Floral Transition PDF

Floral transition ================= Table of Contents {#table-of-contents.TOCHeading} ================= [Floral transition 1](#floral-transition) [Flower organs of a dicotyledonous plant 2](#flower-organs-of-a-dicotyledonous-plant) [Meristematic tissues 2](#meristematic-tissues) [Meristematic t...

Floral transition ================= Table of Contents {#table-of-contents.TOCHeading} ================= [Floral transition 1](#floral-transition) [Flower organs of a dicotyledonous plant 2](#flower-organs-of-a-dicotyledonous-plant) [Meristematic tissues 2](#meristematic-tissues) [Meristematic tissues develop during embryo formation 2](#meristematic-tissues-develop-during-embryo-formation) [Vegetative meristems 2](#vegetative-meristems) [Primary shoot growth (SAM) 2](#primary-shoot-growth-sam) [Flowering growth 2](#flowering-growth) [Steps: vegetative growth to reproductive growth (flowering) 3](#steps-vegetative-growth-to-reproductive-growth-flowering) [Step 1: switch from vegetative to reproductive growth (floral pathways) 3](#step-1-switch-from-vegetative-to-reproductive-growth-floral-pathways) [Competence and Determination 3](#competence-and-determination) [Pathways that enable or promote the floral transition in Arabidopsis 3](#pathways-that-enable-or-promote-the-floral-transition-in-arabidopsis) [Photoperiodism (CO) 4](#photoperiodism-co) [Phytochromes: photoreceptor in photoperiodism 4](#phytochromes-photoreceptor-in-photoperiodism) [CO and FT 4](#co-and-ft) [Autonomous gene pathway (FLC) 5](#autonomous-gene-pathway-flc) [Temperature (cold; FRI) 5](#temperature-cold-fri) [Vernalization (FLC) 5](#vernalization-flc) [Summary: positive and negative regulators 5](#summary-positive-and-negative-regulators) [Step 2: integration of signals from flowering time pathways 6](#step-2-integration-of-signals-from-flowering-time-pathways) [LFY 6](#lfy) Flower organs of a dicotyledonous plant --------------------------------------- - Flowers: structures containing reproductive organs - Function in mediating the union of male and female gametes to produce seeds - Flower organ arrangement is used for classification of flowering plants - After fertilization, portions of the flower develop into a fruit containing the seeds - Sometimes parts of flowers are fused together, e.g. fused carpels **Petals**: coloured leaves (not always coloured); functions in attracting pollinators and assisting pollination **Sepals**: green outer leaves; functions in protection **Stamens**: male reproductive parts, composed of an anther (produces pollen) and filament **Carpels/pistils**: female reproductive parts; stigma (receives pollen), style (guides pollen to ovary), and ovary containing ovules Meristematic tissues -------------------- - Plant growth occurs in meristems, regions of cell division of undifferentiated meristem cells, producing differentiated cells - New plant organs arise from meristems ### Meristematic tissues develop during embryo formation - After the first zygotic division, one cell becomes the embryo, the other the suspensor (links embryo to nutrient tissue of seed) - Torpedo embryonic development stage: - **Cotyledons**: embryonic leaves (dicotyledons have 2) - **Hypocotyl**: part of the stem of embryo - **Shoot meristem**: found between the cotyledons - **Root meristem**: found at root - Thus, most somatic tissues are already present when the embryo is formed - Embryogenesis only forms the root and **shoot apical meristems** (primary meristems) ### Vegetative meristems - **Shoot apical meristem:** produces leaves and more stem (axial/**primary** stem) - **Root apical meristem**: forms roots, and is protected by the root tip as it grows through soil - Vegetative growth is **indeterminant**: - Division of an undifferentiated meristem cell produces one daughter cell that remains undifferentiated, while the other becomes a specialised differentiated cell - Shoot cell in SAM - Root cell in the root apical meristem - These two primary meristems are developed during embryogenesis #### Primary shoot growth (SAM) - **Shoot apical meristem**: dome-structure at the tip of the stem which produces (axial) stem and leaves - Dividing, undifferentiated meristem cells are located in the CZ (very tip) - Below the top (RZ) are differentiated stem (shoot) cells, which the meristem cells gave rise to - The PZ is the regions of the first cell divisions that lead to the formation of leaf primordia - As SAM produces/elongates the stem, it produces internodes between leaf-bearing nodes - At the node, secondary meristems (axillary bud meristems) occur at leaf axils (modified SAMs) - Secondary meristems form lateral branches #### Flowering growth - After the transition from vegetative to floral growth, the apical shoot meristem produces a flower - Thus, a flower is modified stem - Also, the PZ of the shoot meristem give rise to flower primordia instead of leaf primordia - The flower primordia produces all 4 organs of the one flower - The first change seen is sepal primordia (4) instead of leaf primordia (2) - The flower organs are produced from the outer to inner whorls -- sepals first, then sepals, stamens, and carpals - **Thus, the first visual sign of reproductive growth is the formation of sepals (instead of leaf primordia)** Steps: vegetative growth to reproductive growth (flowering) ----------------------------------------------------------- 1. Switch from vegetative to reproductive growth: - Meristem cells must become *competent*, able to respond to floral induction - **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 Studying the regulation of flowering - Use a model plant (e.g. Arabidopsis) - Design an experiment: mutagenize some plants in conditions which promote flowering - Identify mutants: those which deviate in flowering response to promoting conditions (the mutated genes are involved in flower regulation) - Mutant analysis: genetic, clone genes, investigate proteins (mutated genes which changed flowering time are involved in flower regulation) Step 1: switch from vegetative to reproductive growth (floral pathways) ----------------------------------------------------------------------- ### Competence and Determination - Meristem cells in shoot meristems go through two stages as they change from the vegetative to reproductive growth phases - Competent stage: - A meristem is competent if it is able to flower when given the appropriate developmental signals (environmental and endogenous) -- that is, floral induction - **CO, FT, FLC, and FT** regulate the ability to respond to floral stimuli (via **LFY**), thus regulate competence - Determined stage: - After floral induction of competent meristem cells, changing the cell fate of meristem cells results in a determined meristem, which follow the same development program even after removal from its normal position in the plant - The determined meristem cells undergo morphogenesis to produce flower organs (**expressed** stage) - May require additional signals (e.g. hormones) - **WUS, LFY, and CLV** confer determination ### Pathways that enable or promote the floral transition in Arabidopsis - **Enabling pathway**: - Pathways that inhibit repressors governing meristem competence (**FLC** and **FRI**), which inhibit **LFY** - FLC inhibits flowering and instead promotes *vegetative* growth - Include age (autonomous pathway) and vernalization, both which **inhibit FLC** - **Promoting pathway**: - Pathways which promote flowering, by activating **FT** or **LFY** - The enabling pathways regulate the ability of the meristem to respond to floral promotive signals from different environmental and endogenous cues (induce **competence** of meristem cells) \*\*Until vernalisation or the autonomous pathway, FLC is acetylated (FLC expressed), thus suppressing flowering ### Photoperiodism (CO) - The ability to detect day length makes it possible for flowering to occur at a particular time of year, thus allowing for a seasonal response (summer has longer days, winter has shorter days) - Plants monitor day length by measuring the length of the night -- night breaks can cancel the effect of the dark - Short day plants: - Requires **long nights**: flower the duration of the night period exceeds a critical period - Light interruption of night: **prevents** flowering - Long day plants: - Requires **short nights**: flower when duration of night period is shorter than a certain critical period - Light interruption of night: **promotes** flowering - Day-neutral plants: insensitive to day length - **Arabidopsis**: a facultative LDP, flowers early in LDs (**summer**), and later in SDs via the autonomous pathway #### Phytochromes: photoreceptor in photoperiodism - *Night breaks* with lights of different wavelengths (qualities) indicate the involvement of phytochromes in photoperiodism - **Short day plants**: red light treatment during a longer dark period inhibits flowering; reversed by far-red light (promotes) - **Long day plants**: red light treatment during a longer dark period promotes flowering; reversed by far-red light (inhibits) - A phytochrome consists of two proteins, each with two domains: - Pigment chromophore domain: photoreceptor domain (light sensing) - Protein kinase domain: triggers cellular responses - **Sunlight**: more light in the **red** range, resulting in a higher ratio of **Pfr**; opposite for evening light/shade - **Pfr** (active form) signals dark period interruptions: promotes flowering in LDPs and inhibits flowering in SDPs - Pfr reverts slowly back to Pr in darkness or faster by exposure to far-red light #### CO and FT - The circadian clock regulates rhythmic developmental/physiological processes in plants, including flowering time - Phytochromes entrain the clock, as they cycle through active/inactive phases over 24-hour periods depending on light - CO mRNA (gene expression) is regulated under the circadian clock, thus expressed at the same time every day - However, CO protein levels are determined by light signalling (sunlight results in Pfr (active) which results in CO stabilization) - **CO mRNA** (gene expression): peaks **after 12h (afternoon)**, independent of day length - Long day conditions: - The peak in CO mRNA expression is accompanied by an increase in CO protein levels - In LDs, the peak of CO expression coincides with the light period, which stabilises CO and allows it to accumulate - Short day conditions: - CO protein levels remain low throughout the entire 24-hour period, despite expression of CO mRNA at h12+ - The peak in CO mRNA expression coincides with a dark period, as days are shorter - Thus, CO is not stabilized and is instead degraded as it is synthesised, thus does not accumulate - **Ensures CO only accumulates to levels high enough to trigger the flowering pathway when days are sufficiently long** - CO is a transcription regulator that stimulates FT expression, thus promotes flowering Light signalling circadian oscillator CO is expressed in long day conditions, CO is stable activates FT flowering in short day conditions, CO is unstable no flowering ### Autonomous gene pathway (FLC) - **Autonomous pathway**: flowering occurs at a certain age, does not require environmental cues - A group of genes, activated at a certain developmental age, inactivate FLC (FT repression lifted flowering) - For example, under long day conditions (post-vernalization), FLC is inactivated (and CO is expressed), thus flowering is promoted - In short days, FLC is active, thus flowering is repressed - But, the autonomous pathway inactivates FLC independently of light, enabling flowering - Flowering occurs later than in LD conditions, as CO is not activating FT - Thus, FLC no longer inhibits FT ( flowering) - **NOTE**: in the cold, FRI activity overrides the autonomous pathway, so flowering is inhibits even if the flowering age is reached ### Temperature (cold; FRI) - Warmer ambient temperatures promote flowering. However, short cold temperatures prevent flowering - Temperatures below freezing, at which metabolic activity is suppressed are not effective for vernalization - Cold periods repress flowering as a safety mechanism, as flowers are vulnerable to the cold and can't be safely produced - The cold activates FRI protein, which increases FLC expression - FLC then inhibits FT, thus flowering is repressed - FRI action overrides both the autonomous and long day pathways - But, vernalization overrides FRI - Thus, even in conditions promoting flowering, flowering is still repressed in the cold as FLC expression is maintained by FRI - Thus, this pathway can be regarded as the **reversal of competence** ### Vernalization (FLC) - **Vernalization**: process whereby *prolonged* exposures to low temperatures enables flowering if other environmental clues are present (long days) - Thus, vernalisation results in **competence** to flower - At the start of winter, FLC expression is high (flowering repressed) as a result of FRI activation by the cold - However, a *prolonged* cold period indicates winter, and silences FLC via **epigenetic mechanisms** - Once this occurs, the silencing of FLC is maintained (epigenetic changes are **stable**) - **Vernalization overrides FRI function** - Epigenetic histone acetylation results in chromatin compaction, thus FLC is not accessible to FRI for activation - The silencing of FLC increases with the cold period duration, ensuring vernalization (silencing of LFC to levels which permit flowering) only occurs after a duration of cold sufficient to indicate that the winter season has passed - At the end of winter, FT is no longer inhibited by FLC, and long days which occur in spring/summer activate FT via CO - Occurs in plants from temperate climates which experience cold winters - Ensures flowering in spring/summer, rather than autumn - Allows plants to survive winter vegetatively, as flowers are sensitive to the cold - Although inactivation of FLC alone promotes flowering, flowering in vernalization-requiring plants must be accompanied with another environmental signal to activate FT, which ensures that flowering occurs in spring rather than in longer winters ### Summary: positive and negative regulators Step 2: integration of signals from flowering time pathways ----------------------------------------------------------- - Several pathways promote flowering time - Light dependent pathway - Autonomous pathway - Temperature pathway - All of these pathways converge to a floral integration pathway - **LFY**: a central integrator of the floral integration pathway, occurring at the end of the pathway - LFY activates floral meristem identity genes, which start the process of making a flower - These genes then activate floral organ identity genes, resulting in floral morphogenesis ### LFY - All flower pathways regulate the ***timing*** of flowering, and integrate are integrated through LFY - **Thus, FLC expression determines the onset (timing) of flowering** - Further, LFY activate floral meristem and organ identity genes - **Thus, LFY functions in floral meristem identity, and flower organ development** LFY mutants -- regulation of spatial distribution and flower identity - The **onset of flowering** is determined by the amount of LFY expression (also spatial distribution of flowering) - Wild type Arabidopsis: - Have rosette leaves at the base of the plant, and smaller cauline leaves up the stem - Have white flowers - LEAFY mutants: - **Grow leaf-like flowers** instead of the normal white flowers -- flowers fail to develop properly - Have normal rosette and cauline leaves Conclusion: - Flowers went through the switch from vegetative to reproductive growth via the flowering pathways (activated FT/inhibited FLC) -- thus flowering was initiated - However, LFY could not activate the floral **meristem** **identity genes**, and thus the **organ** **identity genes** - Thus, there was "floral growth" but with a **vegetative appearance** - Thus, organs did not grow properly, both in **colour** and **morphology** (leaf-like) -- no patterning response by ABC genes **Onset of flowering (timing)** - Transgenic plants: LFY promoter and GUS reporter gene - **LD conditions**: LFY is expressed very fast -- high expression - **SD conditions**: LFY promotor is activated slower -- takes longer to express the same amount of LFY - **SD and GA hormone**: LFY expressed faster than SD conditions alone (but, still slower than those in LD) **Conclusion: expression of LFY correlates the onset timing of flowering** **Competence (regulating onset of flowering)** - Transgenic plants: different LFY copy numbers - In plants with very high LFY expression (35S promotor), competence to flower occurs within a very short amount of time - Then, with normal promotors, the amount of time taken to reach the competence stage of flowering decreased as copy numbers increased **Conclusion**: expression of LFY correlates with the time taken to reach competence (high LFY = competent earlier)