Root Pole Research Articles

Morphological and histological analysis of anther-derived embryos of coconut (Cocos nucifera L.)

Doubled-haploids are a great source of material for heterozygous coconuts to shorten the time taken for crop improvement through hybridization programs. However, low frequency of embryo conversion and formation of weak plantlets are the major limitations in the protocol. In the present study anther-derived embryos were analysed at cellular level through the histological observations to understand the occurrence of shoot differentiation in those embryos. Attempts were also made to optimise the 6-Benzylaminopurine (BAP) concentration in the regeneration medium. Among the tested BAP levels (5, 15, 25, 35, 45, 55 μM), the regeneration medium containing 25 and 35 μM gave rise to 51.5% sprouted embryos against the control that gave only 22.7%. Among the analysed anther-derived embryos sequential events of differentiation including the formation of provascular strands followed by vascular bundle, differentiation in to the growing point, giving rise to the secondary embryos and polarization in to the shoot and root pole were identified. The blunt embryos devoid any morphological sign of sprouting showed different cellular arrangement viz. haustorial structure without any meristematic point (47%), bipolar with shoot and root meristems (8%) and unipolar with either of the pole (45%). Thick haustorial cover lead to the physical dormancy inhibiting further development of the polar structures. The germinating embryos containing a single shoot gave rise to the healthy plants where as double, multiple or fused shoots formed the week plantlets. The findings can be used for further optimisation of the protocol to achieve a greater plant regeneration frequency. 1. Culture media supplemented with increased levels of 6-Benzylaminopurine (BAP) enhanced the regeneration frequency of anther-derived embryos. 2. Thick haustorial tissue created an obstacle effect on further development of the differentiated shoot apical meristem. 3. Formation of clustered plants through secondary embryogenesis or shoot multiplication at the early stage of plant regeneration caused to give rise to the week plants.

SCARECROW and SHORTROOT control the auxin/cytokinin balance necessary for embryonic stem cell niche specification

ABSTRACTThe root apical meristem is established during embryogenesis, when its organizer, the quiescent center, is specified and the stem cell niche is positioned. The SCARECROW-SHORTROOT heterodimer is essential for quiescent center specification and maintenance. As continuous post-embryonic root growth relies upon the SCARECROW-mediated control of the cytokinin/auxin balance, we investigated the role of SCARECROW and SHORTROOT in controlling cytokinin signaling during embryonic quiescent center specification. We found that from embryogenesis onward both SCARECROW and SHORTROOT antagonize cytokinin signaling, thus repressing the expression of the auxin biosynthetic enzyme ANTRANILATHE SYNTHASE BETA 1. This mechanism prevents detrimental and premature high auxin levels in the QC allowing the establishment of a functional embryonic root pole.

Optimizing FRET-FLIM Labeling Conditions to Detect Nuclear Protein Interactions at Native Expression Levels in Living Arabidopsis Roots.

Protein complex formation has been extensively studied using Förster resonance energy transfer (FRET) measured by Fluorescence Lifetime Imaging Microscopy (FLIM). However, implementing this technology to detect protein interactions in living multicellular organism at single-cell resolution and under native condition is still difficult to achieve. Here we describe the optimization of the labeling conditions to detect FRET-FLIM in living plants. This study exemplifies optimization procedure involving the identification of the optimal position for the labels either at the N or C terminal region and the selection of the bright and suitable, fluorescent proteins as donor and acceptor labels for the FRET study. With an effective optimization strategy, we were able to detect the interaction between the stem cell regulators SHORT-ROOT and SCARECROW at endogenous expression levels in the root pole of living Arabidopsis embryos and developing lateral roots by FRET-FLIM. Using this approach we show that the spatial profile of interaction between two transcription factors can be highly modulated in reoccurring and structurally resembling organs, thus providing new information on the dynamic redistribution of nuclear protein complex configurations in different developmental stages. In principle, our optimization procedure for transcription factor complexes is applicable to any biological system.

Expression of Arabidopsis class 1 phytoglobin (AtPgb1) delays death and degradation of the root apical meristem during severe PEG-induced water deficit.

Maintenance of a functional root is fundamental to plant survival in response to some abiotic stresses, such as water deficit. In this study, we found that overexpression of Arabidopsis class 1 phytoglobin (AtPgb1) alleviated the growth retardation of polyethylene glycol (PEG)-induced water stress by reducing programmed cell death (PCD) associated with protein folding in the endoplasmic reticulum (ER). This was in contrast to PEG-stressed roots down-regulating AtPgb1 that exhibited extensive PCD and reduced expression of several attenuators of ER stress, including BAX Inhibitor-1 (BI-1). The death program experienced by the suppression of AtPgb1 in stressed roots was mediated by reactive oxygen species (ROS) and ethylene. Suppression of ROS synthesis or ethylene perception reduced PCD and partially restored root growth. The PEG-induced cessation of root growth was preceded by structural changes in the root apical meristem (RAM), including the loss of cell and tissue specification, possibly as a result of alterations in PIN1- and PIN4-mediated auxin accumulation at the root pole. These events were attenuated by the overexpression of AtPgb1 and aggravated when AtPgb1 was suppressed. Specifically, suppression of AtPgb1 compromised the functionality of the WOX5-expressing quiescent cells (QCs), leading to the early and premature differentiation of the adjacent columella stem cells and to a rapid reduction in meristem size. The expression and localization of other root domain markers, such as SCARECROW (SCR), which demarks the endodermis and QCs, and WEREWOLF (WER), which specifies the lateral root cap, were also most affected in PEG-treated roots with suppressed AtPgb1. Collectively, the results demonstrate that AtPgb1 exercises a protective role in roots exposed to lethal levels of PEG, and suggest a novel function of this gene in ensuring meristem functionality through the retention of cell fate specification.

RopGEF1 Plays a Critical Role in Polar Auxin Transport in Early Development.

Polar auxin transport, facilitated by the combined activities of auxin influx and efflux carriers to maintain asymmetric auxin distribution, is essential for plant growth and development. Here, we show that Arabidopsis (Arabidopsis thaliana) RopGEF1, a guanine nucleotide exchange factor and activator of Rho GTPases of plants (ROPs), is critically involved in polar distribution of auxin influx carrier AUX1 and differential accumulation of efflux carriers PIN7 and PIN2 and is important for embryo and early seedling development when RopGEF1 is prevalently expressed. Knockdown or knockout of RopGEF1 induces embryo defects, cotyledon vein breaks, and delayed root gravity responses. Altered expression from the auxin response reporter DR5rev:GFP in the root pole of RopGEF1-deficient embryos and loss of asymmetric distribution of DR5rev:GFP in their gravistimulated root tips suggest that auxin distribution is affected in ropgef1 mutants. This is reflected by the polarity of AUX1 being altered in ropgef1 embryos and roots, shifting from the normal apical membrane location to a basal location in embryo central vascular and root protophloem cells and also reduced PIN7 accumulation at embryos and altered PIN2 distribution in gravistimulated roots of mutant seedlings. In establishing that RopGEF1 is critical for AUX1 localization and PIN differential accumulation, our results reveal a role for RopGEF1 in cell polarity and polar auxin transport whereby it imapcts auxin-mediated plant growth and development.

Indirect somatic embryogenesis from transgenic immature leaf of safflower Carthamus tinctorius (Mohler, Roth, Schmidt & Boudreaux, 1967) (Asterales: Asteraceae)

Highly efficient protocol is developed for indirect somatic embryogenesis from transgenic immature leaf of safflower Carthamus tinctorius (Mohler, Roth, Schmidt & Boudreaux, 1967) (Asterales: Asteraceae) explants in solid culture. Here two different half strength mediums like MSG and EMM both were tested in addition to inorganic and organic nitrogen sources and also combination of the reference medium. Inorganic nitrogen source shows about 39.5 number of embryos with 37% embryo germination and organic Nitrogen source like L-glutamine (20 uM) on reference medium influenced 80.5 number of somatic embryos with 44.9% germination. However, maximum of 92.0 number of embryo induction with 52.5% germination was noticed on CH (casein hydrolysate) (1.5 mg/L) containing medium. Combination of inorganic and organic nitrogen sources like NH4NO3 (40 uM) in combinations with reduced form of organic nitrogen casein hydrolysate (CH) (1.5 mg/L) was found to be most suitable for induction of somatic embryos with a maximum of 117 number of somatic embryos with 60.9% germination in immature leaf explants. About 57% plant conversion rate showed in NARI-6 cultivar after 2 weeks of culture. However it showed maximum plant conversion rate about 68% with basal medium along with 1.5% sucrose supplemented with GA3 (1.5 uM) and spermidine (1.0 uM). Germinated embryos with shoot and root poles were isolated and subcultured on suitable root induction medium containing NAA (2.5 uM) + AgNO3 (1.5 uM) showed maximum rooting frequency of about 69.6%.

A study on morphogenesis of roots of Panax stipuleanatus H.T.Tsai et K.M.Feng in vitro and preliminary determination of oleanolic acid in roots

Panax stipuleanatus H.T.Tsai et K.M.Feng which belongs to the genus Panax (Araliaceae family) is an valuable medicinal herbs in Vietnam and China. In the rhizome of Panax stipuleanatus H.T.Tsai et K.M.Feng contains many olean-type triterpenoid saponins, which can improve mental stamina and reduces risks of cancers in humans. Up to now, there is no research on the morphological changes in adventitious root formation from the rhizome of this species. In this study, the callus morphogenesis from the rhizome and the adventitious root morphogenesis from callus were analyzed. The rhizome segments 1-1.5 cm in diameter and 1 cm thick were cultured on MS medium supplemented 30 g/l sucrose, 6 g/l agar and 0.5 mg/l 2.4-D in dark. Callus formed on the surface of the rhizome after four weeks. The first cell divisions occurred in the first two weeks, in secondary cortex parenchyma cells and vascular cambium of the rhizome. 26-week-old callus with slow cell proliferation within the cluster and the long cells outside clusters were transferred to the induced medium supplemented with 0.5 mg/l 2.4-D and 0.1 mg/l TDZ within 6 weeks. The calli were more tight and formed many clusters. The adventitious roots formed 10 weeks after the transfer to MS medium supplemented with 0.5 mg/l NAA. Callus formed spherical structures called embryo-liked but only developed the root pole. Results of the thin layer chromatography using chloroform–methanol (9:1, v/v) showed the presence of oleanolic acid in extracts of adventitious roots fromed from callus-derived rhizomes of Panax stipuleanatus H.T.Tsai et K.M.Feng.

Induction of somatic embryogenesis in explants of shoot cultures established from adult Eucalyptus globulus and E. saligna × E. maidenii trees.

A reproducible procedure for induction of somatic embryogenesis (SE) from adult trees of Eucalyptus globulus Labill. and the hybrid E. saligna Smith × E. maidenii has been developed for the first time. Somatic embryos were obtained from both shoot apex and leaf explants of all three genotypes evaluated, although embryogenic frequencies were significantly influenced by the species/genotype, auxin and explant type. Picloram was more efficient for somatic embryo induction than naphthaleneacetic acid (NAA), with the highest frequency of induction being obtained in Murashige and Skoog medium containing 40 µM picloram and 40 mg l(-1) gum Arabic, in which 64% of the shoot apex explants and 68.8% of the leaf explants yielded somatic embryos. The embryogenic response of the hybrid was higher than that of the E. globulus, especially when NAA was used. The cultures initiated on picloram-containing medium consisted of nodular embryogenic structures surrounded by a mucilaginous coating layer that emerged from a watery callus developed from the initial explants. Cotyledonary somatic embryos were differentiated after subculture of these nodular embryogenic structures on a medium lacking plant growth regulators. Histological analysis confirmed the bipolar organization of the somatic embryos, with shoot and root meristems and closed procambial tissue that bifurcated into small cotyledons. The root pole was more differentiated than the shoot pole, which appeared to be formed by a few meristematic layers. Maintenance of the embryogenic lines by secondary SE was attained by subculturing individual cotyledonary embryos or small clusters of globular and torpedo embryos on medium with 16.11 µM NAA at 4- to 5-week intervals. Somatic embryos converted into plantlets after being transferred to liquid germination medium although plant regeneration remained poor.

Aspectos biométricos, morfoanatômicos e histoquímicos do pirênio de Bactris maraja (Arecaceae)

Bactris maraja (marajá) é uma palmeira frequente nas margens de lagos e igarapés amazônicos, cujos frutos comestíveis são consumidos pelo homem e pela fauna. Os estudos morfoanatômicos das estruturas reprodutivas podem contribuir com informações que subsidiarão estudos taxonômicos, fisiológicos e ecológicos. O objetivo deste estudo foi determinar características biométricas e morfoanatômicas do pirênio, bem como características histoquímicas da semente. Frutos maduros foram coletados e foram determinadas as dimensões, a massa fresca, e o teor de água, e descritas a forma, coloração e textura. O estudo anatômico foi feito pelo método de inclusão em resina. Foram aplicados testes histoquímicos para detecção dos principais metabólitos. Tanto os pirênios quanto as sementes isoladas apresentaram pequena variação biométrica, exceto para a massa fresca. O pirênio varia de obovado a elíptico, com endocarpo espesso, rígido, formado por células esclerificadas de paredes espessas. A semente é obovada depressa a elíptica, com tegumento fino, cartáceo, marrom-claro, aderido ao endocarpo, constituído de células parenquimáticas espessadas e taníferas; endosperma volumoso, sólido, homogêneo, branco, formado por células oblongas, elípticas e ovais transversalmente, e arredondadas longitudinalmente, com reserva proteica e lipídica; embrião lateral, cônico, pequeno, branco-leitoso, com conteúdo proteico e lipídico, polo radicular indiferenciado e polo caulinar com três primórdios foliares.

Plant embryogenesis requires AUX/LAX-mediated auxin influx.

The plant hormone auxin and its directional transport are known to play a crucial role in defining the embryonic axis and subsequent development of the body plan. Although the role of PIN auxin efflux transporters has been clearly assigned during embryonic shoot and root specification, the role of the auxin influx carriers AUX1 and LIKE-AUX1 (LAX) proteins is not well established. Here, we used chemical and genetic tools on Brassica napus microspore-derived embryos and Arabidopsis thaliana zygotic embryos, and demonstrate that AUX1, LAX1 and LAX2 are required for both shoot and root pole formation, in concert with PIN efflux carriers. Furthermore, we uncovered a positive-feedback loop between MONOPTEROS (ARF5)-dependent auxin signalling and auxin transport. This MONOPTEROS-dependent transcriptional regulation of auxin influx (AUX1, LAX1 and LAX2) and auxin efflux (PIN1 and PIN4) carriers by MONOPTEROS helps to maintain proper auxin transport to the root tip. These results indicate that auxin-dependent cell specification during embryo development requires balanced auxin transport involving both influx and efflux mechanisms, and that this transport is maintained by a positive transcriptional feedback on auxin signalling.

High frequency somatic embryogenesis and plant regeneration from hypocotyl and leaf explants of gherkin (Cucumis anguria L.)

Gherkin (Cucumis anguria L.) is an important commercial vegetable crop. An efficient protocol for plantlet regeneration from hypocotyl and leaf explants through direct somatic embryogenesis was developed. High frequency of somatic embryos (21.6 and 34.0) were obtained from hypocotyl and leaf explants when cultured on Murashige and Skoog (MS) salts plus B5 vitamins (MSB5) supplemented with 5% sucrose, 1.5mgL−1 2,4-D, 0.5mgL−1 BAP and 150μM l-glutamine. Initial culturing of embryos in the dark conditions for two weeks, followed by four weeks under light resulted in a higher frequency of embryo formation when compared to continuous light conditions. Histological observation showed that the somatic embryos originated from single cells of the epidermal layer. Histological evidence on formation of shoot and root poles during conversion of the embryos confirmed that these structures were true somatic embryos. Maturation of somatic embryos occurred on plant growth regulator-free MSB5 semi-solid medium containing 3% sucrose and 0.5% (w/v) activated charcoal (AC). Conversion of embryos into plantlets was achieved on MSB5 medium supplemented with 3% sucrose, 0.3% AC and 0.5mgL−1 gibberellic acid (GA3). Sucrose was found to be the best carbon source for SE induction, maturation and germination. Ninety percent of embryos were converted into normal plantlets. The plantlets were successfully acclimatized in the greenhouse with 95% survival rate and transferred to ex vitro conditions which developed with normal phenotypes. This regeneration protocol assured successful embryo induction and plantlet conversion. This is the first report for the induction of direct somatic embryogenesis from hypocotyl and leaf explants of C. anguria. The result of this study is beneficial for genetic transformation and mass clonal propagation.

Local Auxin Sources Orient the Apical-Basal Axis in Arabidopsis Embryos

Establishment of the embryonic axis foreshadows the main body axis of adults both in plants and in animals, but underlying mechanisms are considered distinct. Plants utilize directional, cell-to-cell transport of the growth hormone auxin to generate an asymmetric auxin response that specifies the embryonic apical-basal axis. The auxin flow directionality depends on the polarized subcellular localization of PIN-FORMED (PIN) auxin transporters. It remains unknown which mechanisms and spatial cues guide cell polarization and axis orientation in early embryos. Herein, we provide conceptually novel insights into the formation of embryonic axis in Arabidopsis by identifying a crucial role of localized tryptophan-dependent auxin biosynthesis. Local auxin production at the base of young embryos and the accompanying PIN7-mediated auxin flow toward the proembryo are required for the apical auxin response maximum and the specification of apical embryonic structures. Later in embryogenesis, the precisely timed onset of localized apical auxin biosynthesis mediates PIN1 polarization, basal auxin response maximum, and specification of the root pole. Thus, the tight spatiotemporal control of distinct local auxin sources provides a necessary, non-cell-autonomous trigger for the coordinated cell polarization and subsequent apical-basal axis orientation during embryogenesis and, presumably, also for other polarization events during postembryonic plant life.

Genetic approach towards the identification of auxin–cytokinin crosstalk components involved in root development

Phytohormones are important plant growth regulators that control many developmental processes, such as cell division, cell differentiation, organogenesis and morphogenesis. They regulate a multitude of apparently unrelated physiological processes, often with overlapping roles, and they mutually modulate their effects. These features imply important synergistic and antagonistic interactions between the various plant hormones. Auxin and cytokinin are central hormones involved in the regulation of plant growth and development, including processes determining root architecture, such as root pole establishment during early embryogenesis, root meristem maintenance and lateral root organogenesis. Thus, to control root development both pathways put special demands on the mechanisms that balance their activities and mediate their interactions. Here, we summarize recent knowledge on the role of auxin and cytokinin in the regulation of root architecture with special focus on lateral root organogenesis, discuss the latest findings on the molecular mechanisms of their interactions, and present forward genetic screen as a tool to identify novel molecular components of the auxin and cytokinin crosstalk.

A new method of frequency-dependent network equivalence for power system

Electromagnetic transient simulation for large-scale power system is a time-consuming problem. A new frequency-dependent equivalence method is presented in the paper, which might significantly accelerate power system electromagnetic transient simulation. In the method, an effective algorithm is designed to directly transfer the port admittance determinant of external system’s mixing matrix into admittance rational function; and the step-by-step strategy for the equivalence of actual large system is put forward, which further reduces the calculation quantities needed. Moreover, the study of multiple real root pole characteristics of admittance transfer function of two-port network is performed and a proposition is achieved. Based on the proposition and residue theorem, the equivalence system for external system corresponding to the admittance rational function is obtained. The computation complexity of the step-by-step equivalence method is about o(⌋n/np⌉×t)(⌋⌉ is upper integral operation, n is the total buses number of external system, NP is the total buses number of single step equivalence network, T is single step equivalence time), which indicates that the computation complexity of the method proposed has nearly linear relationship with the buses number of external system, and the method proposed has satisfactory computation speed. Since the mixing matrix of external system includes all the information of external system, therefore, port admittance rational function derived from it can reflect its full frequency characteristic and the equivalence network achieved has high equivalence precision. Moreover, since the port rational function is gained at the condition of the external system without source, which equals stable passive network, it could not show any unstable pole and need not extra measure to make the equivalence system stable. The test results of the samples and comparison with other methods demonstrate that the new method proposed is valid and effective.

Evaluation of different embryogenic systems for production of true somatic embryos in Arabidopsis

Somatic embryogenesis (SE) in Arabidopsis was induced using various systems, including auxin treatment of in vitro cultured explants (immature zygotic embryos, IZEs) and transgenic plants overexpressing embryogenesis-related transcription factors, e.g. LEC2 together with the GUS reporter gene under control of the auxin-induced DR5 promoter. The study indicated that the SE-systems used gave different embryogenic capacities for the production of true embryos. The highest ratio of true embryos (75 %) was found among embryo-like structures in transgenic seedlings overexpressing LEC2. Analysis of in vitro induced SE systems indicated that in somatic embryos produced in response to exogenous auxin treatment the formation of root poles is frequently disturbed. A lack of a properly formed root meristem was observed in 35–80 % of in vitro induced somatic embryos, in dependence on auxin concentration and duration of treatment.

Early Embryogenesis in Flowering Plants: Setting Up the Basic Body Pattern

Early embryogenesis is the critical developmental phase during which the basic features of the plant body are established: the apical-basal axis of polarity, different tissue layers, and both the root pole and the shoot pole. Polarization of the zygote correlates with the generation of apical and basal (embryonic and extraembryonic) cell fates. Whereas mechanisms of zygote polarization are still largely unknown, distinct expression domains of WOX family transcription factors as well as directional auxin transport and local auxin response are known to be involved in early apical-basal patterning. Radial patterning of tissue layers appears to be mediated by cell-cell communication involving both peptide signaling and transcription factor movement. Although the initiation of the shoot pole is still unclear, the apical organization of the embryo depends on both the proper establishment of transcription factor expression domains and, for cotyledon initiation, upward auxin flow in the protoderm. Here we focus on the essential patterning processes, drawing mainly on data from Arabidopsis thaliana and also including relevant data from other species if available.

Bisymmetry in the embryonic root is dependent on cotyledon number and position

In the shoot pole of Arabidopsis embryos, radial symmetry is broken by cotyledon specification. Subsequently, the radial pattern of the embryo axis is converted to bisymmetric. In a recent publication, we showed that distinct boundaries of hormonal signalling output specify the vascular pattern in the root meristem through a mutually inhibitory feedback loop between the hormones auxin and cytokinin. We observed that during embryogenesis, symmetry breakage in the root pole coincided with an influx of auxin from the cotyledons. In this manuscript, we provide genetic data to support the role of the cotyledons in initiating symmetry breaking in the embryonic root pole. Mutants with alterations in cotyledon number fail to establish bisymmetry in the embryo axis. These data further support the idea that input from the cotyledons may be required for the propagation of bisymmetry from the cotyledons to the embryonic root.

Direct somatic embryogenesis and plant regeneration from leaf explants of Limoniumsinensis (Girard) Kuntze

The potentialities of direct somatic embryogenesis and plant regeneration from leaf explants of Limonium sinensis var. Golden Diamond in vitro were investigated. Young whole leaf and cut leaf explants when cultured on MS basal medium supplemented with each of the growth regulators N 6-benzyladenine (BA) (0.44–2.2 μM) or thidiazuron (TDZ) (4.54 μM) alone or in combination with a fixed concentration of α-naphthalene acetic acid (NAA) (1.07 μM) produced somatic embryos directly. More than 90% of the leaf explants produced white, globular somatic embryos on BA (2.2 μM) and NAA (1.07 μM) supplemented MS basal medium within 1 week of inoculation. Most of the embryos matured further and converted after 8 weeks of culture on the same medium. Histological observation showed that the somatic embryos originated from single cells of epidermal layer of leaf. Histological evidence of formation of shoot and root poles during conversion of the embryos confirmed that these structures were true somatic embryos. After conversion the plantlets were further placed on MS medium containing 0.44 μM BA and 4.5 μM IBA for better shoot and root growth. About 90% of the plantlets transferred to the mixture of soil:perlite:vermiculite (1:1:1) in small plastic pots acclimatized successfully. Of these 85.5% plants survived after transferring into earthen pots containing a mixture of soil, coarse sand and cattle manure (1:1:1) under greenhouse or shady open condition.

The Arabidopsis BET bromodomain factor GTE4 regulates the mitotic cell cycle

Proteins containing bromodomains are capable of binding to acetylated histone tails and have a role in recognizing and deciphering acetylated chromatin. Plant BET proteins contain one bromodomain. Twelve BET-encoding genes have been identified in the Arabidopsis genome. Two of these genes have been functionally characterized, one shows a role in seed germination, the other is involved in the establishment of leaf shape. Recently, we characterized a third AtBET gene, named GTE4. We demonstrated that GTE4 is involved in the activation and maintenance of cell division in the meristems and by this controls cell numbers in differentiated organs. Moreover, the quiescent center (QC) identity is partially lost in the apex of the primary root of gte4 mutant, and there is a premature switch from mitosis to endocycling. Genes involved in the retinoblastoma (RB)-E2F pathway, which is important for coupling cell division and cell differentiation in plants and animals, were either up- or down-regulated in the gte4 mutant. In this report we also show that the defect in germination observed in gte4 mutant seeds is not rescued by the action of GA3. Further the root pole of the mutant embryo shows irregular cytokinesis in the procambial stem cells, and the QC of the lateral root shows a partial, but not transient, loss of QC identity. These additional results reinforce the importance of GTE4 in the control of cell proliferation.

Control of Arabidopsis apical–basal embryo polarity by antagonistic transcription factors

Plants, similar to animals, form polarized axes during embryogenesis upon which cell differentiation and organ patterning programs are orchestrated. During Arabidopsis embryogenesis, establishment of the shoot and root stem cell populations occurs at opposite ends of an apical-basal axis. Recent work has identified the PLETHORA (PLT) genes as master regulators of basal/root fate1–3, while the master regulators of apical/shoot fate have remained elusive. Here we show that the PLT1 and PLT2 genes are direct targets of the transcriptional corepressor TOPLESS (TPL) and that PLT1/2 are necessary for the homeotic conversion of shoots to roots in tpl-1 mutants. Using tpl-1 as a genetic tool, we identify the CLASS III HOMEODOMAIN-LEUCINE ZIPPER (HD-ZIP III) transcription factors as master regulators of embryonic apical fate, and show they are sufficient to drive the conversion of the embryonic root pole into a second shoot pole. Furthermore, genetic and misexpression studies reveal an antagonistic relationship between the PLT and HD-ZIP III genes in specifying the root and shoot pole.