Making a Flower - Biology Document PDF

Making a flower =============== Table of Contents {#table-of-contents.TOCHeading} ================= [Making a flower 1](#making-a-flower) [Maintaining the shoot apical meristem (SAM) size 2](#maintaining-the-shoot-apical-meristem-sam-size) [CLV proteins downregulate WUS mRNA 2](#clv-proteins-dow...

Making a flower =============== Table of Contents {#table-of-contents.TOCHeading} ================= [Making a flower 1](#making-a-flower) [Maintaining the shoot apical meristem (SAM) size 2](#maintaining-the-shoot-apical-meristem-sam-size) [CLV proteins downregulate WUS mRNA 2](#clv-proteins-downregulate-wus-mrna) [Areas of CLV and WUS mRNA expression 3](#areas-of-clv-and-wus-mrna-expression) [SAM maintenance: WUS-CLV feedback loop 3](#sam-maintenance-wus-clv-feedback-loop) [Step 3: changing the shoot apical meristem into a floral meristem (change cell fate) 3](#step-3-changing-the-shoot-apical-meristem-into-a-floral-meristem-change-cell-fate) [Step 4: floral organ development (floral organ identity genes) 4](#step-4-floral-organ-development-floral-organ-identity-genes) [ABC model: homeotic genes and whorls 4](#abc-model-homeotic-genes-and-whorls) [Single homeotic mutant phenotypes 4](#single-homeotic-mutant-phenotypes) [Double homeotic mutant phenotypes 5](#double-homeotic-mutant-phenotypes) [Triple homeotic mutant phenotypes 5](#triple-homeotic-mutant-phenotypes) [Change from vegetative to reproductive phase 6](#change-from-vegetative-to-reproductive-phase) **Steps: vegetative growth to reproductive growth (flowering)** 1. Switch from vegetative to reproductive growth: - Meristem cells must become competent (induction to a meristem results in determination) - **Regulated** by the floral pathways: FT, CO, FLC, FRI 2. Integration of signals from various flowering time pathways - LFY integrates all flower signals, and the timing of flowering depends on the degree of LFYs activation - **Regulated** by: LFY (floral integrator) 3. Changing the shoot apical meristem into a floral meristem (change cell fate) - Cell fate of meristem cells changes to a floral one via activation of floral meristem identity genes: **AG, AP, and PI** - The meristem is now determined - **Regulators of meristem fate**: WUS, CLV, LFY 4. Floral organ development - Floral organ identity genes, and their combinations, result in the 4 whorls developing into the 4 flower organs - **Regulators of floral organ identity**: AG, AP, PI, SEP ### Maintaining the shoot apical meristem (SAM) size Shoot apical meristem - Tissue in which mitotic division of undifferentiated meristem cells generates stem, leaves, and **new SAM** - Formed during embryogenesis -- already present in the embryo - Self-organising and self-renewing - SAM (vegetative growth) is indeterminate (continuous organogenesis) - Maintained by products of SAM identity genes **WUSCHEL (WUS) gene** - WUS is a homeodomain transcriptional regulator which acts as a promoter of SAM formation - Regulates meristem indeterminacy (*undifferentiated* identity of meristem cells) - WUS mutants: no SAM present (WT has SAM present between two leaves) - Meristem cells are used up completely after forming leaves/stem **CLAVATA (CLA) genes** - CLAVATA genes regulate the size of the undifferentiated population of meristem cells in SAM by negatively regulating WUS - Thus, the **limit** the **size** of SAM - Overexpression of CLV3 abolishes the shoot meristem completely (WUS expression is inhibited) - CLV mutants: mutations in each CLV genes (separately) all cause **excessive SAM growth** -- enlarged shoot apical meristem - CLV3 mutants: over-proliferation and wild growth of SAM -- tumour-like - CLV1 or CLV2 mutants: whole shoot tip (including the apical meristem) is greatly bigger - **CLV3**: located in the extracellular space (between cells) - **CLV1 and CLV2**: membrane-bound signal receptor proteins, which can be compared to EGF receptors in humans - Extracellular domain of leucine-rich repeats (LRR) - Transmembrane domain - Intracellular cytoplasmic kinase domain - CLV2 has no kinase domain - EGF receptors have a similar structure, but has extracellular cysteine rich repeats instead of LRR +-----------------------------------+-----------------------------------+ | EGF receptor (human) | CLAVATA1 | +===================================+===================================+ | -Extracellular cysteine-rich | -Extracellular leucine-rich | | repeats | repeats | | | | | -Extracellular detection of | -Extracellular detection of | | epidermal growth factor (EGF) | **CLA3** | +-----------------------------------+-----------------------------------+ | -Intracellular tyrosine kinase | -Intracellular serine/threonine | | activity | kinase activity | | | | | -Intracellular transmission of | -Intracellular transmission of | | signal | signal | +-----------------------------------+-----------------------------------+ #### CLV proteins downregulate WUS mRNA - CLV proteins limit the amount of expressed WUSCHEL mRNA (repress WUS), which limits the production of more shoot apical meristem (limits meristem cell population) - Wild type: WUS mRNA is quite limited - CLV mutants: WUS expression is enlarged in shoot meristems -- more SAM is formed - **CLV1 mutant**: WUS mRNA not limited -- extended to cells which usually express CLV1 - **CLV3 mutant**: WUS mRNA not limited -- extends to cells which usually express CLV3 #### Areas of CLV and WUS mRNA expression Meristems (SAM) are structured into zones: - **Central zone**: *undifferentiated cells* (meristem cells) - **Peripheral zone**: new *lateral organ* initiation (leaves and branches; or flowers in floral meristems) - **Rib zone**: formation of new stem/shoot (differentiated meristem cells from the CZ into cells of the stem)  WUS and CLV mRNAs: - WUS and CLV1, 2, and 3 are located in the **CZ** (area of undifferentiated cells), which is divided into 3 layers - CLV3 is *produced* at the surface (in the area where meristem cells originate), but migrates out of these cells down to CLV1/2 expressing cells - CLV1 and 2 are located beneath the CLV3 layer, and surround WUS-expressing cells - WUS is not expressed in CLV1/2 (or CLV3) expressing cells, as these receptors bind CLV3 to downregulate WUS ### SAM maintenance: WUS-CLV feedback loop Occurs in the central zone (CZ) - Remember, the CZ consists of undifferentiated meristem cells - CLV3 is produced in cells on the surface of the CZ, which is the areas in which meristem cells *originate* (divide) - An increase in the number of meristem cells thus increases CLV3 expression - After synthesis, CLV3 leaves their cells and migrates down to CLV1 and CLV2 expressing cells - CLV3 binds to the CLV1/2 receptor, triggering intracellular kinase activity and represses WUS mRNA expression - This limits WUS to a small area, as WUS is only expressed in cells which do not express CLV1/2 - WUS protein feeds back to the surface of the CZ where new meristem cells originate, and promotes SAM formation - **OVERALL**: SAM size and meristem cell number maintained Molecular mechanism of WUS regulation by CLV - CLV3: predicted extracellular protein - CLV1 and CLV2: homo and heterodimer receptors - CLV3 binds to CLV1/2, activating the kinase domain of CLV1 - This activates GTPase - GTPase activates a series of MAP kinase proteins (MAP kinase cascade) - The MAP kinase cascade phosphorylates/activate a *transcription regulator* which suppresses WUS gene expression (mRNA) Step 3: changing the shoot apical meristem into a floral meristem (change cell fate) ------------------------------------------------------------------------------------ - WUS, which promotes meristem cell formation (SAM formation), also activates flower development genes such as AG - WUS is a homeodomain transcription factor which binds to the promotor of AG - AG is only activated when both **LFY** and **WUS** bind - AG expression results in the formation of flower organs (class C gene) LFY: central integrator of all flower promoting pathways - LFY is a central integrator of all flower promoting pathways. Once activated, it interacts with WUS to activate AG - Also, AG functions in the determinate growth of flowers (antagonises WUS). Mutants: "flower within a flower" - WUS functions in meristem production, LFY carries flowering signals -- these functions combine to form the floral meristem - WUS is still needed to produce the meristem, but produces a floral meristem instead because of the interaction with LFY **The meristem is now determined** Step 4: floral organ development (floral organ identity genes) -------------------------------------------------------------- ### ABC model: homeotic genes and whorls - The floral meristem (meristem cells) gives rise to all 4 whorls, thus the *identity* of each whorl is determined by floral organ identity genes (homeotic genes) - The mRNA expression of the genes present in each whorl (and their combinations) define their structural identity - **Homeotic mutations cause organs to develop in wrong places** (homeotic mutations: replace one organ for another) - The ABC model is used to **predict** mutant flower phenotypes Whorls (outside to inside) - Whorl 1: sepals (A) - Whorl 2: petals (A + B) - Whorl 3: stamens (B + C) - Whorl 4: carpels (C) ### Single homeotic mutant phenotypes A and C class mutations: mutually exclusive - A and C expression is mutually exclusive. If A is expressed then C cant be expressed, and vice vera - Ensures that WT flowers only get A in W1/2, and C only in W3/4 - AG is a negative spatial regulator of AP1 gene expression, and vice versa - If A is missing it allows C expression in all 4 whorls, and vice versa (they get replaced by each other) B class mutations - If B is mutated, no alternative gene is expressed - Thus, W1/2 will both be sepals, W3/4 will both be carpels **Class A: Apetala 1 mutant** - A gene is missing: no sepals or petals - C is expressed instead: forms carpals and stamen - Whorls with only C (W1 and 4) become carpals; whorls with B and C (W2 and 3) become stamen Carpels, stamens, stamens, carpals **Class B: Pistillata 1 mutant** - B gene is missing: no petals or stamens - W1 and W2 only have A expression -- both sepals - W3 and W4 only have C expression -- both carpels - Thus, W1 and W4 are wild type; W2 has sepals instead of petals, W3 has carpals instead of stamens Sepals, sepals, carpels, carpels **Class C: Agamous 1 mutant** - Remember, Agamous 1 is the gene activated by LFY and WUS - AG results in formation of flower organs - C gene is missing: no stamens or carpels - A is expressed instead: forms sepals and petals - Pattern of sepal-petal-petal is repeated (second flower forms from the inside of the first) - Produces the start of a new flower in the centre whorl instead of carpals and stamens Sepals, petals, petals; sepals, petals, petals Additional A and B class genes in Arabidopsis - A class: AP1 and AP2 - Mutant AP1 produces the severe phenotype, as AP2 cant completely compensate - B class: Piscillata and Apetala 3 - Only 1 C class ### Double homeotic mutant phenotypes A and B mutants - Only C expressed; all 4 whorls are carpals B and C mutants - Only A expressed; all 4 whorls are sepals A and C Mutants - Only B is expressed in W2 and W3 - W1 and W4 have nothing - Using the ABC model to predict the phenotype is difficult, as B is always expressed in conjunction with either A or C - We can assume whorls with only B will produce something between petals and stamens, and whorls with no genes will produce leaves - As predicted, W2 and W3 show characteristics of both petals and stamens, and W1 and W4 are leaf-like  ### Triple homeotic mutant phenotypes A, B, and C mutants - Predicts all whorls will be leaf-like (no floral identity genes) - As predicted, all 4 whorls show characteristics of leaves (resembles leaves) - These are *true* leaves rather than green flower leaves (sepals), we know this because: - In Arabidopsis, true leaves have trichomes (hair-like strictures), whereas sepals do not - In Arabidopsis, trichomes are Y shaped (in other plants, they're strait) - The leaves seen in the triple mutant have trichomes, thus are true leaves - But, the leaves still take the shape of a flower ### Change from vegetative to reproductive phase - A plant in the vegetative phase must first become competent, that is, able to respond to appropriate developmental signals - Competence is regulated by: CO, FT, FLC, FRI - The competent plant must then become determined, where the meristem is committed to flower development - Occurs through induction (floral signals) whose pathways ultimately activate FLY - Requires regulators of meristem fate: WUS, CLV, LFY - The determined plant then makes a flower (become expressed) - Regulators of floral organ identity: AG, AP, PI, SEP - The plant is now in its flowering phase Mutants - LFY: grow leaf-like flowers - Higher LFY expression -- shorter time to achieve competence to flower - LD conditions -- higher LFY expression - WUS: no SAM present - CLV: excessive SAM growth -- enlarged shoot apical meristem - CLV3: over-proliferation and wild growth of SAM -- tumour-like - CLV1: whole shoot tip (including the apical meristem) is greatly bigger - In both of these, WUS is able to be expressed in usual CLV1 or CLV3 containing cells (more SAM is formed)