FernsGymnospermsAmborellaWaterliliesBasal MagnoliidsMonocotsEudicotsSeed plant Tree of LifeSearch APGIIIhttp://www.mobot.org/mobot/research/apweb/
Where does a multicellular organism come from?• Single-celled embryo• Plant: usually a seed• What to study? – Model species: principles are universal• Comparative analysis
Embryogenesis E Zygote A B C D F in ArabidopsisA. Asymmetric first divisionB-D. Cells have different FATESSuspensor > transverse divisionsEmbryo > precise cell divisionsD. OCTANT: suspensor & embryo lineagesE-F. GLOBULAR: protoderm forms> future epidermis;G. HEART: cotyledons initiated I& root meristem HH. TORPEDO: Gaxis extension > future hypocotylI. COTYLEDONARY:cotyledon greening, vascular tissuesSKILL: Drawing, emphasizing theimportant parts, not artistic quality!TERMINOLOGY:
• From Chun-Ming Liu via biology.kenyon.edu http://tinyurl.com/embryogenesis Arabidopsis Embryogenesis
From Jim Haseloff / Gerd Jürgens, Tübingenhttp://www.plantsci.cam.ac.uk/Haseloff/teaching/PlantSci2_index/notes/notes.htmlhttp://tinyurl.com/Haseloff Lectures 1 and 3 and http://tinyurl.com/embryogenesis
wt wtgnom gnom Disturbed apical basal polarity & No bilateral symmetry Earliest defect at first division of zygote > symmetrical division Division symmetry linked to a change in cell fate Supports hypothesis of asymmetric distribution of cell fate determinants
Mutants are usefultools to understanddevelopment gnom Forms ball-like embryos without apical/basal organs Lacks ability to establish polarity & morphogenesis GNOM GENE INVOLVED IN EMBRYO DEVELOPMENT (POLARITY) Gerd Jürgens, Tübingen
Summary: Two components of embryogenesis1. PATTERN FORMATION Embryo must establish Polarity (apical-basal & radial)> Control of cell divisionEmbryo must achieve correct shape (morphogenesis)> Control of cell division & expansion2. DIFFERENTIATION (Cells in different regions become specializedEg. Chloroplast differentiation in cotyledons/vascular tissues in hypocotyl, radicle & cotyledons)> Control of Cell Fate???
Building the plant bodyplan…….Complex processes repeated with great precision in every developing embryoControl mechanisms must also be very carefully co- ordinated.Is plant cell fate controlled by:(1) Segregation (or inheritance) of determinants at each division?(2) Positional information?
By the end of this lecture you will:1. Use four ways to understand and study developmental processes2. Understand pattern formation and plant embryogenesis3. Know about the structure and development of plant meristems4. Know about totipotency and cell development5. Have insight into cell function and communication
Meristems & Organogenesis Questions… Does form reflect function?What is growth? How is growth controlled? Where do the major organ systems of the plant originate? How are they generated? Brooker Chapters 35 & 36 Campbell & Reece Section 6 Raven section VI Chapters 36 & 42
Evidence for the role of positional information in specifying cell fate PLANT TISSUE CULTURES Cells within differentiated tissues, such as leaf tissue can be induced to REDIFFERENTIATE into a completely new embryo or plant, containing the FULL RANGE OF CELL TYPES ie Change the relative position (local signals) of cells in the leaf totipotency!Haberlandt (Austria): The results of attempts to culture isolated vegetative cellsfrom higher plants in simple nutrient should give insight to the properties andpotentialities which the cell as an elementary organism possesses … “I am notmaking too bold a prediction if I point to the possibility that, in this way, one shouldsuccessfully cultivate artificial embryos from vegetative cells” (1902)
PLANT TISSUE CULTURE Single cell regeneration demonstrates totipotency PLANT CLONINGHerbert Street (Leicester), Ted Cocking/Mike Davey(Nottingham), Nitsch, Steward, Maheshwari, Skoog: definingnutritional and developmental aspects of cultured plantcells: early 1970s
A cell is totipotent if it is has the ability to divide and re-differentiate to form a whole organismWhat does this mean for plants?1. Differentiated plant cells are usually NOT irreversibly committed2. They contain all the genetic information necessary for all aspects of plant development.3. There is no loss of genetic information during development4. Their relative ‘position’ is important in signalling to maintain their fate.
In contrast we easily cannot change the fate of differentiated animal cells without drastic measuresNuclear transplant > genetic re-programming of udder cell nucleus in enucleated egg cytoplasm I am a clone Plant Cells > change of position (external signals) sufficient to re- programme the nucleus Cloning in plants is comparatively easy
What you need to know:• Pattern formation (embryogenesis) – Materials and methods • Model species (Arabidopsis, tobacco) • Experimental biology • Mutants • Evolution • DNA sequence analysis – Growth and development • Essential processes – From the single-celled zygote to the embryo • Asymmetrical first division • Pattern formation and polar/radial symmerty – From the embryo to plant with reiteration of patterns • Positional information in cell fate – Totipotency and regeneration
MERISTEMA spatially restricted region within anorgan in which cell division for growthoccurs Shoot apical meristem Leaf primordium
Meristems are vital!All POSTEMBRYONIC development in plants occurs from meristemsGive rise to all major organ systems> roots, stems, leaves, flowers
Primary meristems• Shoot Apical Meristem• Root Apical Meristem• SAM and RAM produce additional meristematic tissue that increases plant length and produces new organs• Primary meristems produce primary tissues and organs of diverse types
Tissues in plantsD: (Epi-)DermisV: VascularG: Ground
• SAM and RAM both produce – Protoderm – generates dermal tissue – Procambium – produces vascular tissues – Ground meristem – produces ground tissues defined by location• Plant cell specialization and tissue development do not depend much on the lineage of a cell or tissue• Chemical influences are much more important
Stem development and structure• New primary stem tissues arise by the cell division activities of primary meristems located near the bases of SAMs• Epidermis develops at the stem surface – Produces a waxy cuticle (reduces water loss, protects plant)• Cortex – composed of parenchyma tissue – Composed of only one cell type, parenchyma cells – Stores starch in plastids• Stem parenchyma also has the ability to undergo cell division (meristematic capacity) to heal damage
Vegetative growth• Production of tissues by SAM and RAM and growth of mature plant• Plant shoots produce vegetative buds – miniature shoots having a dormant SAM• Under favorable conditions, buds produce new stems and leaves• Indeterminate growth – SAMs continuously produce new stem tissue and leaves as long as conditions are favorable
Plant growth &morphogenesis requires co-ordination of 3 keycellular processes whichoccur within and around the meristemRATE of CELL DIVISIONPLANE of CELL DIVISIONDIRECTION of CELL EXPANSION
How might cell division and expansion be co-ordinated to maintain meristem size and activity?Positional information might be exchangedbetween cellsQuestion…Are cells in the meristem interconnected?
Communication between meristem cell layersPlasmodesmataMembranes from adjacent cellsconnect through a pore inthe cell wall
Summary• SAM is the site of organ initiation• Major activity of the SAM is cell division• Meristem size/shape must be maintained • Otherwise there would be CHAOS!• Involves coordinated control of rates and planes of cell division in different regions• Communication between different cell layers via plasmodesmata which traverse the cell wall
ROOT APICAL MERISTEM (RAM) ORGANISATIONSIMPLER IN ORGANIZATION THAN SHOOT APEXCELLS ARRANGED IN FILESNO LATERAL ORGANS FORMED NEAR APEXROOT CAP PRESENT
HOW DO CELL FILES ARISE?Most division in apical regionLess division below the apexA group of cells that divide infrequently: QUIESCENT CENTREDivisions take place at thePERIPHERY of the QUIESCENTCENTREINITIAL OR STEM CELLSCells DIFFERENTIATE as theyexpand.
ROOT BRANCHINGLATERAL ROOTS emerge further back behind the apical meristem from the PERICYCLE CELL LAYEREstablishment of a NEW MERISTEMDEVELOPMENTAL RESPONSE TO AN ENVIRONMENTAL Epidermis SIGNAL water/nutrient Cortex supplies Endodermis Auxin signalling PERICYCLE Stele
Other meristems [Allow propagation via cloning]KalanchoeMeristems formed at the leaf marginsGenetically identical progeny (mitotic divisions)Vegetative reproduction
• Roots, stems and leaves can function in asexual reproduction – Kalanchoe leaves form plantlets, sucker shoots, potato “eyes”, banana suckers or spears• Apomixis – fruits and seeds are produced in the absence of fertilization – Meiosis produces diploid megaspores (no meiosis II)
Meristems & Organogenesis Where do the major organ systems of the plant originate?Meristem: A spatially restricted regionwithin an organ in which cell division forgrowth occursPOSTEMBRYONIC development in plantsoccurs from meristemsRoot and Shoot Apical MeristemsRate & Plane of cell division; direction ofexpansionCell communication