Fundamentals of Plant Physiology

Description

Fundamentals of Plant Physiology is a distillation of the most important principles and empirical findings of plant physiology. Intended for students seeking an accessible introduction to the field, Fundamentals provides the same high standard of scientific accuracy and pedagogical richness for which Taiz et al.'s popular Plant Physiology and Development, Sixth Edition, is renowned, but in a more concise format and pitched at a slightly lower level. Instructors wishing to focus their lectures on basic plant physiology, as opposed to developmental genetics, should find this new streamlined and simplified text ideally suited for their courses. Cover Brief Contents Table of Contents Preface 1 Plant and Cell Architecture 1-1 Plant Life Processes: Unifying Principles 1-1 1-2 Plant Classification and Life Cycles Box 1.1 Evolutionary Relationships among Plants Plant life cycles alternate between diploid and haploid generations 1-3 Overview of Plant Structure Plant cells are surrounded by rigid cell walls Primary and secondary cell walls differ in their components The cellulose microfibrils and matrix polymers are synthesized via different mechanisms Plasmodesmata allow the free movement of molecules between cells New cells originate in dividing tissues called meristems Box 1.2 The Secondary Plant Body 1-4 Plant Cell Types Dermal tissue covers the surfaces of plants Ground tissue forms the bodies of plants Vascular tissue fom1s transport networks between different parts of the plant 1-5 Plant Cell Organelles Biological membranes are bilayers that contain proteins 1-6 The Nucleus Gene expression involves both transcription and translation Posttranslational regulation determines the life span of proteins 1-7 The Endomembrane System The endoplasmic reticulum is a network of internal membranes Vacuoles have diverse functions in plant cells Oil bodies are lipid-storing organelles Microbodies play specialized metabolic roles in leaves and seeds 1-8 Independently Dividing Semiautonomous Organelles Proplastids mature into specialized plastids in different plant tissues Chloroplast and mitochondrial division are independent of nuclear division 1-9 The Plant Cytoskeleton The plant cytoskeleton consists of microtubules and microfilaments Actin, tubulin, and their polymers are in constant flux in the living cell Microtubule protofilaments first assemble into flat sheets before curling into cylinders Cytoskeletal motor proteins mediate cytoplasmic streaming and directed organelle movement 1-10 Cell Cycle Regulation Each phase of the cell cycle has a specific set of biochemical and cellular activities The cell cycle is regulated by cyclins and cyclin- dependent kinases Mitosis and cytokinesis involve both microtubules and the endomembrane system Summary 2 Water and Plant Cells 2-1 Water in Plant Life 2-2 The Structure and Properties of Water Water is a polar molecule that forms hydrogen bonds Water is an excellent solvent Water has distinctive thermal properties relative to its size Water molecules are highly cohesive Water has a high tensile strength 2-3 Diffusion and Osmosis Diffusion is the net movenent of molecules by random thermal agitation Diffusion is most effective over short distances Osmosis describes the net movement of water across a selectively permeable barrier 2-4 Water Potential The chemical potential of water represents the free-energy status of water Three major factors contribute to cell water potential Water potentials can be ,measured 2-5 Water Potential of Plant Cells Water enters the cell along a water potential gradient Water can also leave the cell in response to a water potential gradient Water potential and its components vary with growth conditions and location within the plant 2-6 Cell Wall and Membrane Properties Small changes in plant cell volume cause large changes in turgor pressure The rate at which cells gain or lose water is influenced by plasma membrane hydraulic conductivity Aquaporins facilitate the movement of water across plasma membranes 2-7 Plant Water Status Physiological processes are affected by plant water status Solute accumulation helps cells maintain turgor and volume Summary 3 Water Balance of Plants 3-1 Water in the Soil A negative hydrostatic pressure in soil water lowers soil water potential Water moves through the soil by bulk flow 3-2 Water Absorption by Roots Water moves in the root via the apoplast, symplast, and transmembrane pathways Solute accumulation in the xylem can generate "root pressure" 3-3 Water Transport through the Xylem The xylem consists of two types of transport cells Water moves through the xylem, by pressure-driven bulk flow Water movement through the xylem requires a smaller pressure gradient than movement through living cells What pressure difference is needed to lift water 100 meters to a treetop? The cohesion-tension theory explains water transport in the xylem Xylem transport of water in trees faces physical changes Plants minimize the consequences of xylem cavitation 3-4 Water Movement from the Leaf to the Atmosphere Leaves have a large hydraulic resistance The driving force for transpiration is the difference in water vapor concentration Water loss is also regulated by the pathway resistances The boundary layer contributes to diffusion resistance Stomata resistance is another major component of diffusional resistance The cell walls of guard cells have specialized features An increase in guard cell turgor pressure opens the stomata 3-5 Coupling Leaf Transpiration and Photosynthesis: Light-dependent Stomatal Opening Stomatal opening is regulated by light Stomatal opening is specifically regulated by blue light 3-6 Water-use efficiency 3-7 Overview: The Soil- Plant- Atmosphere Continuum Summary 4 Mineral Nutrition 4-1 Essential Nutrients, Deficiencies, and Plant Disorders Special techniques are used in nutritional studies Nutrient solutions can sustain rapid plant growth Mineral deficiencies disrupt plant metabolism and function Analysis of plant tissues reveals mineral deficiencies 4-2 Treating Nutritional Deficiencies Crop yields can be improved by the addition of fertilizers Some mineral nutrients can be absorbed by leaves 4-3 Soil, Roots, and Microbes Negatively charged soil particles affect the adsorption of mineral nutrients Soil pH affects nutrient availability, soil microbes, and root growth Excess mineral ions in the soil limit plant growth Some plants develop extensive root systems Root systems differ in form but are based on conmmon structures Different areas of the root absorb different mineral ions Nutrient availability influences root growth Mycorrhizal symbioses facilitate nutrient uptake by roots Nutrients move between mycorrhizal fungi and root cells Summary 5 Assimilation of Inorganic Nutrients 5-1 Nitrogen in the Environment Nitrogen passes through several forms in a biogeochemical cycle Unassimilated ammonium or nitrate may be dangerous 5-2 Nitrate Assimilation Many factors regulate nitrate reductase Nitrite reductase converts nitrite to ammonium Both roots and shoots assimilate nitrate 5-3 Ammonium Assimilation Converting ammonium to amino acids requires two enzymes Ammonium can be assimilated via an alternative pathway Transamination reactions transfer nitrogen Asparagine and glutamine link carbon and nitrogen metabolism 5-4 Amino Acid Biosynthesis 5-5 Biological Nitrogen Fixation Free-living and symbiotic bacteria fix nitrogen Nitrogen fixation requires microanaerobic or anaerobic conditions Symbiotic nitrogen fixation occurs in specialized structures Establishing symbiosis requires an exchange of signals Nod factors produced by bacteria act as signals for symbiosis Nodule formation involves phytohormones The nitrogenase enzyme complex fixes N2 Amides and ureides are the transported forms of nitrogen 5-6 Sulfur Assimilation Sulfate is the form of sulfur transported into plants Sulfate assimilation occurs mostly in leaves Methionine is synthesized from cysteine 5-7 Phosphate Assimilation 5-8 Iron Assimilation Roots modify the rhizosphere to acquire iron Iron cations form complexes with carbon and phosphate 5-9 The Energetics of Nutrient Assimilation Summary 6 Solute Transport 6-1 Passive and Active Transport 6-2 Transport of Ions across Membrane Barriers Different diffusion rates for cations and anions produce diffusion potentials How does membrane potential relate to ion distribution? The Nernst equation distinguishes between active and passive transport Proton transport is a major determinant of the membrane potential 6-3 Membrane Transport Processes Channels enhance diffusion across membranes Carriers bind and transport specific substances Primary active transport requires energy Secondary active transport uses stored energy Kinetic analyses can elucidate transport mechanisms 6-4 Membrane Transport Proteins The genes for many transporters have been identified Transporters exist for diverse nitrogen-containing compounds Cation transporters are diverse Anion transporters have been identified Transporters for metal and metalloid ions transport essential micronutrients Aquaporins have diverse functions Plasma membrane tt+ -ATPases are highly regulated P-type ATPases The tonoplast H+ -ATPase drives solute accumulation in vacuoles H+ -pyrophosphatases a !so pump protons at the tonoplast 6-5 Ion Transport in Stomatal Opening Light stimulates ATPase activity and creates a stronger electrochemical gradient across the guard cell plasma membrane Hyperpolarization of the guard cell plasma membrane leads to uptake of ions and water 6-6 Ion Transport in Roots Solutes n1ove through both apoplast and symplast Ions cross both symplast and apoplast Xylem parenchyma cells participate in xylem loading Summary 7 Photosynthesis: The Light Reactions 7-1 Photosynthesis in Higher Plants 7-2 General Concepts Light has characteristics of both a particle and a wave When molecules absorb or emit light, they change their electronic state Photosynthetic pigments absorb the light that powers photosynthesis 7-3 Key Experiments in Understanding Photosynthesis Action spectra relate light absorption to photosynthetic activity Photosynthesis takes place in complexes containing light-harvesting antennas and photochemical reaction centers The chemical reaction of photosynthesis is driven by light Light drives the reduction of NADP+ and the formation of ATP Oxygen-evolving organisms have two photosystems that operate in series 7-4 Organization of the Photosynthetic Apparatus The chloroplast is the site of photosynthesis Thylakoids contain integral membrane proteins Photosystems I and II are spatially separated in the thylakoid membrane 7-5 Organization of Light-Absorbing Antenna Systems Antenna systems contain chlorophyll and are membrane-associated The antenna funnels energy to the reaction center Many antenna pigment-protein complexes have a common structural motif 7-6 Mechanisms of Electron Transport Electrons from chlorophyll travel through the carriers organized in the Z scheme Energy is captured when an excited chlorophyll reduces an electron acceptor molecule The reaction center chlorophylls of the two photosystems absorb at different wavelengths The PSII reaction center is a multi-subunit pigment-protein complex Water is oxidized to oxygen by PSII Pheophytin and two qui nones accept electrons from PSII Electron flow through the cytochrome b6f complex also transports protons Plastoquinone and plastocyanin carry electrons between photosystem II and photosystem I The PSI reaction center reduces NADp+ Cyclic electron flow generates ATP but no NADPH Some herbicides block photosynthetic electron flow 7-7 Proton Transport and ATP Synthesis in the Chloroplast Summary 8 Photosynthesis: The Carbon Reactions 8-1 The Calvin- Benson Cycle The Calvin-Benson cycle has three phases: carboxylation, reduction, and regeneration The fixation of CO2 via carboxylation of ribulose 1,5-bisphosphate and the reduction of the product 3-phosphoglycerate yield triose phosphates The regeneration of ribulose 1,5-bisphosphate ensures the continuous assimilation of CO2 An induction period precedes the steady state of photosynthetic CO2 assimilation Many mechanis.ms regulate the Calvin-Benson cycle Rubisco activase regulates the catalytic activity of Rubisco Light regulates the Calvin-Benson cycle via the ferredoxin-thioredoxin system Light-dependent ion movements modulate enzymes of the Calvin-Benson cycle 8-2 Photorespiration: The C2 Oxidative Photosynthetic Carbon Cycle The oxygenation of ribulose 1,5-bisphosphate sets in motion the C2 oxidative photosynthetic carbon cycle Photorespiration is linked to the photosynthetic electron transport chain 8-3 Inorganic Carbon-Concentrating Mechanisms 8-4 Inorganic Carbon-Concentrating Mechanisms: The C4 Carbon Cycle Malate and aspartate are the primary carboxylation products of the C4 cycle The C4 cycle assimilates CO2 by the concerted action of two different types of cells Bundle sheath cells and mesophyll cells exhibit anatomica I and biochemical differences The C4 cycle also concentrates CO2 in single cells Light regulates the activity of key C4 enzymes Photosynthetic assimilation of CO2 in C4 plants den1ands more transport processes than in C3 plants In hot, dry climates, the C4 cycle reduces photorespiration 8-5 Inorganic Carbon-Concentrating Mechanisms: Crassulacean Acid Metabolism (CAM) Different mechanisms regulate C4 PEPCase and CAM PEPCase CAM is a versatile mechanism, sensitive to environmental stimuli 8-6 Accumulation and Partitioning of Photosynthates-Starch and Sucrose Summary 9 Photosynthesis: Physiological and Ecological Considerations 9-1 The Effect of leaf Properties on Photosynthesis Leaf anatomy and canopy structure maximize light absorption Leaf angle and leaf movement can control light absorption Leaves acclimate to sun and shade environments 9-2 Effects of light on Photosynthesis in the Intact leaf Light-response curves reveal photosynthetic properties Leaves must dissipate excess light energy Absorption of too much light can lead to photoinhibition 9-3 Effects of Temperature on Photosynthesis in the Intact leaf Leaves must dissipate vast quantities of heat There is an optimal temperature for photosynthesis Photosynthesis is sensitive to both high and low temperatures Photosynthetic efficiency is temperature-sensitive 9-4 Effects of Carbon Dioxide on Photosynthesis in the Intact leaf Atmospheric CO2 concentration keeps rising CO2 diffusion to the chloroplast is essential to photosynthesis CO2 imposes limitations on photosynthesis How will photosynthesis and respiration change in the future under elevated CO2 conditions? Summary 10 Translocation in the Phloem 10-1 Patterns of Translocation: Source to Sink 10-2 Pathways of Translocation Sugar is translocated in phloem sieve elements Mature sieve elements are living cells specialized for translocation Large pores in cell walls are the prominent feature of sieve elements Damaged sieve elements are sealed off Companion cells aid the highly specialized sieve elements 10-3 Materials Translocated in the Phloem Phloem sap can be collected and analyzed Sugars are translocated in a nonreducing form Other solutes are translocated in the phloem 10-4 Rates of Movement 10-5 The Pressure-Flow Model, a Passive Mechanism for Phloem Transport An osmotically generated pressure gradient drives translocation in the pressure-flow model Some predictions of pressure flow have been confirmed, while others require further experimentation There is no bidirectional I transport in single sieve elements, and solutes and water move at the same velocity The energy requirement for transport through the phloem pathway is small in herbaceous plants Sieve plate pores appear to be open channels Pressure gradients in the sieve elements may be modest; pressures in herbaceous plants and trees appear to be similar 10-6 Phloem Loading Phloem loading can occur via the apoplast or symplast Abundant data support the existence of apoplastic loading in some species Sucrose uptake in the apoplastic pathway requires metabolic energy Phloem loading in the apoplastic pathway involves a sucrose-H+ symporter Phloen, loading is symplastic in some species The polymer-trapping model explains symplastic loading in plants with intermediary-type companion cells Phloem loading is passive in several tree species 10-7 Phloem Unloading and Sink-to-Source Transition Phloem unloading and short-distance transport can occur via symplastic or apoplastic pathways Transport into sink tissues requires metabolic energy The transition of a leaf from sink to source is gradual 10-8 Photosynthate Distribution: Allocation and Partitioning Allocation includes storage, utilization, and transport Various sinks partition transport sugars Source leaves regulate allocation Sink tissues compete for avaiLable translocated photosynthate Sink strength depends on sink size and activity The source adjusts over the long term to changes in the source-to-sink ratio 10-9 Transport of Signaling Molecules Turgor pressure and chemical signals coordinate source and sink activities There is no bidirectional transport in single sieve elements, and solutes and water move at the same velocity Plasmodesmata function in phloem signaling Summary 11 Respiration and Lipid Metabolism 11-1 Overview of Plant Respiration 11-2 Glycolysis Glycolysis metabolizes carbohydrates from several sources The energy-conserving phase of glycolysis extracts usable energy Plants have alternative glycolytic reactions In the absence of oxygen, fermentation regenerates the NAD+ needed for glycolytic ATP production 11-3 The Oxidative Pentose Phosphate Pathway The oxidative pentose phosphate pathway produces NADPH and biosynthetic intermediates The oxidative pentose phosphate pathway is redox-regulated 11-4 The Tricarboxylic Acid Cycle Mitochondria are semiautonomous organelles Pyruvate enters the mitochondrion and is oxidized via the TCA cycle The TCA cycle of plants has unique features 11-5 Mitochondrial Electron Transport and ATP Synthesis The electron transport chain catalyzes a flow of electrons from NADH to O2 The electron transport chain has supplen,entary branches ATP synthesis in the mitochondrion is coupled to electron transport Transporters exchange substrates and products Aerobic respiration yields about 60 molecules of ATP per molecule of sucrose Plants have several mechanisms that lower the ATP yield Short-term control of mitochondrial respiration occurs at different levels Respiration is tightly coupled to other pathways 11-6 Respiration in Intact Plants and Tissues Plants respire roughly half of the daily photosynthetic yield Respiratory processes operate during photosynthesis Different tissues and organs respire at different rates Environmental factors alter respiration rates 11-7 Lipid Metabolism Fats and oils store large amounts of energy Triacylglycerols are stored in oil bodies Polar glycerolipids are the main structural lipids in membranes Membrane lipids are precursors of important signaling compounds Storage lipids are converted into carbohydrates in germinating seeds, releasing stored energy Summary 12 Signals and Signal Transduction 12-1 Temporal and Spatial Aspects of Signaling 12-2 Signal Perception and Amplification Signals must be amplified intracellularly to regulate their target molecules Ca2+ is the most ubiquitous second messenger in plants and other eukaryotes Changes in the cytosolic or cell wall pH can serve as second messengers for hormonal and stress responses Reactive oxygen species act as second messengers mediating both environmental and developmental signals 12-3 Hormones and Plant Development Auxin was discovered in early studies of coleoptile bending during phototropism Gibberellins promote stem growth and were discovered in relation to the "foolish seedling disease" of rice Cytokinins were discovered as cell division-promoting factors in tissue culture experiments Ethylene is a gaseous hormone that promotes fruit ripening and other developmental processes Abscisic acid regulates seed maturation and stomatal closure in response to water stress Brassinosteroids regulate floral sex determination, photomorphogenesis, and germination Salicylic acid and jasmonates function in defense responses Strigolactones suppress branching and promote rhizosphere interactions 12-4 Phytohormone Metabolism and Homeostasis Indole-3-pyruvate is the primary intermediate in auxin biosynthesis Gibberellins are synthesized by oxidation of the diterpene ent-kaurene Cytokinins are adenine derivatives with isoprene side chains Ethylene is synthesized from methionine via the intermediate ACC Abscisic acid is synthesized from a carotenoid intermediate Brassinosteroids are derived from the sterol campesterol Strigolactones are synthesized from ￎﾲ-carotene 12-5 Signal Transmission and Cell-Cell Communication 12-6 Hormonal Signaling Pathways The cytokinin and ethylene signal transduction pathways are derived from the bacterial two-component regulatory system Receptor-like kinases mediate brassinosteroid signaling The core ABA signaling components include phosphatases and kinases Plant hormone signaling pathways generally employ negative regulation Protein degradation via ubiquitination plays a prominent role in hormone signaling Plants have mechanisms for switching off or attenuating signaling responses The cellular response output to a signal is often tissue-specific Cross-regulation allows signal transduction pathways to be integrated Summary 13 Signals from Sunlight 13-1 Plant Photoreceptors Photoresponses are driven by light quality or spectral properties of the energy absorbed Plant responses to light can be distinguished by the amount of light required 13-2 Phytochromes Phytochrome is the primary photoreceptor for red and far-red light Phytochrome can interconvert between Pr and Pfr forms 13-3 Phytochrome Responses Phytochrome responses vary in lag time and escape time Phytochrome responses fall into three n1ain categories based on the amount of light required Phytochrome A mediates responses to continuous far-red light Phytochrome regulates gene expression 13-4 Blue-Light Responses and Photoreceptors Blue-light responses have characteristic kinetics and lag times 13-5 Cryptochromes Blue-light irradiation of the cryptochrome FAD chromophore causes a conformational change The nucleus is a primary site of cryptochrome action Cryptochrome interacts with phytochrome 13-6 Phototropins Phototropism requires changes in auxin mobilization Phototropins regulate chloroplast movements Stomatal opening is regulated by blue light, which activates the plasma membrane H+-ATPase 13-7 The Coaction of Phytochrome, Cryptochrome, and Phototropins 13-8 Responses to Ultraviolet Radiation Summary 14 Embryogenesis 14-1 Overview of Embryogenesis 14-2 Comparative Embryology of Eudicots and Monocots Morphological similarities and differences between eudicot and monocot embryos dictate their respective patterns of development Apical-basal polarity is maintained in the embryo during organogenesis Embryo development requires regulated communication between cells Auxin signaling is essential for embryo development Polar auxin transport is mediated by localized auxin efflux carriers Auxin synthesis and polar transport regulate embryonic development Radial patterning guides formation of tissue layers The protoderm differentiates into the epidermis The central vascular cylinder is elaborated by cytokinin-regulated progressive cell divisions 14-3 Formation and Maintenance of Apical Meristems Auxin and cytokinin contribute to the formation and maintenance of the RAM SAM formation is also influenced by factors involved in auxin movement and responses Cell proliferation in the SAM is regulated by cytokinin and gibberellin Summary 15 Seed Dormancy, Germination, and Seedling Establishment 15-1 Seed Structure Seed anaton,y varies widely among different plant groups 15-2 Seed Dormancy There are two basic types of seed dormancy mechanisms: exogenous and endogenous Non-dormant seeds can exhibit vivipary and precocious germination The ABA:GA ratio is the primary determinant of seed dormancy 15-3 Release from Dormancy Light is an important signal that breaks dormancy in small seeds Some seeds require either chilling or after-ripening to break dormancy Seed dormancy can be broken by various chemical compounds 15-4 Seed Germination Germination and postgermination can be divided into three phases corresponding to the phases of water uptake 15-5 Mobilization of Stored Reserves The cereal aleurone layer is a specialized digestive tissue surrounding the starchy endosperm 15-6 Seedling Establishment The development of emerging seedlings is strongly influenced by light Gibberellins and brassinosteroids both suppress photomorphogenesis in darkness Hook opening is regulated by phytochrome, auxin, and ethylene Vascular differentiation begins during seedling emergence Growing roots have distinct zones Ethylene and other hormones regulate root hair development Lateral roots arise internally from the pericycle 15-7 Cell Expansion: Mechanisms and Hormonal Controls The rigid primary cell wall must be loosened for cell expansion to occur Microfibril orientation influences growth directionality of cells with diffuse growth Acid-induced growth and cell wall yielding are mediated by expansins Auxin promotes growth in stems and coleoptiles, while inhibiting growth in roots The outer tissues of eudicot stems are the targets of auxin action The o'linimum lag tin1e for auxin-induced elongation is 10 minutes Auxin-induced proton extrusion loosens the cell wall Ethylene affects microtubule orientation and induces lateral cell expansion 15-8 Tropisms: Growth in Response to Directional Stimuli Auxin transport is polar and gravity-independent The Cholodny-Went hypothesis is supported by auxin movements and auxin responses during gravitropic growth Gravity perception is triggered by the sedimentation of amyloplasts Gravity sensing may involve pH and calcium ions (Ca T) as second messengers Phototropins are the tight receptors involved in phototropism Phototropism is mediated by the lateral redistribution of auxin Shoot phototropism occurs in a series of steps Summary 16 Vegetative Growth and Senescence 16-1 The Shoot Apical Meristem The shoot apical meristem has distinct zones and layers 16-2 Leaf Structure and Phyllotaxy Auxin-dependent patterning of the shoot apex begins during embryogenesis 16-3 Differentiation of Epidermal Cell Types A specialized epidermal lineage produces guard cells 16-4 Venation Patterns in Leaves The primary leaf vein is initiated in the leaf primordium Auxin canalization initiates development of the leaf trace 16-5 Shoot Branching and Architecture Auxin, cytokinins, and strigolactones regulate axillary bud outgrowth The initial signal for axillary bud growth may be an increase in sucrose availability to the bud 16-6 Shade Avoidance Reducing shade avoidance responses can improve crop yields 16-7 Root System Architecture Plants can modify their root system architecture to optimize water and nutrient uptake Monocots and eudicots differ in their root system architecture Root system architecture changes in response to phosphorus deficiencies 16-8 Plant Senescence During leaf senescence, nutrients are remobilized from the source leaf to vegetative or reproductive sinks The developmental age of a leaf may differ from its chronological age Leaf senescence may be sequential, seasonal, or stress-induced The earliest cellular changes during leaf senescence occur in the chloroplast Reactive oxygen species serve as internal signaling agents in leaf senescence Plant hormones interact in the regulation of leaf senescence 16-9 Leaf Abscission The timing of leaf abscission is regulated by the interaction of ethylene and auxin 16-10 Whole Plant Senescence Angiosperm life cycles may be annual, biennial, or perennial Nutrient or hormonal redistribution may trigger senescence in monocarpic plants Summary 17 Flowering and Fruit Development 17-1 Floral Evocation: Integrating Environmental Cues 17-2 The Shoot Apex and Phase Changes Plant development has three phases Juvenile tissues are produced first and are located at the base of the shoot Phase changes can be influenced by nutrients, gibberellins, and epigenetic regulation 17-3 Photoperiodism: Monitoring Day Length Plants can be classified according to their photoperiodic responses Photoperiodism is one of many plant processes controlled by a circadian rhythm Circadian rhythms exhibit characteristic features Circadian rhythms adjust to different day-night cycles The leaf is the site of perception of the photoperiodic signal Plants monitor day length by measuring the length of the night Night breaks can cancel the effect of the dark period Photoperiodic timekeeping during the night depends on a circadian clock A coincidence model links oscillating light sensitivity and photoperiodism Phytochrome is the primary photoreceptor in photoperiodism 17-4 Vernalization: Promoting Flowering with Cold 17-5 Long-distance Signaling Involved in Flowering Gibberellins and ethylene can induce flowering 17-6 Floral Meristems and Floral Organ Development The SAM in Arabidopsis changes with development The four different types of floral organs are initiated as separate whorls Two major categories of genes regulate floral development The ABC model partially explains the determination of floral organ identity 17-7 Pollen Development 17-8 Female Gametophyte Development in the Ovule Functional megaspores undergo a series of free nuclear mitotic divisions followed by cellularization 17-9 Pollination and Double Fertilization in Flowering Plants Two sperm cells are delivered to the female gametophyte by the pollen tube Pollination begins with adhesion and hydration of a pollen grain on a compatible flower Pollen tubes grow by tip growth Double fertilization results in the formation of the zygote and the primary endosperm cell 17-10 Fruit Development and Ripening Arabidopsis and tomato are model systems for the study of fruit development Fleshy fruits undergo ripening Ripening involves changes in the color of fruit Fruit softening involves the coordinated action of many cell wall-degrading enzymes Taste and flavor reflect changes in acids, sugars, and aroma compounds The causal link between ethylene and ripening was demonstrated in transgenic and mutant tomatoes Climacteric and non-climacteric fruits differ in their ethylene responses Summary 18 Biotic Interactions 18-1 Beneficial Interactions between Plants and Microorganisms Other types of rhizobacteria can increase nutrient availability, stimulate root branching, and protect against pathogens 18-2 Harmful Interactions of Pathogens and Herbivores with Plants Mechanical barriers provide a first line of defense against insect pests and pathogens Specialized plant metabolites can deter insect herbivores and pathogen infection Plants store constitutive toxic compounds in specialized structures Plants often store defensive chemicals as nontoxic water-soluble sugar conjugates in specialized vacuoles 18-3 Inducible Defense Responses to Insect Herbivores Plants can recognize specific components of insect saliva Phloem feeders activate defense signaling pathways similar to those activated by pathogen infections Jasmonic acid activates defense responses against insect herbivores Hormonal interactions contribute to plant-insect herbivore interactions JA initiates the production of defense proteins that inhibit herbivore digestion Herbivore damage induces systemic defenses Long-distance electrical signaling occurs in response to insect herbivory Herbivore-induced volatiles can repel herbivores and attract natural enemies Herbivore-induced volatiles can serve as long-distance signals within and between plants Insects have evolved mechanisms to defeat plant defenses 18-4 Plant Defenses against Pathogens Microbial pathogens have evolved various strategies to invade host plants Pathogens produce effector molecules that aid in the colonization of their plant host cells Pathogen infection can give rise to molecular "danger signals" that are perceived by cell surface pattern recognition receptors (PRRs) R proteins provide resistance to individual pathogens by recognizing strain-specific effectors The hypersensitive response is a common defense against pathogens A single encounter with a pathogen may increase resistance to future attacks 18-5 Plant Defenses against Other Organisms Some plant parasitic nematodes form specific associations through the formation of distinct feeding structures Plants compete with other plants by secreting allelopathic secondary metabolites into the soil Some plants are biotrophic pathogens of other plants Summary 19 Abiotic Stress 19-1 Defining Plant Stress Physiological adjustment to a biotic stress involves trade-offs between vegetative and reproductive development 19-2 Acclimation versus Adaptation 19-3 Environmental Stressors Water deficit decreases turgor pressure, increases ion toxicity, and inhibits photosynthesis Salinity stress has both osmotic and cytotoxic effects Temperature stress affects a broad spectrum of physiological processes Flooding results in anaerobic stress to the root Light stress can occur when shade-adapted or shade-acclimated plants are subjected to full sunlight Heavy metal ions can both mimic essential mineral nutrients and generate ROS Combinations of abiotic stresses can induce unique signaling and metabolic pathways Sequential exposure to different a biotic stresses sometimes confers cross-protection Plants use a variety of mechanisms to sense abiotic stress 19-4 Physiological Mechanisms That Protect Plants against Abiotic Stress Plants can alter their morphology in response to abiotic stress Metabolic shifts enable plants to cope with a variety of abiotic stresses Heat shock proteins maintain protein integrity under stress conditions Membrane lipid composition can adjust to changes in temperature and other abiotic stresses Chloroplast genes respond to high-intensity light by sending stress signals to the nucleus A self-propagating wave of ROS mediates systen1ic acquired acclimation Abscisic acid and cytokinins are stress-response hormones that regulate drought responses Plants adjust osmotically to drying soil by accumulating solutes Epigenetic mechanisms and small RNAs provide additional protection against stress Submerged organs develop aerenchyma tissue in response to hypoxia Antioxidants and ROS-scavenging pathways protect cells from oxidative stress Exclusion and internal tolerance mechanisms allow plants to cope with toxic metal and metalloid ions Plants use cryoprotectant molecules and antifreeze proteins to prevent ice crystal formation Summary Glossary Illustration Credits Index