11/05/01BMB/BIOL/ENT Today’s Lecture - Trichome development And Switching into reproductive phase

11/05/01BMB/BIOL/ENT 4302 For more info-- Szymanski DB, Lloyd AM, Marks MD.Progress in the molecular genetic analysis of trichome initiation and morphogenesis in Arabidopsis. Trends Plant Sci May;5(5): Review. Michaels, S. D. and R. M. Amasino Memories of winter: vernalization and the competence to flower. Plant Cell & Environment 23: See--

11/05/01BMB/BIOL/ENT 4303 Plant Trichomes-- insight into cellular differentiation in plants Trichome = hair like structure that extend from the epidermis of aerial tissues Function of trichomes-- boundary layer between epidermis and environment Reduce heat and water loss Protection against herbivory and pathogens Secretion of chemicals or physically limit access

11/05/01BMB/BIOL/ENT 4304 Trichome on Lemon Basil Capitate trichome from Ocimum ssp. (Lemon Basil)

11/05/01BMB/BIOL/ENT 4305 Plumbago-- trichomes protecting nectories ty/webb/BOT201/Angiosperm/plum bago.htm

11/05/01BMB/BIOL/ENT 4306 Example-- Arabidopsis trichomes Fully formed & Developing Developing

11/05/01BMB/BIOL/ENT 4307 Arabidopsis Trichome An Arabidopsis trichome is a single cell Trichomes can be branched The basal portion is called the stalk There are three spikes in the apical portion.

11/05/01BMB/BIOL/ENT 4308 Trichome Number and Distribution Normal plants have leaves with evenly spaced solitary trichomes The Try mutation causes formation of small clusters of 2-5 trichomes; therefore the normal TRY gene regulates the number of trichomes at any specific position

11/05/01BMB/BIOL/ENT 4309 Trichome development-- Steps in development Cell proliferation-- number of cells per trichome Differentiation Inter-cellular communication-- lateral inhibition to get regular spacing Morphogenesis control-- branching Control in development Integration with leaf development (cell division and expansion) Hormone levels Vegetative phase development

11/05/01BMB/BIOL/ENT Restricted Cell Division in Trichome development A normal trichome has endoreduplication (DNA replication) but no mitosis and cytokinesis, THEREFORE-- cell division is inhibited The SIAMESE (SIM) gene is required for this inhibition In sim mutants, trichome cell continue to divide, resulting in multi-celled trichomes

11/05/01BMB/BIOL/ENT Morphology versus Cell Division Because plant cells don’t move, cell division is thought to be critical for plant morphogenesis In the sim mutant, trichomes are multi- cellular, but they have normal morphology This is indicates that alteration of cell division does not automatically change morphology

11/05/01BMB/BIOL/ENT Genetic Regulation of Trichome Initiation Series of checks and balances Initiation Glabrous1 (GL1) Transparent Testa Glabra1 (TTG1) Glabrous3 (GL3) Checks Glabrous1 (GL1) Transparent Testa Glabra1 (TTG1) Triptychon (TRY) Caprice (CPC) TTG1GL3GL1 CPC TTG1TRY GL1 TRICHOME INITIATION

11/05/01BMB/BIOL/ENT GL1 and TTG proteins GL1 encodes a MYB-domain protein; MYB domain proteins are probably transcription factors TTG encodes a WD-40 domain protein; WD-40 domain can interact with other proteins GL1 and TTG proteins regulate the expression of genes needed for trichome initiation and endoreduplication Evidence suggests these form a protein complex that controls trichome initiation

11/05/01BMB/BIOL/ENT Genetic Regulation of Trichome Formation Endoreduplication-- synthesis of DNA w/o cell division Glabrous2 (GL2) Inhibit by Triptychon (TRY) Morphogenesis (aerial expansion of cell and spacing) Glabrous3 (GL3) Glabrous2 (GL2) GL3GL2TRY TRICHOME INITIATION Morphogenesis DNA synthesis in absence of cell division (polyploid)

11/05/01BMB/BIOL/ENT Genes Regulating Early Cell Growth GL2 is needed for the local outgrowth, and encodes a homeodomain protein Many genes regulate the extension growth and morphogenesis. Mutants in these genes have abnormally- shaped trichomes

11/05/01BMB/BIOL/ENT Cytoskeleton and Morphology Actin filament is a major component of cytoskeleton Actin filament form thick cables during trichome growth and disruption of actin filament causes distorted growth Actin filament organization is abnormal in trichome growth mutants: alien, crooked, dis1, gnarled, klunker, and wurm

11/05/01BMB/BIOL/ENT Genes involved in branching Several genes control the branching to trichomes Evidence indicates that they interact with cytoskeleton and microtubule formation Zwichel -- kinesin-like motor protein Furca gene family, Angustifolia and stachel

11/05/01BMB/BIOL/ENT Pathways in trichome development

11/05/01BMB/BIOL/ENT Meristems - What controls the type of structures meristems generate? Vegetative growth verses Reproductive growth (inflorescence and flowers) Shift in plant development is a change in fate of meristem Vegetative Inflorescenceflowers Timing of switch is regulated by environmental factors (light, temperature, gravity, moisture and nutrients)

11/05/01BMB/BIOL/ENT Shoot meristem formation and maintenance [Review article] Michael Lenhard, Thomas Laux Current Opinion in Plant Biology 1999, 2: Meristem identity Vegetative versus Inflorescence?

11/05/01BMB/BIOL/ENT Regulation of vegetative to reproductive transition Internal vs. external factors Signals may vary among plants

11/05/01BMB/BIOL/ENT When to flower? That is the question. Variation among plants in triggers to flower (annual / biennial / perennial) Three independent pathways: Long-day pathway (photoperiod) Autonomous Pathway Vernalization (cold-treatment or extended exposure to cold) Gibberrellin (GA)

11/05/01BMB/BIOL/ENT Photoperiod or daylength controls switch from vegetative growth to reproductive growth is From floral induction to floral shape [Review article] Detlef Weigel. Current Opinion in Plant Biology 1998, 1:55-59.

11/05/01BMB/BIOL/ENT Light Perception-- Red and Far Red Light Plants have several types of light receptors One type is called phytochrome, which can absorb red and far red light In Arabidopsis, there are five isoforms of phytochromes: PhyA, PhyB, PhyC, PhyD, and PhyE

11/05/01BMB/BIOL/ENT Phytochromes Phytochromes have two domains: A light-sensing domain A protein kinase domain Light (photon) probably causes a change of conformation in the light sensing domain, which activates the protein kinase domain

11/05/01BMB/BIOL/ENT Photoperiod-- Controls Constans (Co) gene Affects downstream genes controlling the meristem. Meristem identity genes -- Master regulatory genes of floral initiation Induction of inflorescence Leafy (LFY) Terminal flower (termination of meristem)

11/05/01BMB/BIOL/ENT Autonomous Pathway Flowering is inhibited by Flowering locus C (FLC) Autonomous-pathway genes (FCA, FLD, LD and FVE) promote flowering by down- regulating FLC. In biennial plants, FRI (Frigida) up-regulates FLC leading to delayed flowering (i.e. vegetative growth is prolonged). FRI is dominant over the effects of the autonomous pathway genes.

11/05/01BMB/BIOL/ENT Autonomous Pathway Balance between vegetative and flowering phases, depends upon FLC expression in annual plants Richard M. Amasino, Low FLC, Flower on!Mutant LD, Flower off!

11/05/01BMB/BIOL/ENT Vernalization -- Cold treatment triggers flowering by downregulating FLC expression Flower off! Flower on! Richard M. Amasino,

11/05/01BMB/BIOL/ENT Interaction of pathways to flowering-- as determined by genetic experiments Reeves PH, Coupland G.Analysis of flowering time control in Arabidopsis by comparison of double and triple mutants. Plant Physiol Jul;126(3):

11/05/01BMB/BIOL/ENT Floral initiation -- genetic regulation Meristem identity genes -- Master regulatory genes of floral initiation Induction of inflorescence Leafy (LFY) Unusual floral organs (UFO) Apetala1 (AP1) Cauliflower (CAL) Apetala2 (AP2) Maintenance of flowering - Terminal flower Agamous

11/05/01BMB/BIOL/ENT Master regulatory genes of floral initiation Induction of inflorescence-- switch from vegetative to inflorescence Primary genes-- initial role in floral fate Leafy and Apetala1 -- major roles, LOF -- no floral structures GOF -- precocious flowers form Gene regulation -- transcriptional activators Leafy expressed first, turns on Apetala1 and Cauliflower-- Feedback on each other

11/05/01BMB/BIOL/ENT Floral meristem Identity Genes -- induction of inflorescence Genetic control of shoot and flower meristem behavior [Review article] Sarah J Liljegren, Martin F Yanofsky. Current Opinion in Cell Biology 1996, 8: lof gof

11/05/01BMB/BIOL/ENT Master regulatory genes of floral initiation Maintenance of flowering - How many flowers to form? How to stop making additional flowers within a flower?

11/05/01BMB/BIOL/ENT Maintenance of flowering Two genes of opposite actions Terminal flower -- keeps meristem active forming additional floral meristems (stalk of flowers)-- LOF, one flower Agamous -- terminate cell division and additional generation of floral program within a floral meristem LOF-- flower within a flower, etc.

11/05/01BMB/BIOL/ENT The making of a flower: control of floral meristem identity in Arabidopsis [Review] Pidkowich, Klenz and Haughn. Trends in Plant Science, 1999, 4:2:64-70 Floral Meristem ID genes -- LOF phenotypes

11/05/01BMB/BIOL/ENT Next lecture-- How to make a flower? (Please read Chapter 20 before this lecture)