Gibberellin Signaling in the Endodermis Controls Arabidopsis Root Meristem Size

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Gibberellin Signaling in the Endodermis Controls Arabidopsis Root Meristem Size
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  Current Biology  19 , 1194–1199, July 28, 2009 ª 2009 Elsevier Ltd All rights reserved DOI 10.1016/j.cub.2009.06.023 ReportGibberellin Signaling in the EndodermisControls  Arabidopsis   Root Meristem Size SusanaUbeda-Toma´ s, 1,9, *Ferna´ nFederici, 2,9 IldaCasimiro, 3 Gerrit T.S. Beemster, 4,5,6 Rishikesh Bhalerao, 7 Ranjan Swarup, 1 Peter Doerner, 8 Jim Haseloff, 2 and Malcolm J. Bennett 1, * 1 CentreforPlantIntegrativeBiology,UniversityofNottingham,Nottingham LE12 5RD, UK  2 Department of Plant Sciences, University of Cambridge,Cambridge CB2 3EA, UK  3 Universidad de Extremadura, Facultad de Ciencias, Badajoz06071, Spain 4 Department of Plant Systems Biology, Flanders Institute for Biotechnology (VIB), 9052 Gent, Belgium 5 Department of Biology, University of Antwerp, 2020 Antwerp,Belgium 6 Department of Plant Biotechnology and Genetics, GentUniversity, 9052 Gent, Belgium 7 Umea˚ Plant Science Centre, Department of Forest Geneticsand Plant Physiology, SLU, SE-901 83 Umea˚, Sweden 8 Institute for Molecular Plant Sciences, School of BiologicalSciences, University of Edinburgh, Edinburgh EH9 3JH, UK  SummaryPlant growth is driven by cell proliferation and elongation[1]. The hormone gibberellin (GA) regulates  Arabidopsis  root growth [2–5] by controlling cell elongation [6], but it is currently unknown whether GA also controls root cell prolif-eration. Here we show that GA biosynthetic mutants areunable to increase their cell production rate and meristemsize after germination. GA signals the degradation of theDELLA growth repressor proteins [7–12] GAI and RGA, promoting root cell production. Targeting the expressionof gai (a non-GA-degradable mutant form of GAI) in therootmeristemdisruptscellproliferation.Moreover,express-ing gai in dividing endodermal cells was sufficient to block root meristem enlargement. We report a novel function forGA regulating cell proliferation where this signal acts byremoving DELLA in a subset of, rather than all, meristemcells. We suggest that the GA-regulated rate of expansionof dividing endodermal cells dictates the equivalent rate inotherroottissues.Cellsmustdoubleinsizepriortodividingbutcannotdosoindependently,becausetheyarephysicallyrestrained by adjacent tissues with which they share cellwalls. Our study highlights the importance of probing regu-latory mechanisms linking molecular- and cellular-scaleprocesses with tissue and organ growth responses.Results and Discussion  A germinating seedling must obtain anchorage, water, andnutrients after emerging from its seed coat. Vigorous rootgrowth is essential if the seedling is to rapidly secure theseresources. Root length is determined by the number of dividingcellsandtheirfinalcellsize[1].Rootcellsfirstundergorepeated rounds of division in the root proximal meristem andthensubsequentlyexperiencerapidcellexpansionintheelon-gation-differentiation zone (EDZ; Figure 1 A). In order to maxi-mize root growth after germination, seedlings could increaseeither root cell production rate or final cell size or both.In the model plant  Arabidopsis thaliana , root growth raterapidlyincreasesafterseedgermination( Figure1B;FigureS1 A  available online). We initially investigated the cellular basis for the increased root growth, measuring mature cell length, cellproduction in the meristem, and root meristem size. Maturecell size was essentially constant, whereas the rate of rootcell production increased proportional to the root growthrate; wild-type roots exhibit a doubling in cell production rateover a 4 day period after germination ( Figure 1C; Figures S1D and S1F). Increased cell production can be due to anincreasing number of cells in the meristem because extensivevariations in cell division rates are relatively rare [13]. Indeed,our measurements indicate that root meristem size doublesduring the first days after germination (DAG), then plateausby4–6DAG,dependingonthe  Arabidopsis accession( FiguresS1D–S1F).Similarly, cortical cellnumbers in theroot meristemreached a constant number of between 40 and 60 cells,depending on the  Arabidopsis  accession ( Figures S1E andS1G). Hence, the acceleration in  Arabidopsis  root growth iscorrelated with increasing root meristem size and numbersof meristematic cells ( Figure 1B; Figure S1 ). The plant hormone gibberellin (GA) represents an importantregulator of   Arabidopsis  root growth [2–5]. GA has recently been reported to regulate root cell elongation [6]. However, itis currently unclear whether GA also controls root cell produc-tion. To investigate this possibility, we used pharmacologicaland genetic approaches to test the role of GA in regulation of root meristem size. Reduction of endogenous GA levels bytreating wild-type seedlings with Paclobutrazol (PAC, aninhibitor of GA biosynthesis [14] ) results in a reduced rootgrowth rate ( Figure S1 A). Detailed microscopy measurementsrevealed that PAC treatment caused a reduction in root meri-stem size ( Figures S1D–S1G) and mature cell length ( Figures S1BandS1C).Subsequently,rootsofGAbiosyntheticmutants  ga1-3  [15] and  ga3ox1/ga3ox2  [16] were analyzed. The  ga1-3 mutant failed to increase cell production rate, essentiallyremaining static ( Figure 1C). Similarly, a reduction in cellproduction rate was also detected in  ga3ox1/ga3ox2  mutant( Figure S1K). Both mutants exhibited a smaller root meristemsize compared to wild-type ( Figures 1B–1D; Figure S1H). However,GA3treatmentwasabletofullyrescuerootmeristemsize in the  ga1-3 ,  ga3ox1/ga3ox2  mutants and PAC-treatedseedlings ( Figure 1B; Figures S1D–S1H). Conversely, removal of 7- to 8-day-old  ga1-3  seedlings from GA3-supplementedmediaresultedinareductioninrootmeristemsize( FigureS1L).Hence, GA levels appear to be required to promote and main-tain the increase in root growth rate through control of rootmeristem size.Toinvestigatetherelationshipbetweengibberellinsandrootcelldivision,wemonitoredhowchangesinGAlevelsaffecttheexpression of the mitotic cyclin CycB1;1 (marks G2/M-phase *Correspondence: susana.ubeda-tomas@nottingham.ac.uk (S.U-T.),malcolm.bennett@nottingham.ac.uk (M.J.B.) 9 These authors contributed equally to this work  of the cell cycle [17] ), employing transcriptional and transla-tional fusions CycB1;1::GUS [18] and CycB1;1::GFP, respec-tively. PAC treatment significantly decreased mitotic cellnumber that could be restored to a wild-type level by simulta-neously treating with PAC and GA3 ( Figures 2 A–2I; Figures S2 A–S2D). We also quantified mitotic events in roots of GA biosynthetic mutant’s  ga1-3  and  ga3ox1/ga3ox2 , employingthe newly formed cell wall marker KNOLLE [19]. Immunolocal-ization of KNOLLE revealed a reduced number of cell plates(representing cell division events) in both  ga1-3  and  ga3ox1/  ga3ox2  mutants compared with wild-type roots ( Figures 2J–2M). Our observations (which are consistent with those re-ported in the accompanying manuscript by Achard et al. [20]in this issue of   Current Biology   ) suggest that GA regulatesroot meristem size by promoting mitotic activity.GA acts by destabilizing DELLA proteins like RGA and GAIthat function as growth repressors during  Arabidopsis  seed-ling development [7–12]. This is achieved by GA first binding theGAreceptorGID1[21–23],whichtheninteractswithDELLA  proteins [3–5, 21–24], resulting in their targeted degradation via the SCF/proteosome machinery [8, 25, 26]. To investigate whetherGApromotedrootcelldivisioninaDELLA-dependentmanner, cell production rate studies were performed on theGA-deficient mutant  ga1-3  (  Ler   background), the triple mutant  gai-t6/rga-24/ga1-3  (  Ler   ), and the wild-type (  Ler   ) control [2].We observed that the reduced cell production rate in theGA-deficient  ga1-3  mutant could be restored to a wild-typelevel by mutating the two main DELLA proteins in the  Arabi-dopsis  root (RGA and GAI) in the triple mutant combination  gai-t6/rga-24/ga1-3  ( Figure 1C). Similar results were obtainedinthecaseofthetripleGAreceptormutant  gid1a,b,c inwhichits reduced meristem size could be rescued in the quadruple  rga-24/ gid1a, b, c  mutant (data not shown). Hence GA controls root cell proliferation in a DELLA-dependent manner.Root cell proliferation is first initiated within a population of stem cells surrounding the quiescent center (QC) at the rootapex [27] ( Figure 1 A). Daughter cells that remain in contact with the QC maintain an indeterminate root stem cell identity[28]. In contrast, daughter cells that lose contact with the QCwill acquire a determinate cell identity and undergo a variablenumber of mitotic divisions in the proximal meristem prior to entering the elongation zone and ultimately differentiating A B DC Figure 1. GA Regulates Root Meristem Size and Cell Production Rate(A) Bright-field microscopy image of the  Arabidopsis  root meristem. Two cortical cell files within the proximal meristem (PM) are marked in yellow, and thesame fileswithin the elongation-differentiation zone (EDZ) aremarkedin orange.The distance(indicated as d-a1 and d-a2)between the transition zone (TZ)and the quiescent center (QC, marked in red) correspond to the meristem size (  m m).(B) The GA-deficient mutant  ga1-3  (Col-0 background) showed a reduction in its root growth rate (mm/h), meristem size (  m m), and cortical cell number inmeristem from day 3 to day 6 after germination (DAG). GA treatment (GA3 1  m M) restored the mutant phenotype. Error bars represent SE (n > 15).(C)TheGA-deficientmutant  ga1-3 (Lerbackground)showedareductionincellproductionrate(rootgrowthrate/maturecelllength)thatcouldberecoveredby GA treatment (GA3 1  m M) and by mutating the main root DELLA proteins in the triple mutant  gai-t6/rga-24/ga1-3  (Ler background). Wild-type (Ler back-ground) roots were used as control (n > 20).(D) Root meristem images (bright-field microscopy) of GA-deficient mutants  ga1-3  and  ga3ox1/ga3ox2  (both in Col-0 background) showing a reduction inmeristem size with respect to control (wild-type, Col-0 background) of four DAG seedlings. White and black arrowheads indicate QC and TZ position,respectively. Scale bar represents 25  m m. Statistical significance of differences observed are displayed in Tables S1 A and S1B. Endodermal GA Response Controls Root Meristem Size 1195  [1]. To determine which population of meristematic cells GA acted upon to control root cell proliferation, we disrupted theGA response in either QC or proximal meristematic cells byexpressing a GA-insensitive mutant form of the DELLA proteinGAI (termed gai) in these tissues [6]. Expressing  gai   in alldividing root cells (RCH1 > > gai) resulted in a dramatic reduc-tioninmeristemcellnumber( Figures2N,2O,and2R),whereastargeting  gai   expression to just the QC and columella stemcells (J2341 > > gai) did not affect root meristem cell number compared to the control ( Figures 2P–2R). This result is consis-tent with the lack of an effect of PAC or GA3 treatments onseveral stem cell markers reported in the accompanyingmanuscriptbyAchardetal.[20].Hence,GAappearstocontrolroot meristem size by regulating the proliferation of proximalmeristem cells rather than affecting QC function.The  Arabidopsis  proximal root meristem is composed of lateral root cap, epidermis, cortex, endodermis, and steletissues ( Figure 1 A). GA may control the rate of cell division ina specific tissue or in all proximal meristem tissues simulta-neously. To address this question, we disrupted the GA response either in individual or combinations of root tissuesby targeting  gai   expression employing a selection of tissue or zone-specificGAL4driverlines[6].Weobservedthatexpress-ing  gai   in only a subset of root tissues caused a significantreduction in root growth rate [6] ( Figure 3 A). These included expressing  gai   in the elongation zone (EDZ; J0631 > > gai),cortex/endodermis (J0571 > > gai), and in the endodermis(Q2500 > > gai) tissues ( Figure 3 A; Figure S3 ). In contrast, expressing the wild-type form  GAI  did not affect root growthin any of the cases as described in our previous work [6].We next studied the effect of targeted  gai   expression onmeristem size in detail for each of these lines. Anatomicalmeasurements revealed that the increase in root meristemlength and cell number after germination in the wild-typecontrol was blocked in J0571 > > gai and Q2500 > > gai seed-lings ( Figures 3B and 3C). Hence, GA appears to causea doubling in root meristem size by targeting DELLA degrada-tion in a subset of root meristematic cells in the endodermis.Targeted  gai   expression in the endodermis has recently beenreported to also affect cell elongation and cell morphology inthe EDZ [6] ( Figure 3F). To rule out the possibility that these Figure 2. GA Is Required to Maintain Cell Division in the Proximal Meristem(A–D) Confocal image of radial optical sections of root meristem of trans-genic line CycB1;1::GFP.(E–H)Tangentialopticalsectionsofcorticalcellsfromz-seriesimagestacksused for mitotic cell quantification. All plants were germinated in controlmedium, transferred to the different treatments at 4 DAG, and kept for 48 hr before imaging.(A and E) Control roots at 6 DAG.(B and F) Roots treated with 10  m M GA3.(C and G) Roots treated with 10  m M paclobutrazol (PAC, GA biosynthesisinhibitor).(D and H) Roots treated concurrently with PAC and GA3.(I) Frequency of mitotic cells of roots analyzed in (A)–(H) (n = 20). 6-day-oldplants were imaged after 48 hr of treatment with (from left to right) controlsolution, GA3 10  m M, PAC 10  m M, and the combination of PAC and GA3.In order to measure the frequency of cell division within a region of activeproliferation, 38 cortex cells from the 2 nd to the 20 th position from QC intwo adjacent files of cortex cells were scored in batches of 20 roots for CycB1;1::GFP expression. Propidium iodide was used as a red counter-stain.Asterisk,PAC-treatedrootsshowedasignificantreductionofnumber of mitotic cells (7.1  6  0.80 [SEM] versus 10.1  6  0.94 [SEM] in control;Student’s t test p < 0.05; n = 20).(J–L) Maximal projections of image stacks taken through the whole root viaconfocal microscopy of 4- to 5-day-old Columbia,  ga1-3 , and  ga3ox1/  ga3ox2  mutant seedlings. Green fluorescent marks correspond to cellplates formed in dividing cells. Seedlings were fixed and immunolocaliza-tion experiments performed with anti-Knolle primary antibody and Oregongreen-coupled anti-Rabbit secondary antibody (Invitrogen).(M) The maximal projections of image stacks were used to manuallycount the green fluorescent marks, corresponding to cell plates formed individing cells. Statistical significance of differences observed are displayedin Table S1C.(NandO)RCH1-drivenexpression ofgaiin PMreducesthenumber ofmeri-stematic cells.(N) 7-day-old control root [F1 progeny from RCH1 (Utr) 3 wild-type (Col-0)].(O) 7-day-old RCH1 > > gai root [F1 progeny from RCH1 (Utr)  3  UAS::gai(Col-0)].(P and Q) J2341-driven expression of gai in the QC does not significantlyaffect the number of meristematic cells.(P)6-day-oldcontrolroot[F1progenyfromJ2341(C24) 3 wild-type(Col-0)].(Q) 6-day-old J2341 > > gai root [F1 progeny from J2341 (C24) 3 UAS::gai(Col-0)].(R) Normalized effect of GA response disruption in meristematic cells(RCH1) and stem cell niche (J2341). Asterisk indicates statistical signifi-cance in RCH1 > > gai (Student’s t test p < 4.5048E-44; n = 25).Scale bars represent 40  m m. Current Biology  Vol 19 No 141196  morphological changes in the EDZ are responsible for thedecrease in meristem size, we targeted  gai   expression inEDZ (but not meristem) tissues with the J0631 driver line( Figures S3 A and S3B). In this case, root growth was severelyreduced but no reduction in root meristem size was detectedwhenblockedGAresponsesintheEDZ(J0631>>gai,Figures3B and 3D). Hence, the morphological changes in the EDZdescribedforJ0571>>gaiandQ2500>>gairoots[6]( Figures 3D–3F; Figures S3C and S3D) were not responsible for the reduction in root meristem size in these lines ( Figures 3B–3D). To confirm that gai acted cell autonomously, we alsoexpressed a YFP-tagged gai protein (termed gai-YFP) in thedifferent root meristematic tissues, revealing the spatial spec-ificity of this approach ( Figure S4 ). Therefore, our targeted  gai  expressionresultsindicatethatGAcontrolsrootmeristemsizeby promoting endodermal cell proliferation in the proximalmeristem.In summary, this study and the accompanying paper by Achard et al. [20] provide compelling new evidence that GA regulates  Arabidopsis  root growth by promoting cell prolifera-tion. By increasing the number of root meristematic cells,a greater number of cells are produced, causing root growthto accelerate. The ability to rapidly increase root growth isparticularly important for newly germinated seedlings if theyaretorapidlysecurevitalresourcessuchasanchorage,water,and nutrients after emerging from their seed coat. In this workwe showed how GA is needed during root development after germinationtoattainandmaintainrootmeristemsize,byregu-lating cell proliferation through DELLA protein degradation.Molecular details about DELLA regulation of the cell cyclemachinery are discussed in the accompanying manuscriptbyAchardetal.[20].WereporthowGAcontrolsrootmeristemsizebytargetingDELLAdegradationinasubsetof(ratherthanall) dividing cells. The critical question this observation posesis how dividing cells in the endodermis regulate the prolifera-tion of meristematic cells in adjacent tissues? Cells mustdoubleinsizepriortodividingbutcannotdosoindependently,because they are physically restrained by adjacent tissueswith which they share cell walls ( Figure 4 ). Thus, the rate of expansion of dividing endodermal cells may dictate the equiv-alent rate in other root tissues. We have previously demon-strated that DELLA-regulated endodermal cell expansion israte limiting for other elongation zone tissues [6]. Hence, GA appears to promote root growth by increasing endodermal DEF Figure 3. GA Regulates Endodermal Cell Division Controlling  Arabidopsis  Root Meristem Size(A–D)BlockingGAresponseinendodermis(Q2500>>gai)andendodermis/cortex(J0571>>gai)reducesrootelongationrate(A)andmeristemsize(BandC) by blocking the increase in the number of root meristematic cells. This effect was not observed when blocking GA response in all elongating tissues(J0631 > > gai).(A–C) Graphs representingtime course experiment with roots from 1 to6 days after germination (DAG). Root meristem size was measured as distance (  m m)and as cortical cell number between the transition zone (TZ) and the quiescent center (QC). Error bars represent SE (n > 100 per line).(D)Primaryrootmeristemsofseedlings4DAG.TZandQCpositionsareindicatedbyblackandwhitearrowheads,respectively.Scalebarsrepresent25 m m.(E and F) Blocking GA response in roots reduces cell elongation.(E) Representative image of mature cortical cells showing a reduced length when blocked GA response.(F) Measurements of cortical cell length (  m m) within the mature zone of primary roots in 6 DAG seedlings. The zone selected to take measurements waslocated between the hypocotyl and the differentiation zone of the secondary cell wall of xylem cells. Asterisk, statistically significant differences for valuescompared with wild-type as determined by Student’s t test (p < 0.05). Statistical significance of differences observed are displayed in Tables S1D–S1F. Endodermal GA Response Controls Root Meristem Size 1197  cell expansion in both meristematic and elongation zones,thereby indirectly controlling the rates of division and expan-sion of other root tissues ( Figure 4 ) and root meristem size. Although we provide new evidence that GA regulates  Arabi-dopsis  root cell proliferation, the same signal is inhibitory toshoot apical meristem activity [29, 30]. This marked difference in GA action is likely to reflect distinct differences in (1) theorganization of the tissues and (2) composition of the regula-tory networks that operate in the root and shoot apical meri-stem to control cell proliferation [26, 29, 30]. Experimental ProceduresPlant Material and Treatments  Arabidopsis thaliana  GA mutants with different backgrounds were used asindicated in the text including  ga1-3  in both Columbia-0 (Col-0) and Lands-berg  erecta  (Ler) background,  gai-t6/rga-24  /   ga1-3  (Ler),  ga3ox1/ga3ox2 (Col-0). Transgenic line CycB1;1::GUS was described previously [18] andCycB1;1:: GFP line was constructed by replacing a BamHI-SacI fragmentexcised from pCDG [17], which removed the  uid   A gene, and replacing itwith a BamHI-SacI fragment corresponding to mGFP5. Seeds were surfacesterilized and plated on half (0.5) MS (Murashige Skoog) and agarose 1%(w/v) solidified medium. GA3 and Paclobutrazol (PAC, an inhibitor of GA biosynthesis [14] ) treatments (1  m M) were applied to the media as required.10  m M dosages were used for results presented in Figure 2. In the case of PAC treatments, seeds germinated in 0.5 MS and transferred to PACtreatment after germination (when the root penetrated the endosperm,1.5–2 days after being transfer to growth room). Seedlings grew verticallyin a growth chamber with constant conditions (24  C, 150  m mol/m 2  /s),permitting roots to grow along the surface of the agarose. Targeted Misexpression Approach To map the site of action of GAs in the  Arabidopsis  root meristem, weanalyzed the F1 progenies from crossing the line UAS::gai (in SCL3::GUS,Col-0background)withseveraldriverlines(Q2393,Q2500,J0631,J0951,andJ0571;http://www.plantsci.cam.ac.uk/Haseloff/construction/catalogFrame.html ) in C24 background as described previously [6]. The correspondent F1 progenyfrom crossing UAS::gai with C24wasused as a control for theselines. The line UAS::gai-YFP-9.4 (Col-0 background) was used to cross withthe different driver lines in localizing gai-YFP ( Figure S4 ). Root Growth To assess root growth, the root length of vertically grown seedlings wasmeasured from root tip to hypocotyl base with ImageJ 1.32j freely available( http://rsb.info.nih.gov/ij/  ). Two-tail t tests were performed with MicrosoftExcel software. We used a five point equation to calculate root elongationrates from the root length data [31]. Cell division rate within the proximalmeristem zone (PM, Figure 1 A) and cellular elongation through the elonga-tion-differentiation zone (EDZ, Figure 1 A) determines the final length of primary roots. Root Meristem Size Analysis Root meristem size was expressed as (1) the distance between the quies-cent center (QC) and the transition zone (TZ; indicating the position of thefirst elongating cortical cell), represented as d-a1 and d-a2 in Figure 1 A;(2) the number of cortical cells [32] in a file extending from the QC to theTZ. To calculate the root meristem size, measurements were performedevery day on clarified roots. In the case of the Figure S1L, measurementswere performed in confocal images of roots stained with propidium iodide(Sigma, St. Louis, MO). Image Analysis GUShistochemicalstainingwasperformedbyimmersingrootsinasolution(50 mM sodium phosphate, 1 mM EDTA, 0.5 mM potassium ferrocyanide,0.5 mM potassium ferricyanide, 0.5% Triton X-100, 0.5% dimethylforma-mide,1 mM X-Glc) for 2 hrat 37  C followed by root clarification [33]. Subse-quent imaging with Nomarski optics on a Nikon optiphot-2 microscope anda Leica DFC300 camera performed. Confocal analysis was performed asdescribed previously [6] except for  Figure 2, where images were obtained with a Leica TCS-SP confocal microscope (Leica, Milton Keynes, UK). Inthis case, roots were stained with 10  m g/ml propidium iodide (Sigma) for 15 s, rinsed, and mounted in water. EGFP was excited with the 488 nmline of an argon laser and propidium iodide was excited with the 514 nmline. Fluorescence emission was collected between 505 and 530 nm for EGFP,and606and635nmforpropidiumiodide.Thenumberofmitoticcellswas quantified by manually counting the GFP-positive cells. Immunolocalization 4- to 5-day-old seedlings were fixed, immunolocalization experiments wereperformed as described previously [34] with anti-Knolle primary antibody(1:4000dilution)andOregongreen-coupledanti-Rabbitsecondaryantibody(Invitrogen) (1:200 dilution), and visualization was by confocal microscopy.Maximal projections of image stacks taken 1  m m apart through the wholerootwereused tomanuallycountthegreen fluorescent marks,correspond-ing to cell plates formed in dividing cells, represented in Figures 2J–2M. Supplemental Data SupplementalDataincludefourfiguresandonetableandcanbefoundwiththis article online at http://www.cell.com/current-biology/supplemental/ S0960-9822(09)01296-2. Acknowledgments We thank S.G. Thomas (Rothamsted Research, UK) for   ga3ox1/ga3ox2, ga1-3, gai-t6/rga-24/ga1-3, gid1abc , and  rga-24/gid1abc  mutants, B.Scheresforthe RCH1::GAL4 line, theNottinghamArabidopsis Stock Centre(NASC) for providing selected GAL4 enhancer trap lines used in this study,andBenjaminPeretforassistanceindrawingFigure4.Weacknowledgethesupport of the Biotechnology and Biological Sciences Research Council(S.U.-T. and M.J.B.); BBSRC/EPSRC CISB programme funding (S.U.-T.and M.J.B.); Belgian Scientific policy (BELSPO contract BARN to G.T.S.B.Figure 4. Model for GA Regulating Root Meri-stem SizeEndodermal cells (yellow) in the meristem mustdouble in size prior division and this elongationis GA regulated [6], enabling adjacent tissues toelongate and therefore divide. This increase innumber of divisions will produce a bigger meri-stem (A). In the case of reduced GA levels or reduced GA response in the endodermis, cellswill elongate less and therefore adjacent tissueswill reduce theirelongation, reducing the number of division events, resulting in smaller meristems(B). Black arrows represent cell elongationprocesses in the meristem, black bar headsrepresent inhibition of cell elongation, and redlines represent cell plates corresponding to divi-sion events. The different root tissues repre-sented in the figure are stele (St), pericycle (P),endodermis (En), cortex (C), and epidermis (Ep). Current Biology  Vol 19 No 141198
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