Shoot meristem self-organization and identitySlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Shoot meristem self-organization and identityHow can we use genetics to study shoot meristem organization?How do meristems maintain themselves?How do meristems change their character?Which cells are the stem cells?************* * ****0d 1d 3d 5dEffect of ablation of quiescent center (QC) on tissue-specific marker gene expression(from Sabatini et al., 1999, Cell 99: 463-472)The Quiescent Center is the stem cell nicheVegetative shoot apical meristem (SAM)/outgrowthArabidopsis thaliana (Mouse-ear cress, Thale cress)Small weed, related to mustard - can fit thousands of plants in a small area Short generation time: 6-8 weeks when pushed fast Diploid, self-pollinating, transformableSmall sequenced genome: About 125 Mb and 25,000 genesArabidopsis is closely related to these plantsOther model plantsFinding Arabidopsis mutantsEMS (ethylmethane sulfonate)3. Screen M2 plants for mutants having phenotypes of interest2. Allow M1 plants to self-fertilize, collect M2 seed1. Mutagenize seeds (M1)(from Long et al., 1996, Nature 379: 66-69)Wild-type shoot apex shootmeristemless apexGene expression in embryonic SAMWUSCHEL (WUS) SHOOTMERISTEMLESS (STM)(from Laux et al., 1998, Cell 95: 805-815)wuschel mutant(from Laux et al., 1998, Cell 95: 805-815)Wild typeGene expression in embryo SAMWUSCHEL (WUS) SHOOTMERISTEMLESS (STM)(from Laux et al., 1998, Cell 95: 805-815)WUS expression in adult shoot meristems(from Laux et al., 1998, Cell 95: 805-815)Vegetative shoot meristem Inflorescence and flower meristems(from J. C. Fletcher et al., Science 283, 1911 -1914 (1999)) Wild type clavata3 mutant clv1 and clv3 mutants look similarclavata mutants have enlarged meristemsExpression of CLV1 in Arabidopsis inflorescence shoot meristemClark et al., Cell 89: 575-585 (1997)CLV3 (A-D) and CLV1 (E) expression (from J. C. Fletcher et al., Science 283, 1911 -1914 (1999))CLV3 expression in a clv1 mutant meristem (from J. C. Fletcher et al., Science 283, 1911 -1914 (1999))Model for CLV3 peptide signalingSupport for model:In clv mutant, WUS expression domain expands.clv wus double mutants resemble wus single mutants. If you overexpress WUS, CLV genes are activated and new meristems form.Model for shoot apical meristem (SAM) maintenanceSarkar et al. (2007) Nature 446: 811-814WOX5 is similar in sequence to WUSCHEL.WOX5 is expressed in the root quiescent center.Similar regulation in shoot and root meristemsSarkar et al. (2007) Nature 446: 811-814Root cap stem cells differentiate in a wox5 mutant.Similar regulation in shoot and root meristemsInflorescence meristem:Makes flowers instead of leaves and branchesStem elongates moreVegetative SAMInflorescence SAMFlower meristemsLeaves, branches (lateral meristems)Sepals, petals, stamens, carpelsInduction of floweringShoot meristem identitiesVegetative SAM (indeterminate)Inflorescence SAM (indeterminate)Flower meristems (determinate)Leaves, branches (lateral meristems)Sepals, petals, stamens, carpelsInduction of floweringShoot meristem identitiesVegetative shoot apical meristem (SAM)Flower meristemsShoot apical meristem (SAM)Flower meristem FlowerFlower meristems are normally determinate(They terminate after making a defined number of organs)Arabidopsis flowerSepalStamenGynoecium(fused carpels)PetalVegetative SAM (indeterminate)Inflorescence SAM (indeterminate)Flower meristems (determinate)Leaves, branches (lateral meristems)Sepals, petals, stamens, carpelsInduction of floweringShoot meristem identities1 2 3 44 sepals4 petals6 stamens2 carpelsWhorlsArabidopsis mutants with altered flowersapetala1apetala1cauliflowerpistillataWild typeagamousVegetative SAMInflorescence SAMFlower meristems Meristem identity genes (LFY, AP1, CAL)Leaves, branches (lateral meristems)Sepals, petals, stamens, carpelsInduction of floweringShoot meristem identitiesleafy mutants:partial conversion of flowers to lateral branches and leavesleafy mutant flowers have no petals or stamensleafyWild typeLEAFY is expressed throughout the flower meristem(from Weigel et al., 1992, Cell 69: 843-859)apetala1 mutant flowers lack sepals and petals and have a new flower where a petal should beWild typeapetala1Flowers are converted intoleafy shoots in leafy apetala1 mutantsFlower meristems behave like inflorescence meristems in the apetala1 cauliflower double mutantap1 calap1 calFig. 20.41ap1 calWhy did they call this mutant cauliflower?Vegetative SAMInflorescence SAMFlower meristemsLeaves, branches (lateral meristems)Sepals, petals, stamens, carpels Organ identity genes (AP1, AP2, PI, AP3, AG SEP1, SEP2, SEP3)Induction of floweringShoot meristem
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