PLNT4004 Seed Formation and Development PDF

PLNT4004 Seed Formation and Development Muhammad Farooq PLNT4004 Seed Formation and Development Muhammad Farooq Basic Flower Structure stigma carpel gynoecium locule style...

PLNT4004 Seed Formation and Development Muhammad Farooq PLNT4004 Seed Formation and Development Muhammad Farooq Basic Flower Structure stigma carpel gynoecium locule style Flower is perfect pollen Flower is monoecious ovule ovary anther filament stamen androecium petal corolla sepal perianth receptacle calyx Perianth is complete pedicel Life cycle of a flowering seed plant 2n 2n 2n 1n 2n 2n 4  Male and Female Gametophyte Male Gametophyte – anther Gamete – two sperm cells (in pollen grain or tube) Female Gametophyte – embryo sac Gamete – egg  Anther Pollen sac Pollen mother cells (Microsporocytes) 21-14 Tapetum Anther Epidermis (nutritive) (lily) Raven et al., 1999; Biology of Plants Pollen: Develops from microspores within the sporangia of anthers Pollen sac (a) Development of a male gametophyte (microsporangium) (pollen grain) 1 Each one of the Micro- MEIOSIS microsporangia contains sporocyte diploid microsporocytes (microspore mother cells). Micro- spores (4) 2 Each microsporocyte divides by meiosis to produce four haploid microspores, each of Each of 4 MITOSIS microspores which develops into a pollen grain. Generative cell (will Male 3 form 2 Gametophyte A pollen grain becomes a mature sperm) (pollen grain) male gametophyte when its generative nucleus divides and forms two sperm. This usually Nucleus occurs after a pollen grain lands of tube cell on the stigma of a carpel and the 20 m pollen tube begins to grow. Haploid (1n) Ragweed pollen grain Diploid (2n) 75 m  Pollen Development MICROSPOROGENESIS and MICROGAMETOGENESIS Pollen mother cell (Microsporocyte) (2n) “‘diploid’” Meiosis Nucleus of vegetative cell Generative cell Tetrad Free microspores Mature pollen (n) (n) (n) “‘haploid’”  Tetrad Pollen mother cell Pollen forming  Ovule Development Megasporogenesis and megagametogenesis outer funiculus integument embryo sac inner integument nucellus micropyle  Embryo sacs – Develop from megaspores within ovules (b)Development of a female gametophyte (embryo sac) Mega- 1 sporangium Within the ovule’s Megasporangium is a large Ovule Mega- sporocyte diploid cell called the MEIOSIS megasporocyte (megaspore Integuments mother cell). Micropyle Surviving The megasporocyte divides by megaspore 2 meiosis and gives rise to four Female gametophyte haploid cells, but in most species (embryo sac) only one of these survives as the MITOSIS Ovule Antipodel megaspore. Cells (3) Polar 3 Nuclei (2) Three mitotic divisions of the Egg (1) megaspore form the embryo sac, a multicellular female gametophyte. Integuments Synergids (2) The ovule now consists of the embryo sac along with the surrounding integuments (protective Embryo tissue). sac 100 m Haploid (2n) Diploid (2n)  Megasporogenesis Megasporocyte (mother cell) (2n) (n) (n) (n) Meiosis “Megasporogenesis” Egg Cell Differentiation Buchanan et al., 2000’ Biochemistry and Molecular Biology of Plants  Haploid Egg Antipodal cells Central nuclei Synergid cells Egg cell  Fertilization stigma Pollen tube ovule Fertilization  Double Fertilization Antipodal cells Central nuclei Sperm nuclei Endosperm Egg cell Embryo Synergid cell Growth of the pollen tube and double fertilization Pollen grain Stigma Pollen tube 1 If a pollen grain germinates, a pollen tube 2 sperm grows down the style toward the ovary. Style Polar Ovary nuclei Ovule (containing Egg female gametophyte, or embryo sac) Micropyle 2 The pollen tube Ovule discharges two sperm into Polar nuclei the female gametophyte (embryo sac) within an ovule. Egg Two sperm about to be 3One sperm fertilizes discharged the egg, forming the zygote. The other sperm combines with the two polar nuclei of the embryo Endosperm nucleus (3n) sac’s large central cell, forming (2 polar nuclei plus sperm) a triploid cell that develops into the nutritive tissue called Zygote (2n) endosperm. (egg plus sperm)  Establishment of a body plan Apical-basal axis --cotyledons --shoot meristem --hypocotyl --root& root mer. Single kTime™ and a W) decompressor to see this picture. fertilized Mature radial axis egg embryo --epidermis --cortex/ground tissue --vasculature (phloem and xylem) Precisely controlled cell fate Since plant cells can not move, strictly oriented cell division determines patterning of the embryo. Anticlinal/transverse division: perpendicular to mother axis Periclinal/longitudinal division: parallel to mother cell axis Embryogenesis  Steps for embryo development 1. Elongation of the fertilized egg 2. The first cell division is asymmetric. The apical (top) cell receives most of the cytoplasm and is active in protein synthesis. The apical cell goes on to make the embryo proper The basal cell and its descendants are highly vacuolated. They form the suspensor to connect to the maternal tissue. Apical cell (give rise to most part of embryo Basal cell (give rise to suspensor which provide nutrients 3. The octant stage The apical cell divides twice transversely and once longitudinally to create a sphere of eight cells, an octant. The basal cell divide transversely to create a file of cells, suspensor 4. Dermatogen stage Establish protodermal cells: anticlinal division Suspensor cells undergo programmed cell death, Very tip cell of suspensor differentiated and called hypophysis = gives rise to part of root (root apical meristem) 5. Globular stage The inner cells divide anticlinally and periclinally, endowing the embryo an recognizable axis. suspensor cells further die off and hypophysis differentiate 6. Triangular stage Apical domain: generate two symmetrically positioned cotyledon primordia Basal domain: Form a radially patterned cylinder Initiate root apical meristem from hypophysis 7. Heart stage Apical domain : Cotyledon outgrowth Establishing shoot apical meristem (SAM) Basal domain : Establishing shoot axis Establishing root apical meristem (RAM) Establishing basic tissue types: cortex, provascular tissue, and protoderm Suspensor cells undergo programmed cell death  8. Torpedo stage Enlargement of cotyledons and hypocotyl Vascular differentiation is visible QuickTime™ and a Suspensor cells undergo programmed TIFF celldecompressor (LZW) death are needed to see this picture. QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. 9. Mature embryo Bent cotyledon (for some plants) Cell layers are clearly visible, specifying tissue and organ types Embryo arrests and awaits desiccation and dormancy QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. 28  Embryo development Embryonic origin of seedling structures: controlled by precise cell division, cell elongation and differentiation QuickTime™QuickTime™ and a and a TIFF (LZW)TIFF (LZW) decompressor decompressor are neededare toneeded see thistopicture. see this picture. Pattern establishment: apical/basal and radial axes Establishment of meristem regions shoot apical meristem (SAM) root apical meristem (RAM)  Shoot apical meristem (SAM) Give rise to the shoot system of a plant Initiate in the apical domain of the dermatogen stage (16-cell) and well established at the heart stage.  Root apical meristem (RAM) Give rise to the root system of a plant Initiated from the suspensor as hypophysis at the dermatogen stage and established at the heart stage. Hypophysis develops into root apical meristem. heart stage hypophysis RAM dermatogen stage  From Ovule to Seed After double fertilization – Each ovule develops into a seed – The ovary develops into a fruit enclosing the seed(s)  Embryogenesis Early - Histodifferentiation Mid - Enlargement of the cells Late - Maturation and drying These are overlapping phases ! Seed formation  Developmental Stages Early Mid Late 19.36 pre- globular transition heart torpedo mature globular Buchanan et al, 2000, Biochemistry and Molecular Biology of Plants Simplified Structure of A Mature Seed Seed coat is dead tissue. Seed coat It protects everything inside it. Embryo Embryo is a minute plant. Endosperm Endosperm provides energy for seed germination and early seedling growth.  In a common garden bean, a eudicot – The embryo consists of the hypocotyl, radicle, and thick cotyledons Seed coat Epicotyl Hypocotyl Radicle Cotyledons (a) Common garden bean, a eudicot with thick cotyledons. The fleshy cotyledons store food absorbed from the endosperm before the seed germinates.  The seeds of other eudicots, such as castor beans – Have similar structures, but thin cotyledons Seed coat Endosperm Cotyledons Epicotyl Hypocotyl Hypocotyl Radicle Radicle (b) Castor bean, a eudicot with thin cotyledons. The narrow, membranous cotyledons (shown in edge and flat views) absorb food from the endosperm when the seed germinates.  The embryo of a monocot – Has a single cotyledon, a coleoptile, and a coleorhiza Pericarp fused with seed coat Scutellum (cotyledon) Endosperm Epicotyl Coleoptile Hypocotyl Coleorhiza Radicle (c) Maize, a monocot. Like all monocots, maize has only one cotyledon. Maize and other grasses have a large cotyledon called a scutellum. The rudimentary shoot is sheathed in a structure called the coleoptile, and the coleorhiza covers the young root. Embryogenesis and seed development Embryo development Establishment of patterns Establishment of meristem regions Endosperm development Production of storage materials/signals Preparation for embryo development and seed germination Biochemical analysis Role of hormones in embryogenesis and seed development Cytokinins: high during early embryo development, coinciding with the high rate of cell division IAA: contribute to both cell division, enlargement, and differentiation during embryogenesis GA: regulate cell enlargement during embryogenesis ABA: appear at the late stages of embryo development (embryo maturation). stimulate accumulation of seed storage proteins promote desiccation tolerance Prevent precocious seed germination