Presentation on theme: "Lecture 7 Outline Chimeras II History of “Sports” The Chimera Concept"— Presentation transcript:
1 Lecture 7 Outline Chimeras II History of “Sports” The Chimera Concept Sectorial ChimerasDefinitionsGreen-white-green (GWG) periclinal chimeras
2 History and Application of Grafting Plant CollectorsDarwin’s travelsWinkler’s Chimera ConceptTypes of GraftsModern Applications
3 Grafting occurs naturally Chloroplast genomes can be transferred from one tree to the other across the graft junctionFigure 1A from Stegemann et al., 2012, PNAS 109:2434
4 HistoryExplorers, like Charles Darwin, traveled to new and exotic places and collected ‘treasures’in the form of novel species of plants and animalsNew varieties of plants were highly prized and breeders were therefore motivated to generate new and improved stocksby making interspecific crossesby vegetative propagation (or the generation of a “sport”)
5 HistoryA Sport was defined as, “A spontaneous change or mutation in a part of a plant that created a feature not previously known within the species, their sexual hybrids and descendants”.Sports were notoriously difficult to maintain and would often be unstableThe first reproducible and stable variant was described in 1674 and was called the ‘Bizzarria orange’
6 Pietro Nati ( ), who was the director of the Pisa Botanical Gardens, published one of the first descriptions of a graft chimera known as “La Bizzarria”Resulted from grafting a sour orange scion to a citron stock. A bud arose at the graft junction with a unique mix of parental characteristics.
7 Historical paradox: What was the nature of graft hybrids? Were the resulting hybrid branches the result of sexual hybridization or a union of vegetative tissue (somatic segregation)?The argument against sexual hybridization was the you could not produce such plants by conventional breeding methods.
8 Winkler 1907Did experiments to test the nature of graft hybrids and, using tomato and nightshade plants, reproduced results akin to those seen previouslyGot shoots that were all tomato, all nightshade or 1/2 nightshade and 1/2 tomatoWinkler was responsible for coining the term “chimera” (a fire breathing monster from Greek mythology that had the foreparts of a lion, torso of a goat and the hindquarters of a dragon)
9 ConventionPericlinal chimeras are described according to the relative contribution of graft partners, starting with the L1If plant A is grafted to plant B and a periclinal chimera is produced, the layer contributions are listed beginning with the L1, then the L2 and finally the L3ABA: L1 is A, L2 is B, L3 is ABBA: L1 is B, L2 is B, L3 is AL1L2L3SAM
10 Vegetative Propagation Plants can be maintained by taking cuttings and rooting themBy grafting a cutting onto a stockMaintaining them in tissue cultureGrafting is very important toThe horticultural industry (roses)For maintaining grape varieties for wine production (root stocks resistant to pathogens)The fruit industry (apples)
11 Periclinal and Sectorial Two types of chimerasPericlinal and SectorialAt the SAMWithin Organs
12 Sectorial ChimerasPlants with sectorial meristems show sectors that run vertically down the shootMeristem is divisible into circumferential lineages and therefore there must be a small number of initial cells at the top of the meristem (subset of central zone)Sectors are not usually stable (do not persist for the lifetime of the plant)
13 Sectorial ChimerasPosition of an initial does not restrict the fate of the initial’s descendantsThe position of a marked band is independent of leaf initiation and may or may not encompass a whole leaf or even part of a leaf
14 Within organs reveals division patterns and clonal relationships Sectorial ChimerasWithin organs reveals division patterns and clonal relationshipsDaughter cells arising from one marked cell
15 VariegationSpontaneous sectors can arise from the disruption of pigment biosynthetic genes caused by transposon insertion and excision.
16 Definitions Genotype: The DNA constitution the genetic constitution of a cell, an organism, or an individual (i.e. the specific allele makeup of the individual) (from Wikipedia)b) the actual alleles present in an individualPhenotype: Visible consequencesa) an observable characteristicb) any observable characteristic or trait of an organism (from Wikipedia)
17 Gene expression patterns In theory, every nucleus in a multicellular organism has the same genetic information but expression patterns are different. Genes are turned on and off by multiple factors (temporal/spatial).A direct relationship: gene is on, function is on or gene is off and function is off
18 Factors affecting gene expression Location, location, locationDevelopmental RegulationA gene may be off if it resides in a cell in the L1 but not in the L2 (where you live matters!)Activation due to a change in fateThrough cell division, an L1 daughter cell may end up in an L2 neighbourhood and hence different genes become activated and inactivated (turned on/off)
19 Green-white-green chimeras L1=green, L2=albino, L3=greenConventionally written as GWGWhat does that mean?The L1 cells are genetically green but visibly (phenotypically) white because chloroplasts do not mature (suppressed)The L2 cells are genetically albino or white (genes do not function: they have mutations) and phenotypically whiteThe L3 cells are genetically and phenotypically green (green genes are functional)
20 Green-white-green chimeras The L2 meristematic layer is albinoFrom Leyser and Day, Mechanisms in Plant Development Blackwell Publishing,Chapter 3.
21 L1=green, L2=albino (white), L3=green Genotype of L1, L2 and L3L1=green, L2=albino (white), L3=greenPigments are cell autonomous markers because they are contained within a plastidLeaf in cross-section
22 Phenotype of L1, L2 and L3 L1=white (gene expression suppressed) L2=albino (genes have mutations)L3=green (genes are expressed)
23 Layer displacementsL1 cells that divide periclinally (rare) will give rise to L2 daughter cells that can make chloroplast pigment while L2 cells cannot, even if displaced into the L3. L2 cells invading the L3 occurs more commonly.L1L2L3 layer
24 GWG Chimeras Demonstrate that…. L1 layer is very stable and rarely invades the subjacent L2 layerL2 cells typically remain in the subepidermal layer in most of the leaf but near leaf margins are more likely to become displaced into the L3If L2 cells invade, there is a compensatory reduction in cell divisions in the L3 so that the number of cell layers in the leaf remains constantHence one sees this fairly common pattern of variegation in garden and house plants (white margins and green centre)
25 Sectorial Chimeras are Rare GWG to WWG, GWW or GGG sectors can occur but they tend not to be stableGWW sector would give rise to a white sector on the stem and would run down the vertical axis of the shoot (leaf and stem tissues would be white)GGG sector would produce a solid green band with no variegationSeeds produced in any of these sectors would have the genotype of the L2 only
26 What have we learned so far? Meristem layers are relatively stable (L1>L2>L3).Proteins and RNAs can move between layers and there seems to be directionality.Directionality is determined on a case-by-case basis.There are tissue-specific constraints.SAM
27 Some additional questions Who is in charge? For example, is the coordination of meristem activity due to the response of L2/L3 tissues to signals from the L1?Is there lateral movement of signals within a layer?Are there communication rules and if so, what are the rules?L1L2L3SAM