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Transcript
11/19/2014 Chapter 35: Plant Structure, Growth & Development 1. Vascular Plant Structure 2. Vascular Plant Growth 3. Vascular Plant Development 1. Vascular Plant Structure “Roots & Shoots” Reproductive shoot (flower) Apical bud SHOOT System Node (above ground) Internode Apical bud Shoot Vegetative shoot Leaf system • stems • leaves • flowers, fruits Blade Petiole Axillary bud Stem ROOT System Taproot (below ground) Lateral (branch) roots Root system • taproot (if present) • lateral roots 1 11/19/2014 Overall Organization of Vascular Plants Plants have a hierarchical organization consisting of organs, tissues, and cells: ORGANS • distinct functional structure consisting of multiple types of tissues TISSUES • collection of 1 or more cell types that performs a specific function within an organ 3 Basic Plant Organs Plant organs evolved to obtain nutrients, water and energy on land – below & above ground ROOTS • • absorb water, minerals and other nutrients from the soil anchor & support plant in the ground STEMS • • structural support of plant above ground transport of water & nutrients throughout the plant LEAVES • harvesting light & CO2 for photosynthesis Root Function Roots supply the plant with: CO2 • anchorage in the soil • water • mineral nutrients • carbohydrate storage • roots rely on shoot system for carbohydrates • roots also need access to O2 Minerals O2 H2O *over-watering can suffocate a plant! 2 11/19/2014 Root Structures The first root to emerge during plant development, the primary root, will then give rise to: • lateral roots to increase absorption and anchorage • tiny root hairs to maximize surface area for absorption In may plants, the primary root develops into a prominent tap root which provides: • support for a large, vertical (tall) shoot system • storage for carbohydrates Fibrous Root Structures In some plants, usually monocots, the primary root disappears and a fibrous root system forms which: • increases survival from grazing animals since plant can grow back from remaining roots • retains topsoil Evolutionary Adaptations of Roots In other plants, adventitious roots develop from unusual sources (stems, leaves) which may provide: • greater O2 access in watery environments • greater structural support Prop roots Buttress roots Pneumatophores 3 11/19/2014 Stem Structure and Function Stems support and position the photosynthetic structures (leaves) and reproductive structures (e.g., flowers, cones) to maximize their success. Reproductive shoot (flower) Apical bud Stem structures include: Node • points of leaf attachment called nodes • internodes – the stems between each node Internode Apical bud Leaf Vegetative Shoot shoot system Blade Petiole Axillary bud • apical buds at the shoot tips where growth occurs Stem Taproot • axillary buds which give rise to lateral branches, thorns or flowers Lateral Root (branch) system roots Evolutionary Adaptations of Stems Stems can be modified to serve a variety of functions: • rhizomes which grow just beneath the soil surface and give rise to vertical shoots from axillary buds • stolons that function as “runners” along the soil surface giving rise to new plantlets Rhizome Root Rhizomes Stolon • tubers that serve as storage “sinks” for carbohydrates Tubers Stolons Leaf Structure & Function Leaves are the primary photosynthetic organs. Leaf structures include: Simple leaf • one or more blades • simple leaves have 1 blade • compound leaves have multiple blades called leaflets Axillary bud Petiole Compound leaf • a stalk called a petiole that connects the leaf to a stem • veins that have a branched (dicots) or parallel (monocots) arrangement Leaflet Axillary bud Petiole 4 11/19/2014 Evolutionary Adaptations of Leaves Leaves can be modified for a variety of functions: • reproductive leaves that detach and give rise to a new plant (asexual) • tendrils which cling to larger support structures Tendrils Reproductive leaves • spines to repel herbivores • bulbs that store nutrients Storage leaves Stem Spines 3 Basic Plant Tissue Types Dermal tissue • outer, protective covering of the plant Vascular tissue • transports water, nutrients & gives structural support each of these tissues forms a continuous tissue system throughout the plant Dermal tissue Ground tissue Vascular tissue Ground tissue • everything else! There is also a type of undifferentiated tissue called meristem which we will address later in this chapter. More on Dermal Tissue… In nonwoody plants and structures (e.g., leaves) the dermal tissue is epidermis. epidermis is frequently covered with a waxy cuticle to minimize water loss • some plants also have trichomes in epidermal tissue which provide protection from water loss, intense light and insects Trichomes 300 μm • In woody plants the epidermis develops into a protective laver called periderm (part of the bark). 5 11/19/2014 More on Vascular Tissue… Plant vascular tissue consists of phloem & xylem. Xylem • transports water & minerals upward from the root system to the organs and tissues of the shoot system Phloem • transports photosynthetic products (e.g., sugars) downward to the roots and other parts of the plant Phloem & xylem are organized into vascular bundles or cylinders called steles. More on Ground Tissue… Tissues that are not dermal or vascular are ground tissue which come in 2 general types. Pith • ground tissue found internal to the vascular tissue Cortex • ground tissue found between the dermal and vascular tissue Ground tissues include cells involved in storage, transport, structural support and photosynthesis. Basic Plant Cell Types Plant cells fall into one of 5 general types: • Parenchyma • Collenchyma • Sclerenchyma • Water-conducting cells of xylem • Water-conducting cells of xylem 6 11/19/2014 Parenchyma Cells • have thin primary (1o) cell walls without a secondary (2o) cell wall • the least differentiated plant cell type • the most metabolically active plant cell type • are capable of undergoing cell division and further differentiation Parenchyma cells in a privet (Ligustrum) leaf (LM) 25 μm Collenchyma Cells • provide flexible support in newly formed shoot structures without restraining growth • flexible 1o cell walls with irregular 2o wall thickening Collenchyma cells (in Helianthus stem) (LM) 5 μm Sclerenchyma Cells • provide rigid support due to thick 2o cell walls containing lignin that are dead at maturity • 2 types of sclerenchyma cells: • sclereid cells with very thick 2o cell walls • long and slender fiber cells arranged in threads 5 μm Sclereid cells in pear (LM) 25 μm Cell wall Fiber cells (cross section from ash tree) (LM) 7 11/19/2014 Water-Conducting Xylem Cells Vessel Tracheids 100 μm 2 types of xylem cells, both of which are dead at maturity: TRACHEIDS • found in all xylem vessels • long, thin with tapered ends VESSEL ELEMENTS • wider, less tapered • perforated ends Pits Tracheids and vessels (colorized SEM) Perforation plate Vessel element Vessel elements, with perforated end walls Tracheids Sugar-Conducting Phloem Cells 2 types of phloem cells, both of which are alive at maturity: SIEVE CELLS • found in seedless vascular plants & gymnosperms SIEVE-TUBE ELEMENTS • cells that form sieve tubes in angiosperms • have sieve plates between elements & supporting companion cells 2. Vascular Plant Growth 8 11/19/2014 Meristem Tissue Unlike animals, plants are capable of indeterminate growth – growth throughout the life of the plant. This unlimited growth potential is due to meristem tissue – a special, undifferentiated tissue with unlimited replicative potential. • in contrast, animals and some plant structures (e.g., flowers, thorns) exhibit determinate growth in which they stop growing when they reach a certain size There are 2 types of meristems: • • APICAL MERISTEM LATERAL MERISTEM Apical Meristem Apical meristem is located at the tips of roots and shoots and is responsible for growth in length – what is called primary growth. Shoot tip (shoot apical meristem and young leaves) Axillary bud meristem Root apical meristems • in non-woody (herbaceous) plants, most if not all growth is due to apical meristem • in woody plants (e.g., trees), there is also growth in width, what is referred to as secondary growth… Lateral Meristem Secondary growth in width is due to Primary growth in stems 2 types of lateral meristem: Epidermis Cortex adds new • vascular cambium which Primary phloem layers of phloem & xylem xylem • cork cambium whichPrimary replaces the Pith epidermis with protective periderm Vascular cambium Cork cambium Lateral Secondary growth in stems meristems Cork cambium Periderm Pith Primary xylem Cortex Primary phloem Secondary phloem Vascular cambium Secondary xylem 9 11/19/2014 Primary Growth of Roots Root tips have a protective, non-dividing root cap. Cortex Dermal Ground Vascular Vascular cylinder Epidermis Root hair Just underneath the root cap is the Zone of Cell Division which contains the apical meristem cells. Zone of differentiation Beyond the Zone of Cell Division are 2 zones in successive developmental stages: Zone of elongation Mitotic cells Zone of cell division (including apical meristem) Root cap 100 μm Cortex Endodermis Vascular cylinder Pericycle Core of parenchyma cells Xylem 100 μm (a) Root with xylem and phloem in the center (typical of eudicots) Phloem Endodermis Pericycle Xylem Phloem Monocot Roots • pushes root into soil Zone of Differentiation Epidermis 70 μm Zone of Elongation Dermal Ground Vascular • cells adopt specific fates Eudicot Roots In most eudicot roots, there is a central vascular cylinder (stele) with a “X-shaped” arrangement of xylem as seen in cross section with phloem filling in between the “arms” of the X. Epidermis Cortex Endodermis Vascular cylinder Pericycle In most monocot roots, there is a core of parenchyma cells surrounded by a ring of alternating phloem and xylem vessels. Core of parenchyma cells Xylem Phloem 100 μm (b) Root with parenchyma in the center (typical of monocots) Dermal Ground Vascular 10 11/19/2014 Lateral Root Growth 100 μm Emerging lateral root Epidermis Lateral root Cortex Vascular cylinder Pericycle 1 2 3 Lateral root growth occurs from the meristematic pericycle, the outermost layer of cells in the vascular cylinder just inside the endodermis, the innermost layer of cortex. Primary Growth of Shoots Primary growth of shoot structures occurs from: • apical meristem which lengthens the stem and gives rise to leaf primordia Leaf primordia Young leaf Shoot apical meristem Developing vascular strand • axial meristem which gives rise to new branches from the main stem Axillary bud meristems 0.25 mm Organization of Eudicot Stems Phloem Sclerenchyma (fiber cells) Xylem Ground tissue connecting pith to cortex Pith Cortex Epidermis Vascular bundle (a) Cross section of stem with vascular bundles forming a ring (typical of eudicots) (LM) 1 mm Dermal Ground Vascular In most eudicot stems, the vascular tissue consists of bundles of phloem and xylem arranged in a ring around the central pith tissue. • the xylem is always located inside the phloem adjacent to the pith 11 11/19/2014 Organization of Monocot Stems Ground tissue In most monocot stems, the vascular tissue consists of bundles of phloem and xylem scattered throughout the ground tissue. Epidermis Vascular bundles 1 mm Dermal Ground Vascular (b) Cross section of stem with scattered vascular bundles (typical of monocots) (LM) Leaf Structure Guard cells Cuticle 50 μm Stomatal pore Epidermal cell Sclerenchyma fibers Stoma (b) Surface view of a spiderwort (Tradescantia) leaf (LM) Upper epidermis Palisade mesophyll 100 μm Spongy mesophyll Lower epidermis Bundlesheath cell Cuticle Vein Xylem Phloem Guard cells (a) Cutaway drawing of leaf tissues Dermal Ground Vein Air spaces Guard cells (c) Cross section of a lilac (Syringa) leaf (LM) Vascular Epidermis • outer cell layer on both sides of leaf • secrete waxy cuticle to waterproof the leaf Mesophyll (ground tissue of leaf) • loosely packed photosynthetic parenchyma cells • palisade or spongy arrangement Vascular Bundles • phloem & xylem • surrounded by bundle sheath cells Stomata (singular = “stoma”) • openings for gas exchange, transpiration • regulated by guard cells 12 11/19/2014 (a) Primary and secondary growth in a two-year-old woody stem Pith Primary xylem Vascular cambium Primary phloem Epidermis Cortex Cortex Secondary (2o) Growth of Stems & Roots Epidermis Primary phloem Vascular cambium Vascular ray Primary xylem Secondary xylem Pith Secondary phloem First cork cambium Cork Most recent cork cambium Periderm (mainly cork cambia and cork) Secondary phloem Vascular cambium Bark Cork Secondary xylem Bark Cork cambium Periderm Late wood Early wood Layers of periderm Cork 1 mm Secondary phloem Vascular ray Secondary xylem 1.4 mm Growth ring (b) Cross section of a three-yearold Tilia (linden) stem (LM) All gymnosperms and most eudicots undergo growth in diameter or width – 2o growth. • most monocots undergo primary growth only VASCULAR CAMBIUM • a single-celled ring of meristem between primary xylem and phloem • produces new (secondary) xylem toward the inside and new (secondary) phloem toward the outside CORK CAMBIUM • produces cork cells periderm in place of the original epidermis to produce a protective outer layer More on Vascular Cambium… Vascular cambium Growth X X C P P X X C P Secondary xylem Vascular cambium Secondary phloem X C P X C C After one year of growth After two years of growth 2o phloem and xylem cells form adjacent to the vascular cambium cells, pushing earlier layers further away from the vascular cambium. 13 11/19/2014 Growth Rings Reveal Past Climates In woody stems, spring 2o xylem (spring wood) differs from summer 2o xylem (summer wood), giving the appearance of annual growth rings. • warm & wet = wider ring • cold & dry = narrower ring 2 Ring-width indexes 1.5 1 0.5 0 1600 1700 1800 1900 2000 Year More on Woody Stems… • older xylem that no longer transports fluid = hardwood • newer, active xylem = sapwood • 2o phloem + periderm = bark Growth ring Vascular ray Heartwood Secondary xylem Sapwood Vascular cambium Secondary phloem Bark Layers of periderm 3. Vascular Plant Development 14 11/19/2014 Arabidopsis – A model Plant Much of what we know about plant development comes from studying a tiny weed – Arabidopsis thaliana. Arabidopsis has several advantages that make it very practical to use as a model plant organism: • small size • grows fast • small genome size • easy to genetically modify Genetic Modification of Arabidopsis Agrobacterium tumefaciens, a plant pathogen, is the key: Agrobacterium tumefaciens • • contains the Ti plasmid with a “T DNA” region that is integrated randomly into the host plant cell genome DNA of interest can be “cloned” into the T DNA region and then introduced into the host plant genome 3 1 Ti plasmid Site where restriction enzyme cuts 2 T DNA DNA with the gene of interest Recombinant Ti plasmid Plant with new trait Asymmetrical Cell Division & Cell Fate in Plants Asymmetrical cell division Unspecialized epidermal cell Guard cell “mother cell” Developing guard cells Asymmetrical or “uneven” cell division has been shown to precede the adoption of distinct cell fates in plants as shown in this example. 15 11/19/2014 An Arabidopsis mutant called gnom demonstrates the importance of asymmetric cell division in early plant development: normal • the 1st division of normal plant zygotes is asymmetric and determines the polarity of the plant (i.e., root vs shoot systems) • the 1st division of gnom mutant zygotes is symmetrical and the embryo develops without any polarity – no roots or shoots gnom mutant Genetic Control of Flowering Environmental cues such as day length and temperature trigger flower development in plants such as Arabidopsis. Mutants such as the one shown here have led to the ABC hypothesis of flower development: • the inner whorls of the mutant flower develop into petals and sepals instead of stamens and a carpel Ca St Pe Se Pe Pe Se Pe Se Normal Arabidopsis flower Mutant Arabidopsis flower The ABC Hypothesis of Flowering (a) A schematic diagram of the ABC hypothesis Sepals Petals Stamens A Carpels B C C gene activity B+C gene activity Carpel Petal A+B gene activity Stamen A gene activity Sepal Active BB BB genes: A A C C C C A A Whorls: BB BB C CC CCC C C A A C CC C A A AA AA AB BA AB BA Mutant lacking A Mutant lacking B Mutant lacking C Stamen Carpel Petal Sepal Wild type (b) Side view of flowers with organ identity mutations 16