Plant development revolves aroundaxesJohn Chandler, Judith Nardmann and Wolfgang Werr

Institute of Developmental Biology, University of Cologne, Gyrhofstrasse 17, D-50923 Cologne, Germany

Review

Arabidopsis thaliana has become a paradigm for dicotembryo development, despite its embryology beingnon-representative of dicots in general. The recentcloning of heterologous genes involved in embryonicdevelopment from maize and construction of robustphylogenies has shed light on the conservation oftranscription factor function and now facilitates acomparison of maize and Arabidopsis embryogenesisorthology. In this review, we focus on a comparisonof expression domains of WUSCHEL HOMEOBOXLIKE (WOX), SHOOTMERISTEMLESS (STM), DORN-ROESCHEN (DRN) and CUP-SHAPED COTYLEDON(CUC) genes and their role in axialization in both species,showing that despite significantly divergent modes ofembryogenesis, most notably in terms of axes andplanes of symmetry, there is considerable conservationof function as well as notable differences between maizeand Arabidopsis.

Embryonic patterning: expanding the paradigmEmbryogenesis in plants establishes the basic body planand stem cell populations for the generation of all post-embryonic organs. Arabidopsis thaliana is the best-described species and is a paradigm for dicot embryo de-velopment owing to key features such as the stereotypy ofearly cell divisions and well-characterized gene expressiondomains. Knowledge from Arabidopsis has highlighted theimportance of tightly regulated transcription factor net-works for establishing positional information and a depen-dence on auxin. The increase in gene sequence informationfrom other classical model species such as rice (Oryzasativa), coupled with heterologous gene cloning, hasenabled the isolation of the corresponding genes frommaize (Zea mays). The time is therefore appropriate fora comparative review of the developmental programmes ofboth species.

The aim of this review is not to recapitulate generalembryogenesis, or the function of genes involved, whichhave been covered in several recent reviews [1–3]. Rather,we aim to consolidate the markedly different developmen-tal embryonic programmes of maize and Arabidopsis interms of axialization and compare the expression patternsof Arabidopsis embryonic genes with known maize ortho-logues. The focus on similarities within a core set offundamentally important transcription factors enablesconclusions to be made about evolutionary conservationin the genetic control of embryogenesis and differences in

Corresponding author: Werr, W. (werr@uni-koeln.de).

78 1360-1385/$ – see front matter � 2007 Elsevier Ltd

the respective developmental programmes highlightfuture areas of focus for maize embryo research.

Axes in the Arabidopsis embryoEmbryogenesis in Arabidopsis is rapid with �10 h separ-ating cell divisions and a heart stage embryo being formedwithin 120 h after pollination [4]. Key features ofArabidop-sis embryogenesis include synchronous cell divisions, whichpattern the embryo by the superimposition of axes andplanes of symmetry as it develops (Figure 1a) and thedetermination of cell fate by positional information. Studiesof seedling lethal mutants [5] divide the apical-basal axisinto three domains within the octant stage embryo(Figure 1a) whereby the apical domain generates most ofthe cotyledon tissue and the shoot apical meristem (SAM).The remaining cotyledon tissue, hypocotyl and root apicalinitialswill derive fromthe lower tier of four cells comprisingthe central domain, and the basal region will form thequiescent centre and root cap initials, arising from thehypophysis. Periclinal cell divisions superimpose a secondaxis of radial (inner-outer) symmetry near the 16-cell stage,generating an outer protoderm or epidermis layer, themedial underlying sub-epidermal ground tissue and centralvascular initials throughoutglobularembryogenesis.Radialsymmetry remains evident in the concentric nature of tissuetypes, which persists in the primary root and the hypocotyl.

Cotyledon organogenesis breaks radial symmetry in theapical embryo domain andmarks the transition to bilateralsymmetry, with a central-peripheral axis generated fromthe shoot apical meristem outwards to the expandingcotyledons (Figure 1a). The ab- and adaxialization of coty-ledons in a plane relative to the SAM reflects differen-tiation along this central-peripheral axis. Most dicotstypically have a second plane of symmetry perpendicular(sagittal-longitudinal) to the medial frontal longitudinalplane, depicted in Figure 1a, which splits the SAMbetweenboth cotyledons. In Arabidopsis, the central apical positionof the SAM thus correlates with the ability to convertradial symmetry into two perpendicular planes of bilateralsymmetry. The SAM and opposing root meristem (RM) atthe basal pole still reflect the chalazal-micropylar polarityin the egg cell. It is tempting to consider that the coordinatesystem that provides positional information for the zygoteand the early patterning programmemight be conserved inhigher plant development.

Embryo organization in maizeThe monocots, with a so-called single cotyledon divergefrom the Arabidopsismodel in planes of symmetry and the

. All rights reserved. doi:10.1016/j.tplants.2007.11.010 Available online 11 February 2008

Figure 1. Developmental morphology and axes and planes of symmetry during Arabidopsis and maize embryogenesis. Schematic drawing of successive stages in

Arabidopsis embryo development (a) showing early-, mid-, and late-globular and heart stage embryos, respectively, from left to right. Note the regular cell divisions

generating apical, central and basal domains. Cotyledon (cot) outgrowth flanking the shoot apical meristem (SAM) initiates bilateral symmetry at the heart stage. The

apical-basal axis and the four planes of symmetry following the heart stage are depicted: radial, central-peripheral and two perpendicular planes of bilateral symmetry.

(b) Maize embryo developmental morphology showing the asymmetric division of the zygote, irregular cell divisions in the pro-embryo, and a coleoptilar stage embryo,

side and frontal views. Note the oblique shoot-root axis from shoot apical meristem (SAM) to root meristem (RM) in the coleoptilar side view. The axes (apical-basal, radial

and central peripheral) and two planes of symmetry are depicted for Arabidopsis. The adaxial–abaxial alignment of the maize embryo is in relation to the rachis of the ear.

The single plane of bilateral symmetry in the maize embryo runs parallel (medial) to the adaxial-abaxial axis.

Review Trends in Plant Science Vol.13 No.2

position of the SAM. The maize embryo develops slightlyslower than that of Arabidopsis, with �6–7 days afterpollination necessary for SAM initiation, compared with�4 days inArabidopsis. Inmaize andArabidopsis, only thefirst asymmetric cell division in the zygote is predictable,leading to a small apical cell and larger basal cell(Figure 1b). Subsequent cell divisions in the maize embryoare in random planes, although embryos can be morpho-logically staged.

The embryo develops on the adaxial side of the endo-sperm, which faces the scutellum – possibly the singlemaize cotyledon [6], whereas the SAM is initiated on theopposing flank of the embryo proper oriented away fromthe endosperm. During the coleoptilar stage, the scutellumenlarges and the SAM, encircled by the coleoptile, elabor-ates at a central position when viewed frontally to thescutellum (Figure 1b). The root meristem (RM) initiates ata central basal position of the embryo proper and the root-shoot axis is therefore oriented centro-laterally at an obli-que angle to the apical-basal polarity of the embryo. Themaize embryo has a single axis of bilateral symmetrythrough the midrib of developing leaf primordia, dividingthe scutellum into two mirror halves. In addition to anapical-basal axis and radial (inner-outer) organization, themaize embryo contains an adaxial-abaxial axis, defined in

relation to the rachis of the maize ear, whereby the SAM isoriented towards the rachis and the scutellum at theopposite face of the embryo proper is oriented towardsthe endosperm (Figure 1b). This differs from the adax-ial-abaxial plane in Arabidopsis, which is in relation to theSAM and not based on the orientation of the embryo.Overall, the maize embryo differs fundamentally from thatof Arabidopsis in having a single plane of bilateral sym-metry instead of two, an oblique apical-basal axis and nocentral-peripheral axis.

The molecular marker dramatis personae in Arabidopsis

Genetic analyses have identified multiple gene functionsinvolved in embryonic patterning in Arabidopsis. Here, wefocus on a subset of transcription factors for which maizeorthologues exist, thus enabling a comparison with Arabi-dopsis. These include the WUSCHEL HOMEOBOX LIKE(WOX) and CUP-SHAPED COTYLEDON gene familiesand the SHOOTMERISTEMLESS (STM) and DORN-ROSCHEN (DRN) genes.

The earliest embryo cell-type markers comprise mem-bers of the WOX gene family, which are differentiallyexpressed in the small apical or large basal cell after theasymmetric division of the zygote (WOX2 or WOX8,respectively) [7] or which mark the stem cell organizing

79

Figure 2. Cell-type specification in the maize and Arabidopsis embryo. Expression domains of maize orthologues corresponding to Arabidopsis WUSCHEL HOMEOBOX

LIKE (WOX), CUP-SHAPED COTYLEDON (CUC), DORNROSCHEN (DRN) and SHOOTMERISTEMLESS (STM) genes during maize embryo development (a) compared with

those during Arabidopsis embryogenesis (b). The enlarged inset on the right in (b) illustrates that the STM expression domain encompasses that of WUS and DRN and

mutually excludes that of the CUC genes, which are expressed in the boundary between the SAM and cotyledons. The CUC expression domain represents total CUC1, CUC2

and CUC3 expression. Abbreviations: CZ, central zone; PZ, peripheral zone; an, leaf primordium anlage.

Review Trends in Plant Science Vol.13 No.2

centre (OC) of the SAM or the root quiescent centre (QC)(WUS and WOX5) (Figure 2b). WOX5 can replace WUS inthe shoot OC, suggesting they share stem cell promotingfunctions [8].

The CUC1, CUC2 and CUC3 NAC (NO APICAL MER-ISTEM, ATAF-1, ATAF-2 and CUP-SHAPED COTYLE-DON) family transcription factors have weak single

80

mutant phenotypes but in double mutant combinationsthe SAM is lost and cells of the central zone differentiate,leading to cotyledon fusion [9–11]. CUC3 is expressed inthe upper tier cells of the octant Arabidopsis embryo andlater overlaps with expression of CUC1 and CUC2 in astripe within the central domain that separates the devel-oping cotyledons [10,12]. CUC genes function by activating

Review Trends in Plant Science Vol.13 No.2

STM, which represses cellular differentiation at basallateral margins of the cotyledons, thus enabling completeseparation [10]. At the torpedo stage, STM and CUCexpression domains are mutually exclusive, with STM inthe SAM and CUC gene activity delineating a boundarybetween the SAM and surrounding embryonic tissue sep-arating both cotyledons (Figure 2b). Later in development,STM feedback regulates CUC1 and CUC2 expression [10].

STM is a class I knotted homeobox (KNOX) gene [13]and is orthologous to KNOTTED1 (KN1) of maize [14].After initial expression in a medial stripe in globular andearly heart stage embryos at the sinus between the devel-oping cotyledons, STM expression in later embryogenesisis found in the SAM (Figure 2b) and newly arising second-ary shoot meristems but is downregulated in incipientprimordia when cells acquire the primordial fate, a featuresharedwithKN1 inmaize. Consistent with the absence of aSAM in stm mutants and the lack of transcripts in pri-mordial cells, STM is considered a marker for shoot mer-istem identity [13]. As well as functioning in suppression ofcellular differentiation, STM is also necessary for correctspatial expression of CUC1 and CUC2 [10,12].

DORNROSCHEN (DRN) was identified as a negativeeffector ofSAMfunction [15].DRNand itsparalogueDORN-ROSCHEN-like (DRNL) encode AP2 (APETALA2)/ERF(ETHYLENE RESPONSE FACTOR) type transcriptionfactors, which redundantly control cotyledon organogenesisinArabidopsis [16].DRN is also necessary to specify correctcell divisions in the basal domain and is expressed in theembryo proper from the 2-cell stage throughout globulardevelopment, when transcription then becomes apical andlocalized to the emerging cotyledons. Expression sub-sequently marks the cotyledon tips throughout their de-velopment and is later initiated in the SAM (Figure 2b).Three factors make DRN relevant for comparative studies:(i) its dynamic expression following the first zygotic divisionand its restriction to the apical domain, the cotyledons andsubsequently to the functional SAM; (ii) its role upstream ofauxin transport or perception [16], and (iii) the radializedpin-like phenotype of drn drnl double mutant embryoslacking bilateral symmetry (16). All three aspects implicateDRN function in axis specification.

Comparative maize gene expression and axializationApical-basal polarity of the zygote and the primary

shoot-root axis

In maize, orthologues of WOX2 and WOX5 have beencloned [17]. ZmWOX2A, one of two maize WOX2 paralo-gues, is expressed early in maize embryos in the embryoproper apical domain, and then laterally on the adaxialside of the embryo proper where it becomes restricted tothe outer L1 layer and pre-patterns the SAM [18](Figure 2a). ZmWOX2A can therefore be considered amarker for embryo proper cell fate and the implementationof lateral adaxial positional information pre-patterning theposition of the SAM.Maize also has twoWOX5 paralogues:analogous to WOX5 in Arabidopsis, ZmWOX5B marks theroot quiescent centre and is expressed in a central basaldomain of the embryo proper subtended by vacuolizedsuspensor cells [18] (Figure 2a). The activation ofZmWOX5B at the late proembryo or early transition stage

indicates root specification, whereas ZmWOX2A expres-sion shifts to a lateral position, thus realizing positionalinformation of the adaxial-abaxial axis. No ZmWOX2Bexpression could be detected in the maize embryo andZmWOX5A transcripts were localized to the suspensor,suggesting a function outside the morphogenetic axis.

Strikingly, WOX2 in Arabidopsis and ZmWOX2A inmaize are both expressed in the zygote, are associatedwith apical cell fate and also pre-pattern the location ofthe SAM, thereby interpreting adaxial positional infor-mation in maize. However, ZmWOX5B retains centralpositional information for the RM inmaize [18] (Figure 2a).

According to PCR-based approaches, maize containsthree ZmWOX9 paralogues but no WOX8 orthologue,which is also absent in the genomes of rice or Populusand there is no evidence for ZmWOX9A, B or C expressionin the zygote. However, following the coleoptilar stage,ZmWOX9A, B or C are differentially expressed in thesuspensor: ZmWOX9C in outer cell layers of the suspensorand ZmWOX9A and ZmWOX9B more generally through-out the suspensor (Figure 2a) [18]. ZmWOX5A, the para-logue of the QC-specific ZmWOX5B, is first activated in theendosperm basal transfer layer, which lies between theendosperm and the maternal placental tissue, and then inthe central suspensor region of the leaf stage 1 embryo,thereby suggesting that some cellular identity is sharedbetween both tissue types. In general, analysis of themaizeWOX gene family highlights remarkably conserved func-tions in the case of ZmWOX2A and ZmWOX5B in maizeand WOX2 and 5 in Arabidopsis. However, additionalWOX2 and 5 paralogues in maize (ZmWOX2B and 5A)and additional WOX9 paralogues as well as an apparentlack of any WOX8 orthologue suggest functional diversifi-cation has also occurred.

Delineation of meristem boundaries and CUC genes

In Arabidopsis, CUC genes function to define cotyledonboundaries and pre-pattern the transition to bilateralsymmetry of the embryo. In maize, a heterologous cloningapproach identified a single CUC3 orthologue and para-logous genes ZmNAM1 and ZmNAM2 as putative CUC1/CUC2 orthologues [19]. In contrast to Arabidopsis, theexpression of maize CUC orthologues begins later, andis concomitant with KN1 expression in the early transitionstage embryo, instead of preceding it as CUC expressionprecedesSTM transcription inArabidopsis. The boundary-specific nature of maize CUC2 and CUC3 orthologues ismaintained, with transcripts excluded from the SAM butdelineating boundaries for surrounding embryonic tissue,including the coleoptile or true leaf primordia derived fromSAM activity (Figure 2a). This conservation in expressionis striking given the markedly different leaf developmentprogramme in maize compared with Arabidopsis [20].However, the lack of association with early embryonicdecisions before SAM initiation suggests that the regulat-ory network in maize has adapted significantly, possiblylinked to the need to establish and translate adaxial-abaxial positional information into the initiation of SAMand scutellum to opposing faces of the maize pro-embryo.Unfortunately, a bilateral species comparison cannotdetermine whether CUC pre-patterning has been lost in

81

Review Trends in Plant Science Vol.13 No.2

the course of maize evolution or has been newly recruitedin Arabidopsis.

Meristem formation: the SHOOTMERISTEMLESS and

KNOTTED1 orthology

As well as initiating SAM formation and consolidating theapical pole in Arabidopsis, STM has an important function(along with CUC gene function) in the establishment ofplanes of bilateral symmetry or central-peripheral pos-itional information.

Expression of the maize STM orthologue,KN1, begins inearly transition stage embryos and is associated with thedifferentiation of small, cytoplasm-rich cells at the adaxialface of the embryo proper [21] (Figure 2a). In contrast to theSAM-specific expression ofSTM in theArabidopsis embryo,theKN1 expression domain comprises a sector that extendsfrom the lateral periphery (pre-patterned by ZmWOX2A)towards the centre of the embryo and includes the prospec-tive RM (Figure 2a). In later embryonic stages, the invertedcup-shaped KN1 domain connects the SAM and RM andcontains the expression domain ofZmWOX5B in amutuallyexclusive pattern (Figure 2a). In addition, KN1 is not tran-scribed in the L1 layer of the SAM, but the protein trafficsfrom sub-epidermal cells to the outer L1 layer [22]. There-fore, not taking into account the movement of the KN1protein, the expression of KN1 and STM in the maize andArabidopsis SAM is analogous. Similarly, because STM isinitially expressed outside the SAM and suppresses pre-mature differentiation of cells between the cotyledons inArabidopsis, KN1 expression outside the SAM in maizeensures that cells of the morphogenic shoot-root axis aremaintained in an undifferentiated state. It seems thatKN1transcription in the early transition stage embryo is acti-vated according to adaxial and central positional infor-mation [21] and the mutually exclusive expressionpatterns of ZmWOX5B and KN1 suggest subsequent per-ception of apical-basal polarity towards the coleoptilarstage.

DORNROSCHEN (DRN) and scutellum fateThe putative maize DRN orthologue, ZmDRN [23] is firstexpressed at the scutellum face of the embryo proper in themaize late proembryo. During the coleoptilar stage, thisspecific and localized expression pattern shifts to the SAMand leaf primordium anlage (Figure 2a). Therefore, theexpression domains of ZmDRN are reminiscent of those ofDRN inArabidopsis,marking the incipient cotyledons in thelate globular embryo and subsequently the SAM during thetorpedo stage. This correlation is particularly striking if thescutellumisseenasbeinganalogous toasingle cotyledon [6].However, one difference is that whereasDRN expression isexpressed from the two-cell Arabidopsis embryo onwards,this early expression is lacking in the early maize embryo.Thismightbeassociatedwithdifferences in the rolesofDRNand ZmDRN in axis specification in Arabidopsis and maizeand with differences in bilateral symmetry, one plane ofwhich is lacking in the maize embryo. Significantly, thecoincidence of DRN expression with maxima of auxin con-centration and/or response and the involvement of DRN inauxin signalling [16] raises the question of whether auxinhas a similar role in the maize embryo.

82

Integration of positional information in Arabidopsis

and maizeGiven the conserved gene expression domains betweenArabidopsis and maize, but bearing in mind the additionalsub-functionalization in the WOX family in maize andaxialization differences, any discussion of transcriptionfactors involved in embryonic patterning requires com-ment on the integration of positional information.

Since the observation that exogenous auxin inhibitorscause embryo defects and alter bilateral symmetry, [24]much evidence has positioned auxin as a key signal mol-ecule integrating positional patterning information in Ara-bidopsis [25–28]. Via dynamic fluxes in PIN auxin effluxprotein production in the early Arabidopsis embryo, tran-sient auxin maxima help to establish hypophysis identity,thereby further consolidating the apical-basal axis [29–32].Polar auxin transport also creates local auxin maximaassociated with cotyledon outgrowth, thereby breakingembryonic radial symmetry and establishing planes ofbilateral symmetry [24]. In addition to auxin redistribu-tion, auxin biosynthesis essentially controls embryonicpatterning: higher order knockouts in YUCCA genesencoding flavin monooxygenases cause a failure to estab-lish a hypocotyl and RM [33]. In addition, auxin restrictsgene expression, for example CUC gene expression in theperipheral domain [34,35]. Two putative PIN1 orthologueshave recently been isolated from maize [36], but their roleduring embryogenesis has not been established. Thisrepresents a significant knowledge deficit in maize, whichshould be rectified by the characterization of PIN functionand by determining whether auxin flux contributes toapical basal polarity or the establishment of the adaxial-abaxial axis. This will aid the interpretation of apparentlyconserved transcription factor cascades in maize, forexample those involvingCUC, STM,DRN andWOX genes.

In addition to auxin and promoters controlling geneexpression domains, additional factors might define andhone boundaries: microRNAs control genes involved inArabidopsis embryonic patterning, such as the class IIIHD-ZIP (homeodomain zipper) genesPHV (PHAVOLUTA)and PHB (PHABULOSA) [37] or CUC1 and CUC2, whichare controlled by miR165 and 166 in the abaxial leafdomain [38,39], or miR164 at the SAM boundary [40].The corresponding microRNA target sites are conservedin maize orthologues [19,41] and might contribute to theestablishment of steady-state embryonic expressiondomains. CUC genes in Arabidopsis are also regulatedby a chromatin remodelling ATPase [42] and by TCP(TEOSINTE BRANCHED1, CYCLOIDEA, PCF1) tran-scription factors [43]. Many microRNAs are conservedamong plants [44], however, whether they represent con-served levels of gene regulation during plant embryogen-esis is still an open question – methodology forinvestigating microRNA expression using in situ hybrid-ization in Arabidopsis and maize is being developed [45].

Functional redundancy in embryogenesisIn Arabidopsis, many pathways affecting embryonic pat-terning are functionally redundant, including those invol-ving the CUC, PIN, WOX, class III HD-ZIP and DRN andDRNL genes. In maize, although orthologues for some

Review Trends in Plant Science Vol.13 No.2

Arabidopsis WOX genes have not been found, there issignificant conservation in the genetic repertoire. Basedon expression patterns, gene duplications in maize haveenabled sub-functionalization of paralogues, for exampleZmWOX 5A and B or ZmWUS1 and 2 [18,46]. Other dupli-cated loci still act redundantly, for example NARROW-SHEATH1 and 2 in the leaf, equivalent to PRESSEDFLOWER in Arabidopsis [47,48] and NS1 and ZmWOX3Ain the margins of the coleoptile [18]. Some gene families inmaizeare smaller than theirArabidopsis counterparts, suchas thePIN familywith only twomembers inmaize, andgeneorthologues might be also absent, as with DRNL. Alterna-tive hypothesesmight explain these differences: polar auxintransport might be less important in specifying maize axesor a lack of gene duplication eventsmight not have providedpotential for sub-functionalization in these gene families.

Concluding remarks and future perspectivesComparative evolutionary developmental biologyaddresses questions concerning the conservation and evol-ution of regulatory networks [49]. A bilateral comparisonbetween the repertoire of gene orthologues within themaize and Arabidopsis developmental paradigms enablesdiscrimination of conserved components of developmentalpathways from those that have evolved and led to gene sub-functionalization.

Embryonic patterning is closely linked to axialization:in Arabidopsis, STM and the WOX genes contribute toapical-basal polarity, the CUC genes contribute to central-peripheral axis differentiation and also contribute to bilat-eral symmetry, together with DRN and DRNL. A majordifference in maize is the additional adaxial-abaxialpolarity, which is reflected in a single plane of bilateralsymmetry compared with two perpendicular planes inArabidopsis. However, despite variant patterning, thebasic transcription factors of regulatory pathways showremarkable similarity in their expression patterns. Forexample, ZmDRN expression in the scutellum and theSAM is analogous to DRN expression in Arabidopsis coty-ledons and in the SAM, and expression ofWOX2 andWOX5orthologues in maize pre-pattern the SAM and RM as theydo in Arabidopsis. It seems that expression patterns havebeen conserved despite gene duplications enabling de novosub-functionalization, as is evident for ZmWOX5B. Un-derstanding the basic principles of plant embryonic pat-terning in maize is valuable because of its importance as acrop plant and as a model for grasses. The large repertoireof maize embryo and developmental mutants and expand-ing genome resources offer the potential to manipulateplant architecture. It is pertinent to recognize that Arabi-dopsis is not representative of dicot embryo development,with respect to a single cell file suspensor, early stereotypiccell division patterns or staging: many species, such asPhaseolus, have disorganized cell divisions redolent ofmaize development [50] and some dicots abort one cotyle-don [51]. The study of other plant species might not onlyuncover novel patterning genes but also uncover signallingpathways masked in Arabidopsis.

The data summarized here are based on in situ hybrid-ization, however, higher resolution of transcriptional pro-filing in Arabidopsis embryos is now attainable using laser

capture microdissection [52], facilitating the temporalcharacterization of transcriptional profiles in specificsub-embryo cell types [53]. In addition, in vitro embryoculture [54] allows real-time imaging to monitor geneexpression as for example in inflorescence meristems[55], to detect subtle changes in dynamic expression pro-files. With this state-of-the-art technology, plant embryodevelopment is now accessible and presents an opportunityfor elucidating fundamental principles of cellular decision-making or differentiation in plant development with anevolutionary perspective.

AcknowledgementsWe gratefully acknowledge funding for this work from the DeutscheForschungsgemeinschaft via SFB572 and SFB680.

References1 Laux, T. et al. (2004) Genetic regulation of embryonic pattern

formation. Plant Cell 16, S190–S2022 Jenik, P.D. et al. (2007) Embryonic patterning in Arabidopsis thaliana.

Annu. Rev. Cell Dev. Biol. 23, 207–2363 Vernoud, V. et al. (2005) Maize embryogenesis. Maydica 50, 469–4834 Jenik, P.D. et al. (2005) Interaction between the cell cycle and

embryonic patterning in Arabidopsis uncovered by a mutation inDNA polymerase. Plant Cell 17, 3362–3377

5 Mayer, U. et al. (1991) Mutations affecting body organisation in theArabidopsis embryo. Nature 353, 402–407

6 Abbe, E.C. et al. (1951) The growth of the shoot apex in maize: internalfeatures. Am. J. Bot. 38, 744–752

7 Haecker, A. et al. (2004) Expression dynamics of WOX genes mark cellfate decisions during early embryonic patterning in Arabidopsisthaliana. Development 131, 657–668

8 Sarkar, A.K. et al. (2007) Conserved factors regulate signalling inArabidopsis thaliana shoot and root stem cell organisers. Nature446, 811–814

9 Aida, M. et al. (1997) Genes involved in organ separation inArabidopsis: an analysis of the cup-shaped cotyledon mutant. PlantCell 9, 841–857

10 Aida, M. et al. (1999) Shoot apical meristem and cotyledon formationduring Arabidopsis embryogenesis: interaction among the CUP-SHAPED COTYLEDON and SHOOT MERISTEMLESS genes.Development 126, 1563–1570

11 Vroemen, C.W. et al. (2003) TheCUP-SHAPEDCOTYLEDON3 gene isrequired for boundary and shoot meristem formation in Arabidopsis.Plant Cell 15, 1563–1577

12 Takada, S. et al. (2001) The CUP-SHAPED COTYLEDON1 gene ofArabidopsis regulates shoot apical meristem formation. Development128, 1127–1135

13 Long, J.A. et al. (1996) A member of the KNOTTED class ofhomeodomain proteins encoded by the STM gene of Arabidopsis.Nature 379, 66–69

14 Kerstetter, R.A. et al. (1997) Loss-of-function mutations in the maizehomeobox gene, knotted1, are defective in shoot meristemmaintenance. Development 124, 3045–3054

15 Kirch, T. et al. (2003) The DORNROSCHEN/ENHANCER OFSHOOT REGENERATION1 gene of Arabidopsis acts in the controlof meristem cell fate and lateral organ development. Plant Cell 15,694–705

16 Chandler, J.W. et al. (2007) The AP2 transcription factorsDORNROSCHEN and DORNROSCHEN-LIKE redundantly controlArabidopsis embryo patterning via interaction with PHAVOLUTA.Development 134, 1653–1662

17 Nardmann, J. and Werr, W. (2007) The evolution of plant regulatorynetworks: what Arabidopsis cannot say for itself. Curr. Opin. PlantBiol. 10, 653–659

18 Nardmann, J. et al. (2007) WOX gene phylogeny in Poaceae: acomparative approach addressing leaf and embryo development.Mol. Biol. Evol. 24, 2474–2484

19 Zimmermann, R. and Werr, W. (2005) Pattern formation in themonocot embryo as revealed by NAM and CUC3 orthologues fromZea mays L. Plant Mol. Biol. 58, 669–685

83

Review Trends in Plant Science Vol.13 No.2

20 Poethig, R.S. (1997) Leaf morphogenesis in flowering plants. Plant Cell9, 1077–1087

21 Smith, G.L. et al. (1995) Expression of knotted1 marks shoot meristemformation during maize embryogenesis. Dev. Genet. 16, 344–348

22 Lucas, W.J. et al. (1995) Selective trafficking of KNOTTED1homeodomain protein and its mRNA through plasmodesmata.Science 270, 1980–1983

23 Zimmermann, R. and Werr, W. (2007) Transcription of the putativemaize orthologue of the Arabidopsis DORNROSCHEN gene marksearly asymmetry in the proembryo and during leaf initiation in theshoot apical meristem. Gene Expr. Patterns 7, 158–164

24 Liu, C. et al. (1993) Auxin polar transport is essential for theestablishment of bilateral symmetry during early plantembryogenesis. Plant Cell 5, 621–630

25 Teale, W.D. et al. (2006) Auxin in action: signalling, transport and thecontrol of plant growth and development. Nat. Rev. Mol. Cell Biol. 7,847–859

26 De Smet, I. and Jurgens, G. (2007) Patterning the axis in plants – auxinin control. Curr. Opin. Genet. Dev. 17, 337–343

27 Berleth, T. et al. (2007) Towards the systems biology of auxin-transport-mediated patterning. Trends Plant Sci. 12, 151–159

28 Boutte, Y. et al. (2007) Mechanisms of auxin-dependent cell and tissuepolarity. Curr. Opin. Plant Biol. 10, 616–623

29 Friml, J. et al. (2003) Efflux-dependent auxin gradients establish theapical-basal axis of Arabidopsis. Nature 426, 147–153

30 Friml, J. et al. (2006) Apical-basal polarity: why plants cells don’t standon their heads. Trends Plant Sci. 11, 12–14

31 Weijers, D. et al. (2005) Maintenance of embryonic auxin distributionfor apical-basal patterning by PIN-FORMED-dependent auxintransport in Arabidopsis. Plant Cell 17, 2517–2526

32 Jenik, P.D. and Barton, M.K. (2005) Surge and destroy: the role ofauxin in plant embryogenesis. Development 132, 3577–3585

33 Cheng, Y. et al. (2007) Auxin synthesized by the YUCCA flavinmonooxygeneases is essential for embryogenesis and leaf formationin Arabidopsis. Plant Cell 19, 2430–2439

34 Furutani, M. et al. (2004) PIN-FORMED1 and PINOID regulateboundary formation and cotyledon development in Arabidopsisembryogenesis. Development 131, 5021–5030

35 Treml, B.S. et al. (2005) The gene ENHANCER OF PINOID controlscotyledon development in the Arabidopsis embryo. Development 132,4063–4074

36 Carraro, N. et al. (2006) ZmPIN1a and ZmPIN1b encode two novelputative candidates for polar auxin transport and plant architecturedetermination of maize. Plant Physiol. 142, 254–264

37 Prigge, M.J. et al. (2005) Class III homeodomain-leucine zipper genefamily members have overlapping, antagonistic and distinct roles inArabidopsis development. Plant Cell 17, 61–76

38 McConnell, J.R. et al. (2001) Role of PHABULOSA and PHAVOLUTAin determining radial patterning in shoots. Nature 411, 709–713

Free journals for dev

The WHO and six medical journal publishers hav

Research Initiative, which enables nearly 70 of the

to biomedical literature

The science publishers, Blackwell, Elsevier, Harc

International Health and Science, Springer-Verlag a

and the British Medical Journal in 2001. Initially, m

free or at significantly reduced prices to universit

institutions in developing countries. In 2002, 22 ad

journals are now available. Currently more than 7

Gro Harlem Brundtland, the former director-gen

‘‘perhaps the biggest step ever taken towards red

and poor co

For more information, vis

84

39 Bao, N. et al. (2004) MicroRNA binding sites in Arabidopsis class IIIHD-ZIP mRNAs are required for methylation of the templatechromosome. Dev. Cell 7, 653–662

40 Laufs, P. et al. (2004) MicroRNA regulation of the CUC genes isrequired for boundary size control in Arabidopsis meristems.Development 131, 4311–4322

41 Juarez,M.T. et al. (2004)microRNA-mediated repression of rolled leaf1specifies maize leaf polarity. Nature 428, 84–88

42 Kwon, C.S. et al. (2006) A role for chromatin remodelling in regulationof CUC gene expression in the Arabidopsis cotyledon boundary.Development 133, 3223–3230

43 Koyama, T. et al. (2007) TCP transcription factors control themorphology of shoot lateral organs via negative regulation of theexpression of boundary-specific genes in Arabidopsis. Plant Cell 19,473–484

44 Willmann,M.R. and Poethig, S.R. (2007) Conservation and evolution ofmiRNA regulatory programs in plant development. Curr. Opin. PlantBiol. 10, 503–511

45 Kidner, C. and Timmermans,M. (2006) In situ hybridization as a tool tostudy the role of microRNAs in plant development. Methods Mol. Biol.342, 159–179

46 Nardmann, J. and Werr, W. (2006) The shoot stem cell niche inangiosperms: expression patterns of WUS orthologues in rice andmaize imply major modifications in the course of mono- and dicotevolution. Mol. Biol. Evol. 23, 2492–2504

47 Matsumoto, N. and Okada, K. (2001) A homeobox gene, PRESSEDFLOWER, regulates lateral axis-dependent development ofArabidopsis thaliana. Genes Dev. 15, 3355–3364

48 Nardmann, J. et al. (2004) Themaize duplicate genes narrow sheath1and narrow sheath2 encode a conserved homeobox gene function in alateral domain of shoot apical meristems. Development 131, 2827–2839

49 Bowman, J.L. et al. (2007) Green genes-comparative genomics of thegreen branch of life. Cell 129, 229–234

50 Wardlaw, C.W. (1955) Embryogenesis in plants. Methuen & Co. LTD,(London)

51 Cook, M.T. (1903) The development of the embryo sac and embryo ofClaytonia Virginica. Ohio Naturalist 3, 349–353

52 Casson, S. et al. (2005) Laser capturemicrodissection for the analysis ofgene expression during embryogenesis of Arabidopsis. Plant J. 42,111–112

53 Spencer, M.W. et al. (2007) Transcriptional profiling of the Arabidopsisembryo. Plant Physiol. 143, 924–940

54 Sauer, M. and Friml, J. (2004) In vitro culture of Arabidopsis embryoswithin their ovules. Plant J. 40, 835–843

55 Heisler, M.G. et al. (2005) Patterns of auxin transport and geneexpression during primordium development revealed by liveimaging of the Arabidopsis inflorescence meristem. Curr. Biol. 15,1899–1911

eloping countries

e launched the Health InterNetwork Access to

world’s poorest countries to gain free access

through the internet.

ourt Worldwide STM group, Wolters Kluwer

nd John Wiley, were approached by the WHO

ore than 1500 journals were made available for

ies, medical schools, and research and public

ditional publishers joined, and more than 2000

0 publishers are participating in the program.

eral of the WHO, said that this initiative was

ucing the health information gap between rich

untries’’.

it www.who.int/hinari