plant J 2002; 32:353 - 360
- Maruyama Nakashita A, Nakamura Y, Yamata T, Takahashi H. A novel regulatory pathway of sulphate uptake in Arabidopsis roots: implication of CRE1/WOL/AHK4-mediated cytokinin-dependent regulation. Plant J 2004; 38:779 - 789