The auxin responsive AP2/ERF transcription factor CROWN ROOTLESS5 is involved in crown root initiation in rice through the induction of OsRR1, a type-A response regulator of cytokinin signaling.

Cytokinin is known to have negative effects on de novo auxin-induced root formation. However, the regulatory mechanisms of root initiation by both cytokinin and auxin are poorly understood. In this study, we characterized a rice mutant, termed crown rootless5 (crl5), which produced fewer crown roots and displayed impaired initiation of crown root primordia. The expression of CRL5, which encodes a member of the large AP2/ERF transcription factor family protein, was observed in the stem region where crown root initiation occurs. Exogenous auxin treatment induced CRL5 expression without de novo protein biosynthesis, which also required the degradation of AUX/IAA proteins. A putative auxin response element in the CRL5 promoter region specifically interacted with a rice ARF, demonstrating that CRL5 may be a direct target of an ARF, similar to CRL1/ADVENTITIOUS ROOTLESS1 (ARL1) that also regulates crown root initiation. A crl1 crl5 double mutant displayed an additive phenotype, indicating that these two genes function in different genetic pathways for crown root initiation. In addition, ProACT:CRL5/WT showed a cytokinin-resistant phenotype for crown root initiation, and also up-regulated the expression of two negative regulators of cytokinin signaling, OsRR1 and OsRR2, which were downregulated in crl5. Transgenic plants that over-expressed OsRR1 under the control of the CRL5 promoter in a crl5 mutant background produced a higher number of crown roots than the crl5 plant. Taken together, these results indicate that auxin-induced CRL5 promotes crown root initiation through repression of cytokinin signaling by positively regulating type-A RR, OsRR1.

[1]  R. Fischer,et al.  Plant organ size control: AINTEGUMENTA regulates growth and cell numbers during organogenesis. , 2000, Proceedings of the National Academy of Sciences of the United States of America.

[2]  M. Estelle,et al.  The IAA1 protein is encoded by AXR5 and is a substrate of SCF(TIR1). , 2004, The Plant journal : for cell and molecular biology.

[3]  J. Reed,et al.  Control of auxin-regulated root development by the Arabidopsis thaliana SHY2/IAA3 gene. , 1999, Development.

[4]  Ottoline Leyser,et al.  Auxin regulates SCFTIR1-dependent degradation of AUX/IAA proteins , 2001, Nature.

[5]  Bruno Müller,et al.  Cytokinin and auxin interaction in root stem-cell specification during early embryogenesis , 2008, Nature.

[6]  Makoto Matsuoka,et al.  Production and characterization of auxin-insensitive rice by overexpression of a mutagenized rice IAA protein. , 2006, The Plant journal : for cell and molecular biology.

[7]  Yoshiaki Inukai,et al.  CRL4 regulates crown root formation through auxin transport in rice , 2008 .

[8]  Tom Beeckman,et al.  Cytokinins Act Directly on Lateral Root Founder Cells to Inhibit Root Initiation[W] , 2007, The Plant Cell Online.

[9]  Ping Wu,et al.  ARL1, a LOB-domain protein required for adventitious root formation in rice. , 2005, The Plant journal : for cell and molecular biology.

[10]  B. Krizek,et al.  AINTEGUMENTA and AINTEGUMENTA-LIKE6 Act Redundantly to Regulate Arabidopsis Floral Growth and Patterning1[C][W][OA] , 2009, Plant Physiology.

[11]  T. Komari,et al.  Efficient transformation of rice (Oryza sativa L.) mediated by Agrobacterium and sequence analysis of the boundaries of the T-DNA. , 1994, The Plant journal : for cell and molecular biology.

[12]  AXR2 encodes a member of the Aux/IAA protein family. , 2000, Plant physiology.

[13]  H. Kitano,et al.  Rice plant development: from zygote to spikelet. , 2005, Plant & cell physiology.

[14]  N. Chua,et al.  The Arabidopsis Auxin-Inducible Gene ARGOS Controls Lateral Organ Size Article, publication date, and citation information can be found at www.plantcell.org/cgi/doi/10.1105/tpc.013557. , 2003, The Plant Cell Online.

[15]  H. Fukaki,et al.  Lateral root formation is blocked by a gain-of-function mutation in the SOLITARY-ROOT/IAA14 gene of Arabidopsis. , 2002, The Plant journal : for cell and molecular biology.

[16]  J. Ecker,et al.  Type-A Arabidopsis Response Regulators Are Partially Redundant Negative Regulators of Cytokinin Signaling Online version contains Web-only data. , 2004, The Plant Cell Online.

[17]  Brigitte Courtois,et al.  Genetic control of root development in rice, the model cereal. , 2010, Trends in plant science.

[18]  B. Krizek.  Ectopic expression of AINTEGUMENTA in Arabidopsis plants results in increased growth of floral organs. , 1999, Developmental genetics.

[19]  M. Estelle,et al.  The IAA 1 protein is encoded by AXR 5 and is a substrate of SCFTIR 1 , 2004 .

[20]  Kazuo Shinozaki,et al.  Identification of CRE1 as a cytokinin receptor from Arabidopsis , 2001, Nature.

[21]  M. Sauer,et al.  The maize (Zea mays L.) RTCS gene encodes a LOB domain protein that is a key regulator of embryonic seminal and post-embryonic shoot-borne root initiation. , 2007, The Plant journal : for cell and molecular biology.

[22]  P. Perez,et al.  AINTEGUMENTA, an APETALA2-like gene of Arabidopsis with pleiotropic roles in ovule development and floral organ growth. , 1996, The Plant cell.

[23]  J. Sheen,et al.  Two-component circuitry in Arabidopsis cytokinin signal transduction , 2001, Nature.

[24]  Renze Heidstra,et al.  PLETHORA proteins as dose-dependent master regulators of Arabidopsis root development , 2007, Nature.

[25]  G. Hagen,et al.  Auxin-responsive gene expression: genes, promoters and regulatory factors , 2002, Plant Molecular Biology.

[26]  R. O. Morris,et al.  Molecular and biochemical characterization of a cytokinin oxidase from maize. , 2001, Plant physiology.

[27]  R. Amasino,et al.  The PLETHORA Genes Mediate Patterning of the Arabidopsis Root Stem Cell Niche , 2004, Cell.

[28]  Masaki Furuya,et al.  OsARF1, an auxin response factor from rice, is auxin-regulated and classifies as a primary auxin responsive gene , 2002, Plant Molecular Biology.

[29]  F. B. Pickett,et al.  Mutations in the AXR3 gene of Arabidopsis result in altered auxin response including ectopic expression from the SAUR-AC1 promoter. , 1996, The Plant journal : for cell and molecular biology.

[30]  B. Krizek,et al.  DNA binding properties of the Arabidopsis floral development protein AINTEGUMENTA. , 2000, Nucleic acids research.

[31]  F. Skoog,et al.  A revised medium for the growth and bioassay with tobacco tissue culture , 1962 .

[32]  Klaus Palme,et al.  The PIN auxin efflux facilitator network controls growth and patterning in Arabidopsis roots , 2005, Nature.

[33]  Joseph J. Kieber,et al.  Cytokinin signaling: two-components and more. , 2008, Trends in plant science.

[34]  Makoto Matsuoka,et al.  Crown rootless1, Which Is Essential for Crown Root Formation in Rice, Is a Target of an AUXIN RESPONSE FACTOR in Auxin Signalingw⃞ , 2005, The Plant Cell Online.

[35]  B. Krizek,et al.  AINTEGUMENTA-like (AIL) genes are expressed in young tissues and may specify meristematic or division-competent states , 2005, Plant Molecular Biology.

[36]  Jitendra P Khurana,et al.  Bmc Plant Biology Molecular Characterization and Differential Expression of Cytokinin-responsive Type-a Response Regulators in Rice (oryza Sativa) , 2005 .

[37]  Limin Huang,et al.  The WUSCHEL-Related Homeobox Gene WOX11 Is Required to Activate Shoot-Borne Crown Root Development in Rice[C][W] , 2009, The Plant Cell Online.

[38]  Nenggang Zhang,et al.  Initiation and Elongation of Lateral Roots in Lactuca sativa , 1999, International Journal of Plant Sciences.

[39]  J. Ecker,et al.  Multiple Type-B Response Regulators Mediate Cytokinin Signal Transduction in Arabidopsisw⃞ , 2005, The Plant Cell Online.

[40]  A. Theologis,et al.  ARF7 and ARF19 Regulate Lateral Root Formation via Direct Activation of LBD/ASL Genes in Arabidopsis[W] , 2007, The Plant Cell Online.

[41]  Mukesh Jain,et al.  Structure and expression analysis of early auxin-responsive Aux/IAA gene family in rice (Oryza sativa) , 2005, Functional & Integrative Genomics.

[42]  Nori Kurata,et al.  Identification and characterization of cytokinin-signalling gene families in rice. , 2006, Gene.

[43]  Renze Heidstra,et al.  Cytokinins Determine Arabidopsis Root-Meristem Size by Controlling Cell Differentiation , 2007, Current Biology.

[44]  Takashi Aoyama,et al.  A Genetic Framework for the Control of Cell Division and Differentiation in the Root Meristem , 2008, Science.

[45]  B. Han,et al.  Genome-wide analysis of the auxin response factors (ARF) gene family in rice (Oryza sativa). , 2007, Gene.

[46]  S. Tabata,et al.  ARR1, a Transcription Factor for Genes Immediately Responsive to Cytokinins , 2001, Science.

[47]  S. Iwahori,et al.  KNOX homeodomain protein directly suppresses the expression of a gibberellin biosynthetic gene in the tobacco shoot apical meristem. , 2001, Genes & development.

[48]  Lu Wang,et al.  Adventitious root formation in rice requires OsGNOM1 and is mediated by the OsPINs family , 2009, Cell Research.

[49]  H. Kouchi,et al.  Isolation and characterization of novel nodulin cDNAs representing genes expressed at early stages of soybean nodule development , 1993, Molecular and General Genetics MGG.

[50]  J. Fox,et al.  Cytokinin oxidase from wheat: partial purification and general properties. , 1989, Plant physiology.

[51]  The AINTEGUMENTA gene of Arabidopsis required for ovule and female gametophyte development is related to the floral homeotic gene APETALA2. , 1996, The Plant cell.

[52]  G. Hagen,et al.  ARF1, a transcription factor that binds to auxin response elements. , 1997, Science.