Plant Form and Function - Campbell Biology PDF

Campbell Biology, 12th Edition Esparza 1 Neil A. Campbell, et.al Unit 6: Plant Form and Function Table of Contents Chapter 35: Vascular Plant Structure, Growth, and Development 2 Concept 35.1 Plants have a...

Campbell Biology, 12th Edition Esparza 1 Neil A. Campbell, et.al Unit 6: Plant Form and Function Table of Contents Chapter 35: Vascular Plant Structure, Growth, and Development 2 Concept 35.1 Plants have a hierarchical organization consisting organs, tissues, and cells 3 Concept 35.2 Different meristems generate new cells for primary and secondary growth 4 Concept 35.3 Primary growth lengthens roots and shoots 5 Concept 35.4 Secondary growth increases the diameter of stems and roots in woody plants6 Concept 35.5 Growth, morphogenesis, and cell differentiation produce the plant body 7 Chapter 36: Resource Acquisition and Transport in Vascular Plants 8 Concept 36.1 Adaptations for acquiring resources were key steps in the evolution of vascular plants 8 Concept 36.2 Different mechanisms transport substances over short or long distances 9 Concept 36.3 Transpiration drives the transport of water and minerals from roots ot shoots via the xylem 9 Concept 36.4 The rate of transpiration is regulated by stomata 10 Concept 36.5 Sugars are transported from sources to sinks via the phloem 10 Concept 36.6 The symplast is highly dynamic 11 Chapter 37: Soil and Plant Nutrition 11 Concept 37.1 Soil containing a living, complex ecosystem 11 Concept 37.2 Plant roots absorb many types of essential elements from the soil 12 Concept 37.3 Plant nutrition often involves relationships with other organisms 12 Chapter 38: Angiosperm Reproduction and Biotechnology 14 Concept 38.1 Flowers, double fertilization, and fruits are key features of the angiosperm life cycle 14 Concept 38.2 Flowering plants reproduce sexually, asexually, or both 15 Concept 38.3 People modify crops by breeding and genetic engineering 16 Chapter 39: Plant Responses to Internal and External Signals 17 Concept 39.1 Signal transduction pathways link signal reception to response 17 Concept 39.2 Plants use chemicals to communicate 17 Concept 39.3 Responses to light are critical for plant success 17 Concept 39.4 Plants respond to a wide variety of stimuli other than light 17 Concept 39.5 Plants respond to attacks by pathogens and herbivores 17 Campbell Biology, 12th Edition Esparza 2 Neil A. Campbell, et.al Unit 6: Plant Form and Function Chapter 35: Vascular Plant Structure, Growth, and Development Concept 35.1 Plants have a hierarchical organization consisting organs, tissues, and cells Vascular Plant Organs: Roots, Stems, and Leaves Roots, stems, and leaves formed a root system and a shoot system to accommodate the need to absorb water & nutrients underground and acquire sunlight above ground ○ Root - organ that anchors the plant, absorbs mineral & water, and stores nutrients Primary root is the first root from the embryo, branching out into lateral roots, enhancing the ability to absorb nutrients Taproot system consists of a main, vertical root (taproot) that lengthens the plant, giving it more access to sunlight FIbrous root system is a thick mass of slender roots in which the main root dies early, forcing roots (adventitious) to emerge from the stem Growing from the tips of roots are extension of root hairs, which increases surface area for water + mineral absorption ○ Stem - organ bearing leaves and buds in which its role is to maximize photosynthesis & facilitate reproduction with pollen and fruit dispersal Consists of a system of nodes (where the leave grows) & internodes (between nodes) Growth concentrated at the apical bud of the shoot; axillary bud gives rise to branches Have experienced modification (e.g. stolons in asexual reproduction) Leaf - main photosynthetic organ that facilitates gas exchange Consists of a flattened blade & a stalk (petiole) Its vascular tissue (veins) may be parallel in monocots & branched in eudicots May be simple (one leaflet) or compound (multiple leaflets). Campbell Biology, 12th Edition Esparza 3 Neil A. Campbell, et.al Unit 6: Plant Form and Function Dermal, Vascular, and Ground Tissues Tissue system - continuous & consisting of dermal, vascular, and ground tissues ○ Dermal tissue - outer protective covering of the leaf Usually forms a single layer of tissue (epidermis) coated with a waxy cuticle, which lowers desiccation Periderm may replace epidermis in older roots Specialized epidermal cells (guard cells in gas exchange & trichomes in preventing water loss) Vascular tissue facilitates transport of nutrients & provides mechanical support Xylem - conducts water & minerals upward from roots → shoots Phloem - transports sugars to needed areas (roots or developing leaves) Stele - vascular tissue in roots (vascular spindle) or stems (vascular bundles) Ground tissue - specialized for storage, photosynthesis, support, etc. internally (pith) & externally (cortex) Common Types of Plant Cells Plant cells undergo cell differentiation to yield specialization in structure & function ○ Parenchyma cells - perform metabolic functions (photosynthesis) Relatively undifferentiated and thin-walled cells May contain colorless starch-containing plastids called amyloplasts ○ Collenchyma cells - provide structural, but flexible support Unevenly thickened primary walls that support young, growing parts of the plant ○ Sclerenchyma cells - provide structural but rigid support Thick secondary cell wall with lignin Helps support mature, non growing plant parts Campbell Biology, 12th Edition Esparza 4 Neil A. Campbell, et.al Unit 6: Plant Form and Function May die at functional maturity May be sclereids (boxy, harden leaf parts) and fibers (stranded) ○ Water-conducting cells - tracheids (wider) & vessel elements (thinner) - are dead & lignified by maturity, but leave behind a conduit for water transport Vessel elements have perforation plate to enable free water movement through long pipes called vessels ○ Sugar-conducting cells - Sieve-tube elements (devoid of internal organelles) Composed of sieve plates (between sieve-tube elements) & companion cells (connected by plasmodesmata) that function in transport of sugars through the phloem of angiosperms Concept 35.2 Different meristems generate new cells for primary and secondary growth Plants grow throughout their lives (indeterminate growth) due to undifferentiated tissue (meristems) while some stop growing at a certain point (determinate growth) ○ Apical meristems enable growth in length (primary growth) at root/shoot tips Located in the apical bud, where it produces altering internodes and leaf-bearing nodes Campbell Biology, 12th Edition Esparza 5 Neil A. Campbell, et.al Unit 6: Plant Form and Function ○ Lateral meristems enable growth in thickness (secondary growth) along roots & shoots that have ceased primary growth Vascular cambium and vascular tissue (secondary xylem/phloem) Secondary xylem responsible for most thickening Cork cambium replaces epidermis with tougher peridermis ○ Primary meristems - tissues that form during primary growth Consists of Protoderm (dermal), ground meristem (ground), & procambium (vascular) Flowering plants grouped as annuals (life cycle complete in a year or less), biennials (two growing seasons before death), or perennials (live many years) Concept 35.3 Primary growth lengthens roots and shoots Primary Growth of Roots Root apical meristem make a root cap (covers the meristem) Campbell Biology, 12th Edition Esparza 6 Neil A. Campbell, et.al Unit 6: Plant Form and Function ○ Three stages of primary growth (progressively moves away from the root tip) Zone of cell division - stem cells of root apical meristems form new cells Zone of Elongation - primary lengthening of the plant Zone of Differentiation - new root cells specialized in structure & function Protoderm gives rise to the epidermis and root hairs Ground meristem forms ground tissue along the cortex, which allows extracellular diffusion of water & nutrients from root hairs ○ Endodermis - innermost layer of the cortex that regulates passage of substance from soil → vascular cylinder Procambium produces the vascular cylinder (xylem + phloem surrounded by a cell layer called the pericycle) ○ Eudicots = roots with xylem and phloem in the center ○ Monocots = roots with parenchyma in the center ○ Lateral roots emerge from the pericycle Primary Growth of Shoots Shoot apical meristem elongate & give rise the proctoderm, ground meristem, and the procambium ○ As roots branch, the closer an axillary bud gets to an apical bud, the more inhibited it is (apical dominance) Campbell Biology, 12th Edition Esparza 7 Neil A. Campbell, et.al Unit 6: Plant Form and Function Removing the apical meristem stimulates growth in the axillary buds, thus making a plant bushier. ○ As stems grow, the epidermis, ground tissue (mostly parenchyma), and vascular tissue (arise from axillary buds without disrupting other tissue) form Eudicot stems = ringed vascular bundles Monocot stems = scattered vascular bundles Leaves form from leaf primordia, projections along the sides of shoot apical meristem ○ Leaf epidermis contains stomata - pores that regulate gas exchange & transpiration - in which guard cells open/close them ○ Mesophyll - a leaf’s ground tissue formed between epidermal layers Palisade mesophyll, located under the upper epidermal layer, contain chloroplast-rich cells like parenchyma Spongy mesophyll, located above the under epidermal layer, contain chloroplast-deficient cells Gasses (CO2 & O2) circulate here ○ Bundle-sheath cells enclose the veins & facilitate transport between vascular tissue and mesophyll Campbell Biology, 12th Edition Esparza 8 Neil A. Campbell, et.al Unit 6: Plant Form and Function Concept 35.4 Secondary growth increases the diameter of stems and roots in woody plants Secondary growth consists of tissue produced by vascular & cork cambiums ○ Occurs simultaneously in woody plants ⬆️ Vascular Cambium and Secondary Vascular Tissue Vascular cambium adds secondary xylem/phloem to the stem, its diameter ○ Contain stem cells oriented parallel (mature cells) or perpendicular (vascular rays) to the axis of stem/root Secondary xylem forms rigid, lignified walls (gives wood its rigidity) ○ Early (spring) wood has thick secondary xylem & thin cell walls ○ Late (summer) wood has thick-walled cells providing structural support Develops a distinct growth ring used to estimate a tree’s age (dendrochronology) and the climate it experienced ○ Older layers of secondary xylem (heartwood) become inactive, whereas younger layers (sapwood) still conduct water Campbell Biology, 12th Edition Esparza 9 Neil A. Campbell, et.al Unit 6: Plant Form and Function Secondary phloem does not accumulate as extensively Cork Cambium and Production of Periderm In the earliest stages of secondary growth, epidermis is deposited ○ Cork Cambium (arises from the pericycle) forms the periderm, which includes layers of cork cells and itself Bark - tissue external to the vascular cambium ○ Composed of secondary phloem & periderm Periderm contain openings called lenticels that allow for gas exchange despite a waxy periderm Evolution of Secondary Growth Studies of Arabidopsis thaliana show primary & secondary growth are evolutionary related with developmental genes controlling these processes Concept 35.5 Growth, morphogenesis, and cell differentiation produce the plant body Development - series of changes that give rise to tissues, organs, and organisms ○ Affected by genotype and phenotype (developmental plasticity) ○ Involves growth, morphogenesis, and cell differentiation Arabidopsis thaliana chosen as a model organism for genetic experiments because: ○ Short generation time, rapid maturation, abundant seeds, and a small genome ○ Provides information on the effect of certain genes on plant development Campbell Biology, 12th Edition Esparza 10 Neil A. Campbell, et.al Unit 6: Plant Form and Function Growth: Cell Division and Cell Expansion Cell division may be asymmetric, generating cells with different fates ○ Ex: epidermal cell asymmetrically divided into a large epidermal cell & a small guard cell Plant cells grow mainly through water uptake in a large central vacuole ○ Provides an energetically inexpensive way to increase vacuolar sap ○ Orientation of expansion perpendicular to cellulose microfibrils Morphogenesis and Pattern Formation Pattern formation - development of specific structures in specific locations ○ Two hypotheses: Lineage-based mechanisms determine morphology Position-based mechanisms in proximity to neighboring cells determine a plant cell’s fate; proven to be MORE LIKELY ○ In animals, lineage-based mechanisms with Hox genes determine cell fate ○ KNOTTED-1 gene determines leaf shape Gene Expression and the Control of Cell Differentiation Regulation of gene expression results in different phenotype despite identical genome ○ Activation/Inactivation of developmental genes determined by neighboring cells Ex: GLABRA-2 gene, responsible for root hair distribution, is expressed depending on how many cortical cells border it Campbell Biology, 12th Edition Esparza 11 Neil A. Campbell, et.al Unit 6: Plant Form and Function Shift in Development: Phase Changes Plants develop in phase from a juvenile vegetative stage → adult reproductive stage ○ Phase changes - changes in morphology due to transitional phases in the shoot apical meristem ○ Node formation will determine if leaves are juvenile or adult Genetic Control of Flowering Involves transition from vegetative growth → reproductive growth (flowering) ○ Controlled by the protein products of flower-inducing genes Formation of concentric “circles” of plant organs determined by position ABC hypothesis - proposes that three classes of genes direct the development of four types of floral organs ○ A active = sepals, A and B active = petals, B and C = stamens, C active = carpels ○ A gene inhibits expression of C and vice versa Type of plant cell What it does How structure fits function Campbell Biology, 12th Edition Esparza 12 Neil A. Campbell, et.al Unit 6: Plant Form and Function Parenchyma Schlermchya Collenchyma Xylem Phloem Chapter 36: Resource Acquisition and Transport in Vascular Plants Concept 36.1 Adaptations for acquiring resources were key steps in the evolution of vascular plants Evolutionary success of plants lies in its ability to acquire resources from aboveground & underground ○ Early plants were nonvscular & absorbed resources directly from water ○ After taller plant arose and they needed a way to minimize water loss, natural selection favored those w/ roots & long-distance transport (xylem & phloem) Stems support the leaves and act as conduits for transport ○ Taller = less shade, more light ○ Branching = efficient harvesting of light Leaves conduct photosynthesis and facilitate gas exchange ○ More water availability = larger leaves ○ Arrrange of leafs (phyllotaxy) is important for light capture May be alternate, opposite, or whorled ○ Surface area of a leaf can promote/hinder light abortion Campbell Biology, 12th Edition Esparza 13 Neil A. Campbell, et.al Unit 6: Plant Form and Function Leaf area index describe ratio between upper/lower leaf surace If too high, nonreproducive leaves undergo self-pruning’ ○ Consequences of lead orientation depend on the environment Horizontal leaves efficient in low-light condition ○ Shoot adaptatiosn accommodate photosynthesis & stomatal water loss Roots “mine” soil for nutrients and form mutualsitic relationships (e.g. mycorrhizae) Concept 36.2 Different mechanisms transport substances over short or long distances Apoplast and Symplast: Transport Continuums Plant tissue consists of everything outside the plasma membrane (apoplast) and the cytosol & plasmodesmata (symplast) Water & solvents can travel through the apoplastic, symplastic, or transmembrane route Short-Distance Transport of Solute & Water across Plasma Membranes All plants have a selectively permeable membrane with general types of transport protein, but specific differences when compared to animals ○ H+ is involved in basic transport mechanisms instead of Na+ Ex: H+/sucrose cotransporter ○ Slow Ion-gated channels & action potentials using Ca2+ channels Osmosis - diffusion of free water across a membrane ○ Water potential (Ψ) predicts water flow based on solute conc. & pressure Water movement: high water potential → low water potential Campbell Biology, 12th Edition Esparza 14 Neil A. Campbell, et.al Unit 6: Plant Form and Function Measured using megapascal (MPa), a unit of pressure Pure water in an container = 0MPa Ψ=ΨS +ΨP ○ Solute potential (ΨS ) becomes more negative as solute conc. increases Negative impact on water potential ○ Pressure potential (ΨP) depends on atmospheric pressure If increased, the protoplast (living parts of the cell) pushes against the cell wall and exerts turgor pressure If cellular Ψ > environmental Ψ, it undegroes plasmolysis and water diffuses out If cellular Ψ < environmental Ψ, then the cell is turgid ○ Losing turgor pressure results in wilting Water molecules move through transport protein called aquaporins Long Distance Transport: Bulk Flow Bulk flow - movement of liquid in response to a pressure gradient (not ΨS) ○ Occurs in xylem cells & within the sieve-tube elements of the phloem ○ Must faster than diffusion or active transport; occurs in dead cells ○ Does NOT require energy; SUN-POWERED Concept 36.3 Transpiration drives the transport of water and minerals from roots ot shoots via the xylem Root cells (aided by root hairs) absorb water and minerals Water & minerals pass from the soil → root cortex → vascular cylinder (mediated by the innermost endodermis in the cortex) → xylem ○ If already in the symplast, endodermis allows passage ○ If coming from the apoplast, the Casparian strip blocks entry Mediate entry/exit of water & minerals Campbell Biology, 12th Edition Esparza 15 Neil A. Campbell, et.al Unit 6: Plant Form and Function Water & minerals in the xylem (xylem sap) move to stems & leaves through veins ○ Involves the loss of a significant amount of water (transpiration) ○ Root pressure (from water → cortex) “pushes” the xylem sap, resulting in guttation (exudation of water) Minimal in comparison to the forces of transpiration ○ Cohesion-tension hypothesis - transpiration provides the “pull” of xylem, which is transcribed by cohesion between water molecules Transpirational pull results as H-bonding forms a water molecule chain that attachs to the cell wall (adhesion) & other water molecules (cohesion) Driven by a water potential gradient from opposite ends of xylem Broken water chain (cavitation) may be reestablished via root pressure Campbell Biology, 12th Edition Esparza 16 Neil A. Campbell, et.al Unit 6: Plant Form and Function Concept 36.4 The rate of transpiration is regulated by stomata Increased surface area for efficient photosynthesis also result in hefty transpiration Stomata (pores where transpiration occurs) are opened/closed by guard cells ○ Stomatal density regulated genetically (shade tolerance) & environmentally (low CO2 = high stomatal density) Stomatal opening (high turgor p.) & closing (low turgor p.) results form K+ absorption ○ K+ uptake open stoma & creates a membrane potential, increasing turgidity ○ Loss of K+ closes stoma, resulting in osmotic loss of water Stimuli for the opening/closing of stomata include: ○ Light stimulate guard cells to absorb K+ and become turgid ○ CO2 depletion cause stomatal opening if there is sufficient water ○ Circadian rhythms that cycle every 24 hours ○ Environments stimulating evaporation may cause stomatal closing Campbell Biology, 12th Edition Esparza 17 Neil A. Campbell, et.al Unit 6: Plant Form and Function ○ Abscisic acid (ABA), a plant hormone, signals stomatal closing in water deficiency, inhibiting photosynthesis If transpiration > water uptake, then wilting and evaporative cooling occurs Xerophytes - plants that have adapted to dry environments ○ Ex: reduced leaves and CAM photosynthesis Concept 36.5 Sugars are transported from sources to sinks via the phloem Translocation - transport of photosynthesis products (photosynthates) ○ Occurs in sieve-tubes between sieve plates ○ Phloem sap moves from sugar sources (mature leaves, fully developed organs) to sugar sinks (growing plant organs) Loading of sucrose into phloem: mesophyll → symplast → sieve-tube elements (could also move to apoplast from symplast) Sucrose → sieve-tubes & companion cells via H+/sucrose cotransporter ○ Mechanisms involves bulk flow driven by positive pressure (pressure flow) Pressure building (from sugar accumulation & resulting osmotic flow) at source and its reduction at the sink drives it ○ Uses living cells instead of dead cells like xylem transport Campbell Biology, 12th Edition Esparza 18 Neil A. Campbell, et.al Unit 6: Plant Form and Function If sinks > source, plant may abort some sinks (self-thinning) Concept 36.6 The symplast is highly dynamic Symplast is responsible for changes in plant transport processes ○ Plasmodesmatal number changes in response to changes in turgor pressure, cytosloic Ca2+ levels, cytosolic pH, and differentiation ○ Despite small pore size, plant viruses produce viral movement proteins, which acts as plasmodsmatal regulators Interconnectedness only occurs between symplastic domains ○ Phloem acts as an “information highway” for macromolecules & viruses Faciliates systemic communication throughout the plant ○ Electrical signaling between cells occurs through the phloem (acts as a nerve) Chapter 37: Soil and Plant Nutrition Concept 37.1 Soil containing a living, complex ecosystem Soil Texture Depends on particle size, which arise from mechanical & chemical weathering ○ Becomes mixed with organisms & organic matter (humus), forming topsoil (horizon A) Most fertile topsoil (loams) provide the most plant growth Topsoil Composition Composed of water, air, inorganic matter (minerals), and organic matter ○ Inorganic components Soil particles bind easily to cations (K+, Ca2+, Mg2+) rather than anions Campbell Biology, 12th Edition Esparza 19 Neil A. Campbell, et.al Unit 6: Plant Form and Function In roots, cation exchange - cations in soil particles are displaced by other caitons (especially H+) - occurs Ex: Clay, (-), increase cation exchange and water retention ○ Organic components Humus, organisms (e.g. earthworms), etc. Soil Conservation and Sustainable Agriculture Soil managements, by fertilization (addition of mineral nutrients into soil) & other practices, helped prepare modern societies ○ Sustainable agriculture - commitment to farming practices that promote conservation, environmental safety, and profitability Irrigation promotes plant growth with a supply of water (from aquifers) Unsustainable, causes land subsidence + soil salinization Ferilization reduces soil depletion Uses factory fertilizers rich in N, P, & K (N-P-K raito) and organic fertilizers Soil pH adjustments made based on a crop’s mineral needs Prefer slightly acidic pH (8), too acidic = Al3+ toxicity Erosion reduced using no-till agriculture (seeds with little disturbance to soil) Phytoremediation - use of plants to detoxify soil Concept 37.2 Plant roots absorb many types of essential elements from the soil Essential elements are needed for a plant’s life cycle ○ Determined by hydroponic culture (plants grown in mineral solutions) ○ Macronutrients required in large amounts and constitute important organic compounds C, H, O, N, P, and S (*K, Ca2+, Mg2+) ○ Micronutrients required in small amounts and function as cofactors Cl-, Fe, Mn, B, Zn, Cu, Ni, and Mo (Na in C4/CAM) Campbell Biology, 12th Edition Esparza 20 Neil A. Campbell, et.al Unit 6: Plant Form and Function Symptoms of mineral deficiency depend on its function & mobility and age & species of the individual ○ Macronutrients (especially N) more common as micronutrients can easily be replenished ○ Can affect older leaves more if the mineral is highly mobile Global climate change has decreased effective photosynthesis & nutrient absorption Concept 37.3 Plant nutrition often involves relatiobshiops with outer organisms Plant derive nutrition with the help of bacteria, animals, fungi, and more Bacteria and Plant Nutrition Rhizobacteria closely associate with plant roots or live in the rhizosphere, soil in close proximity to plant roots ○ Plant yield nutrition while rhizobacteria antibiotic protection, absorption of toxins, and N2 fixation ○ May be free-living or in between plant cells (endophytes) Campbell Biology, 12th Edition Esparza 21 Neil A. Campbell, et.al Unit 6: Plant Form and Function Nitrogen cycle - series of natural processes that produce usable nitrogen compounds (NH4+ & NO3 2-) for living organisms and their reentry into air & soil ○ Involves nitrifiying bacteria & ammonifying bacteria fixating atmospheric N2 and humus, respectively Nitrogen fixation - reduction of N2 into NH3 via nitrogenase ○ N2 + 8e – 1 8 H+ 1 16 ATP → 2 NH3 + H2 + 16 ADP + 16 P ○ Legume-Rhizobium mutualisitc relationships change root structure Nodules - rhizobium-infected plant cells in which rhizobium form bacteroids, providing an anaerobic environment for N2 fixation Leghemoglobin acts as an “oxygen buffer” that provide more anaerobic conditions Respond and communicate via chemical signals Crop rotations - nonlegume is grown and a legume is planted to replenish N2 in soil ○ Seeds exposed to bacteria beforehand to ensure relation with Rhizobium Fungi and Plant Nutrition Campbell Biology, 12th Edition Esparza 22 Neil A. Campbell, et.al Unit 6: Plant Form and Function Mycorrhizae - plant-fungus mutualism ○ Plants provide sugar; fungi yield efficient water uptake & many minerals ○ Early plants lacked roots while early fungi lacked photosynthesis, providing an evolutionary basis for this relationship Ectomycorrhizae form sheathes of mycelia over the surface of the root; 10% Arbuscular mycorrhizae embed mycelia into the root cells; 85% ○ Pentrate cell wall and invaginate, forming arbuscles Produces good crop yields and an in-depth understanding of ecological interactions Epiphytes, Parasitic Plants, and Carnivorous Plants Epiphyte - nonparasitic plants that grow on another plant (e.g. bromeliads) Parasitic plants absorb nutrients form their plant host (via projections called haustoria) Carnivorous plants are photosynthetic & consume insects and other critters ○ Use various mechanisms (e.g. digestive enzymes) to capture prey ○ Ex: Venus Flytrap (Dionaea muscipula) “See Hopkins, California — Mighty good” (CHOPKNSCaMg) - a mnemonic for remembering the nine plant macronutrients, as shown below. See = C = carbon; H = H = hydrogemm, O = O = oxygen Chapter 38: Angiosperm Reproduction and Biotechnology Concept 38.1 Flowers, double fertilization, and fruits are key features of the angiosperm life cycle Involves an alternation of generation in which the “three Fs” - flowers, double fertlization, and fruits - are key traits of angiosperm life cycles Flower Structure and Function Campbell Biology, 12th Edition Esparza 23 Neil A. Campbell, et.al Unit 6: Plant Form and Function Flower - sporphyitc structure specified for reproduction ○ Four floral organs includes Carpel (pistil) - innermost megasporophyll that produces ovules Ovary (one or two ovules) at the base and a long neck (style) with sticky stigma to capture pollen May have single (simple pistil) or fused carpels (compound pistil) Stamen - second innermost microsporphyll containing a filament stalk & a terminal anther (microsporangia produce pollen) Petals - sterile modifed leaves designed ot attract pollinators Sepals - sterile modified leaves protecting floral byds ○ Floral organs connected by a stem part called the receptacle ○ Complete flowers have all four floral organs while incomplete flowers lack some ○ Inflorescences - showy clusters of flowers Methods of Pollination Pollination - transfer (via wind, wind, or animals) of pollen to the ovule-bearing structure of a seed plant ○ Natural selection favors floral structures that enhance pollination while also adeptness of the pollinator to pollinate (an example of coevolution) Ex: long proboscis of an insect & tubular flowers The Angiosperm Life Cycle In the megasporangium of each ovule, megasporophytes produce four megaspores in which one undergoes mitosis to produce an embryo sac Anther in the stamen produces four microsporangia, which contain diploid microsporoctes that undergo meiosis to yield four haploid microspores ○ Microspores form pollen grains (male gametophyte) in which the generative cell forms two sperm & the tube cell makes the pollen tube Pollen grain transferred to stigma (pollination) where sperm meets the female gametophyte via a pollen tube Campbell Biology, 12th Edition Esparza 24 Neil A. Campbell, et.al Unit 6: Plant Form and Function ○ One sperm fertilizes the egg to form a diploid zygote while the other sperm combines with two polar nuclei, yielding a triploid endosperm Fertilized ovule develops into a nutrient-rich seed; ovary forms fruit (aids in protection & seed dispersal) ○ Seed germination produces a sporophyte, which resets the life cycle Seed Development and Structure Endosperm mitotically divides into a “supercell”, comprising the food supply of a seed Zygote divides into a terminal cell (becomes embryo) & a basal cell (forms suspensor) ○ Proembryo develops rudimentary organs (e.g. cotyledons, shoot/root apex) Seed consists of an embryo, food supply, and a protective seed coat ○ May enter dormancy & cease metabolism and growth ○ Eudicots consist of two cotyledons in which the hypocotyl terminates at the radicle (stem) and the epicotyl is above it Food supply stored in endosperm or cotyledons ○ Monocots contain a cotyledon (scutellum) that absorbs nutrients from endosperm & an embryo protected by the coleptile and coleorhiza Seed dormancy, broken by environmental cues, ensures optimal conditions for germin. Sporphyte Development from Seed to Mature plant Imbibition, water uptake due to a dry seed, intiates seed germination ○ Dicots: Radicle (embryonic root) → hypocotyl → cotyledon & foliage leaves → hypocotyl (pulls cotyledons out of the roots) ○ Monocots: radicle & coleoptile (helps plant grow straight up)→ foliage leaves Primary & secondary growth, faciiated by meristematic cells, forms plant organs and flowers (if cues cause a floral meristem to form) Campbell Biology, 12th Edition Esparza 25 Neil A. Campbell, et.al Unit 6: Plant Form and Function Fruit Structure and Function Fruit - mature ovary in a flower; protect seeds & aid in seed dispersal ○ Ovary wall may thicken to form the pericarp, wilt away, or harden Simple fruits - derived from simple carpels Aggregate fruits - derived from many separate carepls of one flower Multiple fruits develops from many cerpels formed by flower clusters (inflorescense) Accessory fruits - derived from non-ovary parts Concept 38.2 Flowering plants reproduce sexually, asexually, or both Asexual reproduction (vegatative production) generates a clone identical to the parent plant Mechanism of Asexual Reproduction Mehanisms include: ○ Fragmentation of plants into whole plants (e.g. root system of an aspen tree can give rise to shoots that become separate shoot systems) ○ Apomixis - diploid cell in ovule forms embryo & ovules form seeds (uses seed dispersal, a sexual reproduction mechanism) Campbell Biology, 12th Edition Esparza 26 Neil A. Campbell, et.al Unit 6: Plant Form and Function Advantages & Disadvantages of Asexual and Sexual Reproduction Allows for quick proliferation of “fit” genetic traits in stable environments ○ When unstable, sexual reproduction is better (for genetic variation) as identical progeny are subject to a bottleneck Self-perilization possible in crop plants, but not flowering plants Mechanisms That Prevent Self-Fertilization Mechanisms include: ○ Having make and female flowers on separate plants (dioecious species) ○ Stamen & styles of different heights on different plants (“pin” & “thrum” plants) ○ Self-incompatibility - biochemically rejecting pollen similar to itself Gametophytic self-incompatibility - S genes (recognizes “self” in plants) in pollen determine blocking of fertilization Sporophytic self-incompatibility - S genes in sporphytoc tissue & stigma Totipotent - a cell that can divide and sexually produce a genetic clone Vegetative propagation = human-induced vegetative production ○ In the cuttings, the wounded end contains totipotent cells called a callus Form between the scion (twig grafted on stock) & stock (provide roots) where cell differentiation complete unification of grafted individuals ○ In test-tube cloning, a root forms a callus, which differentiates into a new plant Concept 38.3 People modify crops by breeding and genetic engineering In plant breeding, farmers select desirable traits ○ Completed through cross-pollination & hybridization ○ Natural gene modification occurs via horizontal gene transfer in which a transgene is passed from one organism to another Biotech & genetic engineering can use intermediate species to produce genes from an extinct species, which can be incorporated into another organisms (GMOs) ○ Can increase crop yields, reducing world hunger and malnutrition ○ Biofortification increases the nutritional value of crops ○ Herbicid glyphosphate created an environmental pressure, producing resistance in bacteria Campbell Biology, 12th Edition Esparza 27 Neil A. Campbell, et.al Unit 6: Plant Form and Function Biofuels derived from biomass (total mass of organic matter in a group) can reabsorb CO2 in the atmosphere, reducing CO2 emissions Many criticisms (effects on hhuans health, nontagret organsism, and potential for transgenes escape) surround GMOs Chapter 39: Plant Responses to Internal and External Signals Concept 39.1 Signal transduction pathways link signal reception to response Etiolation - physical adaptations for growing in darkness ○ Ex: plant shoot with unexpanded leaves to preserve water & energy De-etiolation (“greening”) - acquires typical plant traits (e.g. expanded leaves) in light Reception Ligand binds to a receptor (e.g. phytochrome in light detection during de-etiolation) Transduction Amplification of a weak signal with the help of secondary messengers (e.g. Ca2+) ○ In response to light, guanylyl cyclase is activated, leading to rapid increase of Ca2+ Ex: role of phytochrome in de-etiolation (greening) response Campbell Biology, 12th Edition Esparza 28 Neil A. Campbell, et.al Unit 6: Plant Form and Function Response Activated pathway affects cellular functions ○ Transcriptional regulation of genes that encode specific proteins ○ Post-translational modifications in pre-existing proteins Resulting proteins may be involved in photosynthesis (e.g. chlorophyll production) Concept 39.2 Plants use chemicals to communicate Mobile information molecules (hormones) are internal regulators of plant growth Hormone - signaling molecule that is transported to specific parts of the body, where it binds to a target cell’s receptor and initiates a response ○ In plants, known as a plant growth regulator to remain inclusive ○ Plant hormones control growth and development Plant Hormones Auxin induces cell elongation, plant development, fruits growth; abundant in shoot tips and young leaves; polar movement from tip to base ○ Functions in tropism (response toward or away stimuli), specifically phototropism (toward or away light) and gravitropism ○ Acid growth hypothesis explains how auxin promotes cell wall acidification, which activates expansins that loosen the cell wall and make it flexible ○ Used as herbicides Campbell Biology, 12th Edition Esparza 29 Neil A. Campbell, et.al Unit 6: Plant Form and Function Cytokinins stimulate cell division + differentiation, modify apical dominance (with auxin & strigolactones), promote lateral bud formation, and slow plant aging ○ Produced mainly in roots and transported to other organs ○ In a callus, high auxin and low cytokinin = roots Gibberellins (GA) promote stem elongation, pollen tube formation, fruit growth, and seed germination (helps break dormancy & acquire nutrients) Abscisic Acid (ABA) inhibits plant growth ○ High levels stimulate seed dormancy, low levels result in germination Campbell Biology, 12th Edition Esparza 30 Neil A. Campbell, et.al Unit 6: Plant Form and Function ○ In droughts, it closes stomata to conserve water, preventing wilting Ethylene involved in response to mechanical stress, senescence, leaf abscission, and fruit ripening through a positive feedback ○ Stimulates the triple response (slowing of stem elongation + stem thickening + horizontal growth due to curvature) Brassinosteroids promote cell elongation, xylem differentiation, & inhibit leaf absiccion ○ Absiccion layer is where a leaf separates from the stem Jasmonates mediate plant defense & development ○ Ex: jasmonate (JA) and methyl jasomanate (MeJA) Strigolactones regulate apical dominance, seed germination, and mycorrhizae Plant Hormone Major Responses Auxin (IAA) Produced in shoot/root spical meristems & young leaves Found in developing seeds/fruits Stimulates cell elongation; regulates branch- ing and organ bending (phototropism and gravitropism); inhibits growth of auxiliary buds Cytokinins Produced in roots Stimulate plant cell division and differentia- tion; promote axillary bud growth; slows aging and deterioration Gibberellins Produced in meristems of apical buds/roots, young leaves, and developing seeds Promote stem elongation; help seeds break dormancy and use stored reserves; helps form the pollen tube Campbell Biology, 12th Edition Esparza 31 Neil A. Campbell, et.al Unit 6: Plant Form and Function Abscisic acid (ABA) Produced in almost all plant cells Promotes stomatal closure in response to drought; promotes seed dormancy Ethylene Produced by most parts of a leaf Mediates senescence, leaf abscission, fruit ripening, and obstacle avoidance by shoots (the triple response) Brassinosteroids Found in all plant cells Chemically similar to the sex hormones of animals; induce cell elongation and division Jasmonates Found in several parts of a lead Mediate plant defenses against insect herbi- vores; regulate a wide range of physiological processes Strigolactones Produced in roots due to low phosphate or high auxin Regulate apical dominance, seed germina- tion, and mycorrhizal associations Concept 39.3 Responses to light are critical for plant success Photomorphogenesis - plant development triggered by light Action spectrums allow scientists to determine which photoreceptors (pigments) are involved in a specific process Campbell Biology, 12th Edition Esparza 32 Neil A. Campbell, et.al Unit 6: Plant Form and Function ○ Blue-light photoreceptors - pigments that absorb blue wavelengths Involved in phototropism (controlled by phototropin), stomatal opening, & hypocotyl elongation ○ Phytochrome photoreceptors absorb red (Pr) & far-red (Pfr) wavelengths Red stimulates seed germination while far-red inhibits it Last flash of light determines response; effects are reversible Mediated by Pr ⟷ Pfr interconversion ○ High ratio of Pfr to Pr = light = germination ○ Low ratio of Pfr to Pr = dark = no germination Helps detect light & initiate “shade avoidance” in high light intensity Circadian rhythms are internal 24-hour periods unaffected by the environment ○ Constant conditions = deviation from 24 hours (in rhythms, not clockwork) ○ Controlled by (-) feedback loops involving “clock genes” & transcription factors ○ Light photoreceptors (mentioned above) determine these rhythms Photoperiodism and Responses to Seasons Photoperiodism - physiological response to day (photoperiods) & night lengths ○ Short-day plants require a light period shorter than a critical length ○ Long-day plants require a light period longer than a critical length ○ Day-neutral plants flower at maturity, regardless of photoperiod Flowering in short-day (long-night) & long-day (short-night) plants controlled by night lengths ○ Short-day: night > critical dark period = no flowering ○ Long-day: night < critical dark period = flowering ○ Dark periods interrupted by red light, but reversed by far-red light Campbell Biology, 12th Edition Esparza 33 Neil A. Campbell, et.al Unit 6: Plant Form and Function Vernalization - treatment of cold to induce flowering (e.g. in winter wheat) Florigen (encoded by FLOWERING LOCUS (FT) gene) induces flowering in grafted short-day & long-day plants ○ Flowering plant induces flowering in non-flowering plant Concept 39.4 Plants respond to a wide variety of stimuli other than light Gravity Gravitropism - response to gravity ○ Positive gravitropism in roots and negative gravitropism in shoots Campbell Biology, 12th Edition Esparza 34 Neil A. Campbell, et.al Unit 6: Plant Form and Function ○ Detected using the settling of statoliths (starch-filled plastids) to lower parts of the cytoplasm Mechanical Stimuli Thigmomorphogenesis - response to mechanical stresses ○ May result in rapid leaf movements, ○ Response spreads via electrical impulses (action potentials) Environmental Stresses Environmental Stress Major Response Drought Reduce water loss by stomatal closing, ABA production Campbell Biology, 12th Edition Esparza 35 Neil A. Campbell, et.al Unit 6: Plant Form and Function Flooding Formation of air tubes (via ethylene that kills root cortec cells) that help roots survive oxygen deprivation Salt Avoiding osmotic water loss by producing solutes tolerated at high concentrations (halophytes adapted with salt glands). Heat Heat-shock proteins produced, which reduce protein denaturation at high temperatures; stomatal closing Cold Adjusting membrane fludiityl avidoung osmotic water loss; producing antifreeze proteins Concept 39.5 Plants respond to attacks by pathogens and herbivores Biotic stresses such as herbivores & pathogen necessitated defense systems Defenses Against Pathogens First line of defense is the physical barriers of the epidermis and periderm When viruses invade the plant: ○ Pathogen-associated molecular patterns (PAMPs)-triggered immunity identify PAMPs using toll-like receptorsfound in viruses and bacteria Leads to production of phytoalexins and cell wall toughening Considered a plant’s innate immune system ○ Effector-triggered immunity was created due to the presence effectors (pathogen-encoded proteins) Disease resistance (R) genes encode R proteins that are activated by effectors, which induce local (hypersensitive response) or general (systematic response) defense Hypersensitive response creates localized lesions (brown spots) and specific as it attacks a pathogen’s cell wall, metabolism, and reproduction. Campbell Biology, 12th Edition Esparza 36 Neil A. Campbell, et.al Unit 6: Plant Form and Function Systematic acquired resistance is nonspecific and produces salicylic acid (activates a pathway that responds to infection) Defenses Against Herbivores Herbivory - animals eating plants ○ Creates openings for infection, stunts growth, etc. ○ Physical (e.g. thorns) & chemical (e.g. toxic compounds) defenses combat this Factor Example of plant response Light Seed germination in response to red light