37 plantnutrition text

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Plant Nutrition

• Plants require certain chemical elements to complete their life cycle

• Plants derive most of their organic mass from the CO2 of air

– But they also depend on soil nutrients such as water and minerals

Figure 37.2

CO2, the sourceof carbon for

Photosynthesis,diffuses into

leaves from theair through

stomata.

Throughstomata, leavesexpel H2O andO2.

H2O

O2

CO2

Roots take inO2 and expelCO2. The plantuses O2 for cellularrespiration but is a net O2 producer.

O2

CO2

H2O

Roots absorbH2O and

minerals fromthe soil.

Minerals


Macronutrients and Micronutrients

• More than 50 chemical elements

– Have been identified among the inorganic substances in plants, but not all of these are essential

• A chemical element is considered essential

– If it is required for a plant to complete a life cycle


• Researchers use hydroponic culture

– To determine which chemicals elements are essential

Figure 37.3

TECHNIQUE Plant roots are bathed in aerated solutions of known mineral composition. Aerating the water provides the roots with oxygen for cellular respiration. A particular mineral, such as potassium, can be omitted to test whether it is essential.

RESULTS If the omitted mineral is essential, mineral deficiency symptoms occur, such as stunted growth and discolored leaves. Deficiencies of different elements may have different symptoms, which can aid in diagnosing mineral deficiencies in soil.

Control: Solutioncontaining all minerals

Experimental: Solutionwithout potassium

APPLICATION In hydroponic culture, plants are grown in mineral solutions without soil. One use of hydroponic culture is to identify essential elements in plants.


• Essential elements in plants

Table 37.1


• Nine of the essential elements are called macronutrients

– Because plants require them in relatively large amounts (CHNOPS plus K, Ca, and Mg)

• The remaining eight essential elements are known as micronutrients

– Because plants need them in very small amounts (function mainly as cofactors, Fe, Mn, Cu, and Zinc)


Symptoms of Mineral Deficiency

• The symptoms of mineral deficiency

– Depend partly on the nutrient’s function

– Depend on the mobility of a nutrient within the plant


• The most common deficiencies

– Are those of nitrogen, potassium, and phosphorus

Figure 37.4

Phosphate-deficient

Healthy

Potassium-deficient

Nitrogen-deficient


• Soil quality is a major determinant of plant distribution and growth

• Along with climate

– The major factors determining whether particular plants can grow well in a certain location are the texture and composition of the soil

• Texture

– Is the soil’s general structure

• Composition

– Refers to the soil’s organic and inorganic chemical components


Fertilizers

• Commercially produced fertilizers

– Contain minerals that are either mined or prepared by industrial processes

• “Organic” fertilizers

– Are composed of manure, fishmeal, or compost


• Nitrogen is often the mineral that has the greatest effect on plant growth

• Plants require nitrogen as a component of

– Proteins, nucleic acids, chlorophyll, and other important organic molecules


Soil Bacteria and Nitrogen Availability

• Nitrogen-fixing bacteria convert atmospheric N2

– To nitrogenous minerals that plants can absorb as a nitrogen source for organic synthesis

Figure 37.9

Atmosphere

N2

Soil

N2 N2

Nitrogen-fixingbacteria

Organicmaterial (humus)

NH3

(ammonia)

NH4+

(ammonium)

H+

(From soil)

NO3–

(nitrate)Nitrifyingbacteria

Denitrifyingbacteria

Root

NH4+

Soil

Atmosphere

Nitrate and nitrogenous

organiccompoundsexported in

xylem toshoot system

Ammonifyingbacteria


Improving the Protein Yield of Crops

• Agriculture research in plant breeding

– Has resulted in new varieties of maize, wheat, and rice that are enriched in protein

• Such research

– Addresses the most widespread form of human malnutrition: protein deficiency


• Plant nutritional adaptations often involve relationships with other organisms

• Two types of relationships plants have with other organisms are mutualistic

– Symbiotic nitrogen fixation

– Mycorrhizae


The Role of Bacteria in Symbiotic Nitrogen Fixation

• Symbiotic relationships with nitrogen-fixing bacteria

– Provide some plant species with a built-in source of fixed nitrogen

• From an agricultural standpoint

– The most important and efficient symbioses between plants and nitrogen-fixing bacteria occur in the legume family (peas, beans, and other similar plants)


• Along a legumes possessive roots are swellings called nodules

– Composed of plant cells that have been “infected” by nitrogen-fixing Rhizobium bacteria

Figure 37.10a

(a) Pea plant root. The bumps onthis pea plant root are nodules containing Rhizobium bacteria.The bacteria fix nitrogen and obtain photosynthetic productssupplied by the plant.

Nodules

Roots


• Inside the nodule

– Rhizobium bacteria assume a form called bacteroids, which are contained within vesicles formed by the root cell

Figure 37.10b

(b) Bacteroids in a soybean root nodule. In this TEM, a cell froma root nodule of soybean is filledwith bacteroids in vesicles. The cells on the left are uninfected.

5 m

Bacteroidswithinvesicle


• The bacteria of a nodule

– Obtain sugar from the plant and supply the plant with fixed nitrogen

• Each legume

– Is associated with a particular strain of Rhizobium


• Development of a soybean root nodule

Figure 37.11

Infectionthread

Rhizobiumbacteria

Dividing cellsin root cortex

Bacteroid

2 The bacteria penetrate the cortex within the Infection thread. Cells of the cortex and pericycle begin dividing, and vesicles containing the bacteria bud into cortical cells from the branching infection thread. This process results in the formation of bacteroids.

Bacteroid

Bacteroid

Developingroot nodule

Dividing cells in pericycle

Infectedroot hair

1

2

3

Nodulevasculartissue

4

3 Growth continues in the affected regions of the cortex and pericycle, and these two masses of dividing cells fuse, forming the nodule.

Roots emit chemical signals that attract Rhizobium bacteria. The bacteria then emit signals that stimulate root hairs to elongate and to form an infection thread by an invagination of the plasma membrane.

1

4 The nodule develops vascular tissue that supplies nutrients to the nodule and carries nitrogenous compounds into the vascular cylinder for distribution throughout the plant.


The Molecular Biology of Root Nodule Formation

• The development of a nitrogen-fixing root nodule

– Depends on chemical dialogue between Rhizobium bacteria and root cells of their specific plant hosts


Symbiotic Nitrogen Fixation and Agriculture

• The agriculture benefits of symbiotic nitrogen fixation

– Underlie crop rotation

• In this practice

– A non-legume such as maize is planted one year, and the following year a legume is planted to restore the concentration of nitrogen in the soil


Mycorrhizae and Plant Nutrition

• Mycorrhizae

– Are modified roots consisting of mutualistic associations of fungi and roots

• The fungus

– Benefits from a steady supply of sugar donated by the host plant

• In return, the fungus

– Increases the surface area of water uptake and mineral absorption and supplies water and minerals to the host plant


The Two Main Types of Mycorrhizae

• In ectomycorrhizae

– The mycelium of the fungus forms a dense sheath over the surface of the root

Figure 37.12a

a Ectomycorrhizae. The mantle of the fungal mycelium ensheathes the root. Fungal hyphae extend from the mantle into the soil, absorbing water and minerals, especially phosphate. Hyphae also extend into the extracellular spaces of the root cortex, providing extensive surface area for nutrient exchange between the fungus and its host plant.

Mantle(fungal sheath)

Epidermis Cortex Mantle(fungalsheath)

Endodermis

Fungalhyphaebetweencorticalcells (colorized SEM)

100 m(a)


• In endomycorrhizae

– Microscopic fungal hyphae extend into the root

Figure 37.12b

Epidermis Cortex

Fungalhyphae

Roothair

10 m

(LM, stained specimen)

Cortical cells

Endodermis

Vesicle

Casparianstrip

Arbuscules

2 Endomycorrhizae. No mantle forms around the root, but microscopic fungal hyphae extend into the root. Within the root cortex, the fungus makes extensive contact with the plant through branching of hyphae that form arbuscules, providing an enormous surface area for nutrient swapping. The hyphae penetrate the cell walls, but not the plasma membranes, of cells within the cortex.

(b)


Agricultural Importance of Mycorrhizae

• Farmers and foresters

– Often inoculate seeds with spores of mycorrhizal fungi to promote the formation of mycorrhizae

• Some plants

– Have nutritional adaptations that use other organisms in nonmutualistic ways

– Epiphytes, Parasitic Plants, and Carnivorous Plants


• Exploring unusual nutritional adaptations in plants

Figure 37.13

Staghorn fern, an epiphyte

EPIPHYTES

PARASITIC PLANTS

CARNIVOROUS PLANTS

Mistletoe, a photosynthetic parasite Dodder, a nonphotosynthetic parasite

Host’s phloem

Haustoria

Indian pipe, a nonphotosynthetic parasite

Venus’ flytrapPitcher plants Sundews

Dodder


Angiosperm Reproduction and Biotechnology

• The parasitic plant Rafflesia arnoldii

– Produces enormous flowers that can produce up to 4 million seeds

Figure 38.1


• Pollination enables gametes to come together within a flower

• In angiosperms, the dominant sporophyte

– Produces spores that develop within flowers into male gametophytes (pollen grains)

– Produces female gametophytes (embryo sacs)


• An overview of angiosperm reproduction

Figure 38.2a, b

Anther attip of stamen

Filament

AntherStamen

Pollen tube

Germinated pollen grain(n) (male gametophyte)on stigma of carpel

Ovary (base of carpel)

Ovule

Embryo sac (n)(female gametophyte)

FERTILIZATIONEgg (n)

Sperm (n)

Petal

Receptacle

Sepal

Style

Ovary

Key

Haploid (n)

Diploid (2n)

(a) An idealized flower.

(b) Simplified angiosperm life cycle.See Figure 30.10 for a more detailedversion of the life cycle, including meiosis.

Mature sporophyteplant (2n) withflowers

Seed(developsfrom ovule)

Zygote(2n)

Embryo (2n)(sporophyte)

Simple fruit(develops from ovary)

Germinatingseed

Seed

CarpelStigma


Flower Structure

• Flowers

– Are the reproductive shoots of the angiosperm sporophyte

– Are composed of four floral organs: sepals, petals, stamens, and carpels


Gametophyte Development and Pollination

• In angiosperms

– Pollination is the transfer of pollen from an anther to a stigma

– If pollination is successful, a pollen grain produces a structure called a pollen tube, which grows down into the ovary and discharges sperm near the embryo sac


• Pollen

– Develops from microspores within the sporangia of anthers

3 A pollen grain becomes a mature male gametophyte when its generative nucleus divides and forms two sperm.This usually occurs after a pollen grain lands on the stigma of a carpel and the pollen tube begins to grow. (SeeFigure 38.2b.)

Development of a male gametophyte (pollen grain)

(a)

2 Each microsporo-cyte divides by meiosis to produce four haploid microspores, each of which develops into a pollen grain.

Pollen sac(microsporangium)

Micro-sporocyte

Micro-spores (4)

Each of 4microspores

Generativecell (willform 2sperm)

MaleGametophyte(pollen grain)

Nucleus of tube cell

Each one of the microsporangia contains diploid microsporocytes (microspore mother cells).

1

75 m

20 m

Ragweedpollengrain

Figure 38.4a

MEIOSIS

MITOSIS

KEYto labels

Haploid (2n)Diploid (2n)


Keyto labels

MITOSIS

MEIOSIS

Ovule

Ovule

Integuments

Embryosac

Mega-sporangium

Mega-sporocyte

Integuments

Micropyle

Survivingmegaspore

AntipodelCells (3)

PolarNuclei (2)

Egg (1)

Synergids (2)

Development of a female gametophyte (embryo sac)

(b)

Within the ovule’smegasporangium is a large diploid cell called the megasporocyte (megasporemother cell).

1

Three mitotic divisions of the megaspore form the embryo sac, a multicellular female gametophyte. The ovule now consists of the embryo sac along with the surrounding integuments (protective tissue).

3

Female gametophyte(embryo sac)

Diploid (2n)

Haploid (2n) Figure 38.4b

100

m

The megasporocyte divides by meiosis and gives rise to fourhaploid cells, but in most species only one of these survives as the megaspore.

2

• Embryo sacs

– Develop from megaspores within ovules


Mechanisms That Prevent Self-Fertilization

• Many angiosperms

– Have mechanisms that make it difficult or impossible for a flower to fertilize itself

Figure 38.5

Stigma

Antherwith

pollen

Stigma

Pin flower Thrum flower


• The most common anti-selfing mechanism in flowering plants

– Is known as self-incompatibility, the ability of a plant to reject its own pollen

• Researchers are unraveling the molecular mechanisms that are involved in self-incompatibility


• Some plants

– Reject pollen that has an S-gene matching an allele in the stigma cells

• Recognition of self pollen

– Triggers a signal transduction pathway leading to a block in growth of a pollen tube


Double Fertilization

• After landing on a receptive stigma

– A pollen grain germinates and produces a pollen tube that extends down between the cells of the style toward the ovary

• The pollen tube

– Then discharges two sperm into the embryo sac

– After fertilization, ovules develop into seeds and ovaries into fruits


• In double fertilization

– One sperm fertilizes the egg

– The other sperm combines with the polar nuclei, giving rise to the food-storing endosperm


Stigma

Polarnuclei

Egg

Pollen grain

Pollen tube

2 sperm

Style

Ovary

Ovule (containingfemale gametophyte, orembryo sac)

Micropyle

Ovule

Polar nuclei

Egg

Two spermabout to bedischarged

Endosperm nucleus (3n) (2 polar nuclei plus sperm)

Zygote (2n)(egg plus sperm) Figure 38.6

• Growth of the pollen tube and double fertilization

If a pollen graingerminates, a pollen tube

grows down the styletoward the ovary.

1

The pollen tubedischarges two sperm into

the female gametophyte(embryo sac) within an ovule.

2

One sperm fertilizesthe egg, forming the zygote.

The other sperm combines withthe two polar nuclei of the embryo

sac’s large central cell, forminga triploid cell that develops into

the nutritive tissue calledendosperm.

3


From Ovule to Seed

• After double fertilization

– Each ovule develops into a seed

– The ovary develops into a fruit enclosing the seed(s)


Endosperm Development

• Endosperm development

– Usually precedes embryo development

• In most monocots and some eudicots

– The endosperm stores nutrients that can be used by the seedling after germination

• In other eudicots

– The food reserves of the endosperm are completely exported to the cotyledons


Embryo Development

• The first mitotic division of the zygote is transverse

– Splitting the fertilized egg into a basal cell and a terminal cell

Figure 38.7

Ovule

Terminal cell

Endospermnucleus

Basal cell

Zygote

Integuments

Zygote

Proembryo

CotyledonsShootapexRootapex

Seed coat

Basal cell

Suspensor

EndospermSuspensor


Structure of the Mature Seed

• The embryo and its food supply

– Are enclosed by a hard, protective seed coat

In a common garden bean, a eudicot

The embryo consists of the hypocotyl, radicle, and thick cotyledons

Figure 38.8a

(a) Common garden bean, a eudicot with thick cotyledons. The fleshy cotyledons store food absorbed from the endosperm before the seed germinates.

Seed coat

Radicle

Epicotyl

Hypocotyl

Cotyledons


• The seeds of other eudicots, such as castor beans

– Have similar structures, but thin cotyledons

Figure 38.8b

Seed coat

Endosperm

Cotyledons

Epicotyl

Hypocotyl

Radicle

(b) Castor bean, a eudicot with thin cotyledons. The narrow, membranous cotyledons (shown in edge and flat views) absorb food from the endosperm when the seed germinates.

Figure 38.8b

Seed coat

Endosperm

Cotyledons

Epicotyl

Hypocotyl

Radicle

(b) Castor bean, a eudicot with thin cotyledons. The narrow, membranous cotyledons (shown in edge and flat views) absorb food from the endosperm when the seed germinates.


• The embryo of a monocot

– Has a single cotyledon, a coleoptile, and a coleorhiza

Figure 38.8c

(c) Maize, a monocot. Like all monocots, maize has only one cotyledon. Maize and other grasses have a large cotyledon called a scutellum. The rudimentary shoot is sheathed in a structure called the coleoptile, and the coleorhiza covers the young root.

Scutellum(cotyledon)

Coleoptile

Coleorhiza

Pericarp fusedwith seed coat

Endosperm

Epicotyl

Hypocotyl

Radicle


From Ovary to Fruit

• A fruit

– Develops from the ovary

– Protects the enclosed seeds

– Aids in the dispersal of seeds by wind or animals


• Fruits are classified into several types

– Depending on their developmental origin

Figure 38.9a–c

Simple fruit. A simple fruit develops from a single carpel (or several fused carpels) of one flower (examples: pea, lemon, peanut).

(a) Aggregate fruit. An aggregate fruit develops from many separate carpels of one flower (examples: raspberry, blackberry, strawberry).

(b) Multiple fruit. A multiple fruit develops from many carpels of many flowers (examples: pineapple, fig).

(c)

Pineapple fruitRaspberry fruitPea fruit

Stamen

Carpel(fruitlet) Stigma

Ovary

Raspberry flower

Eachsegmentdevelopsfrom thecarpel ofone flower

Pineapple inflorescence

Stamen

CarpelsFlower

Ovary

StigmaStamen

Ovule

Pea flower

Seed


Seed Germination

• As a seed matures

– It dehydrates and enters a phase referred to as dormancy


From Seed to Seedling

• Germination of seeds depends on the physical process called imbibition

– The uptake of water due to low water potential of the dry seed

• Seed dormancy

– Increases the chances that germination will occur at a time and place most advantageous to the seedling

• The breaking of seed dormancy

– Often requires environmental cues, such as temperature or lighting cues


Figure 38.10a

Foliage leaves

Cotyledon

Hypocotyl

Radicle

Epicotyl

Seed coat

Cotyledon

Hypocotyl Cotyledon

Hypocotyl

Common garden bean. In common garden beans, straightening of a hook in the hypocotyl pulls the cotyledons from the soil.

(a)

• The radicle

– Is the first organ to emerge from the germinating seed

• In many eudicots

– A hook forms in the hypocotyl, and growth pushes the hook above ground


• Monocots

– Use a different method for breaking ground when they germinate

• The coleoptile

– Pushes upward through the soil and into the air

Figure 38.10b

Foliage leaves

ColeoptileColeoptile

Radicle

Maize. In maize and other grasses, the shoot grows straight up through the tube of the coleoptile.

(b)


• Many flowering plants clone themselves by asexual reproduction

• Many angiosperm species

– Reproduce both asexually and sexually

• Sexual reproduction

– Generates the genetic variation that makes supposed evolutionary adaptation possible

• Asexual reproduction in plants

– Is called vegetative reproduction


Mechanisms of Asexual Reproduction

• Fragmentation

– Is the separation of a parent plant into parts that develop into whole plants

– Is one of the most common modes of asexual reproduction


Vegetative Propagation and Agriculture

• Humans have devised various methods for asexual propagation of angiosperms

• Many kinds of plants

– Are asexually reproduced from plant fragments called cuttings

• In a modification of vegetative reproduction from cuttings

– A twig or bud from one plant can be grafted onto a plant of a closely related species or a different variety of the same species


• In a process called protoplast fusion

– Researchers fuse protoplasts, plant cells with their cell walls removed, to create hybrid plants

Figure 38.13 50 m


• Plant biotechnology is transforming agriculture

• Plant biotechnology has two meanings

– It refers to innovations in the use of plants to make products of use to humans

– It refers to the use of genetically modified (GM) organisms in agriculture and industry


Artificial Selection

• Humans have intervened

– In the reproduction and genetic makeup of plants for thousands of years


• Maize

– Is a product of artificial selection by humans

– Is a staple in many developing countries, but is a poor source of protein

Figure 38.14


• Interspecific hybridization of plants

– Is common in nature and has been used by breeders, ancient and modern, to introduce new genes


Reducing World Hunger and Malnutrition

• Genetically modified plants

– Have the potential of increasing the quality and quantity of food worldwide

Figure 38.15Ordinary rice

Genetically modified rice

Figure 38.16


The Debate over Plant Biotechnology

• There are some biologists, particularly ecologists

– Who are concerned about the unknown risks associated with the release of GM organisms (GMOs) into the environment

• One concern is that genetic engineering

– May transfer allergens from a gene source to a plant used for food


Possible Effects on Nontarget Organisms

• Many ecologists are concerned that the growing of GM crops

– Might have unforeseen effects on nontarget organisms

• Perhaps the most serious concern that some scientists raise about GM crops

– Is the possibility of the introduced genes escaping from a transgenic crop into related weeds through crop-to-weed hybridization

• Despite all the issues associated with GM crops

– The benefits should be considered


Plant Responses to Internal and External Signals

• For example, the bending of a grass seedling toward light

– Begins with the plant sensing the direction, quantity, and color of the light

Figure 39.1


• Signal transduction pathways link signal reception to response

• Plants have cellular receptors

– That they use to detect important changes in their environment

• For a stimulus to elicit a response

– Certain cells must have an appropriate receptor


• A potato left growing in darkness

– Will produce shoots that do not appear healthy, and will lack elongated roots

• These are morphological adaptations for growing in darkness

– Collectively referred to as etiolation

Figure 39.2a

(a) Before exposure to light. Adark-grown potato has tall,spindly stems and nonexpandedleaves—morphologicaladaptations that enable theshoots to penetrate the soil. Theroots are short, but there is littleneed for water absorptionbecause little water is lost by theshoots.


• After the potato is exposed to light

– The plant undergoes profound changes called de-etiolation, in which shoots and roots grow normally

Figure 39.2b

(b) After a week’s exposure tonatural daylight. The potatoplant begins to resemble a typical plant with broad greenleaves, short sturdy stems, andlong roots. This transformationbegins with the reception oflight by a specific pigment,phytochrome.


• The potato’s response to light

– Is an example of cell-signal processing

Figure 39.3

CELLWALL

CYTOPLASM

1 Reception 2 Transduction 3 Response

Receptor

Relay molecules

Activationof cellularresponses

Hormone orenvironmentalstimulus

Plasma membrane


Reception

• Internal and external signals are detected by receptors

– Proteins that change in response to specific stimuli


Transduction

• Second messengers

– Transfer and amplify signals from receptors to proteins that cause specific responses


Figure 39.4

1 Reception 2 Transduction 3 Response

CYTOPLASM

Plasmamembrane

Phytochromeactivatedby light

Cellwall

Light

cGMP

Second messengerproduced

Specificproteinkinase 1activated

Transcriptionfactor 1 NUCLEUS

P

P

Transcription

Translation

De-etiolation(greening)responseproteins

Ca2+

Ca2+ channelopened

Specificproteinkinase 2activated

Transcriptionfactor 2

• An example of signal transduction in plants

1 The light signal isdetected by thephytochrome receptor,which then activatesat least two signaltransduction pathways.

2 One pathway uses cGMP as asecond messenger that activatesa specific protein kinase.The otherpathway involves an increase incytoplasmic Ca2+ that activatesanother specific protein kinase.

3 Both pathwayslead to expressionof genes for proteinsthat function in thede-etiolation(greening) response.


Response

• Ultimately, a signal transduction pathway

– Leads to a regulation of one or more cellular activities

• In most cases

– These responses to stimulation involve the increased activity of certain enzymes


Transcriptional Regulation

• Transcription factors bind directly to specific regions of DNA

– And control the transcription of specific genes

• Post-translational modification

– Involves the activation of existing proteins involved in the signal response


• Plant hormones help coordinate growth, development, and responses to stimuli

• Hormones

– Are chemical signals that coordinate the different parts of an organism


The Discovery of Plant Hormones

• Any growth response

– That results in curvatures of whole plant organs toward or away from a stimulus is called a tropism

– Is often caused by hormones


A Survey of Plant Hormones


• In general, hormones control plant growth and development

– By affecting the division, elongation, and differentiation of cells

• Plant hormones are produced in very low concentrations

– But a minute amount can have a profound effect on the growth and development of a plant organ


Auxin

• The term auxin

– Is used for any chemical substance that promotes cell elongation in different target tissues

• Auxin transporters

– Move the hormone out of the basal end of one cell, and into the apical end of neighboring cells

• According to a model called the acid growth hypothesis

– Proton pumps play a major role in the growth response of cells to auxin


Expansin

CELL WALL

Cell wallenzymes

Cross-linkingcell wallpolysaccharides

Microfibril

H+ H+

H+

H+

H+

H+

H+

H+

H+

ATP Plasma membrane

Plasmamembrane

Cellwall

NucleusVacuole

Cytoplasm

H2O

Cytoplasm

• Cell elongation in response to auxin

Figure 39.8

1 Auxinincreases the

activity ofproton pumps.

4 The enzymatic cleavingof the cross-linkingpolysaccharides allowsthe microfibrils to slide.The extensibility of thecell wall is increased. Turgorcauses the cell to expand.

2 The cell wallbecomes more

acidic.

5 With the cellulose loosened,the cell can elongate.

3 Wedge-shaped expansins, activatedby low pH, separate cellulose microfibrils fromcross-linking polysaccharides. The exposed cross-linkingpolysaccharides are now more accessible to cell wall enzymes.


Lateral and Adventitious Root Formation

• Auxin

– Is involved in the formation and branching of roots

• An overdose of auxins

– Can kill eudicots

• Auxin affects secondary growth

– By inducing cell division in the vascular cambium and influencing differentiation of secondary xylem


Cytokinins

• Cytokinins

– Stimulate cell division and differentiation

– Are produced in actively growing tissues such as roots, embryos, and fruits

– Work together with auxin

• Cytokinins retard the aging of some plant organs

– By inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues


Control of Apical Dominance

• Cytokinins, auxin, and other factors interact in the control of apical dominance

– The ability of a terminal bud to suppress development of axillary buds

Figure 39.9a

Axillary buds


• If the terminal bud is removed

– Plants become bushier

Figure 39.9b

“Stump” afterremoval ofapical bud

Lateral branches


Gibberellins

• Gibberellins have a variety of effects

– Such as stem elongation, fruit growth, and seed germination

• In stems

– Gibberellins stimulate cell elongation and cell division

– Both auxin and gibberellins must be present for fruit to set


• Gibberellins are used commercially

– In the spraying of Thompson seedless grapes

Figure 39.10


• After water is imbibed, the release of gibberellins from the embryo– Signals the seeds to break dormancy and germinate

Germination

Figure 39.11

2 2 The aleurone responds by synthesizing and secreting digestive enzymes thathydrolyze stored nutrients inthe endosperm. One exampleis -amylase, which hydrolyzesstarch. (A similar enzyme inour saliva helps in digestingbread and other starchy foods.)

Aleurone

Endosperm

Water


GA

GA

-amylase

Radicle

Sugar

1 After a seedimbibes water, theembryo releasesgibberellin (GA)as a signal to thealeurone, the thinouter layer of theendosperm.

3 Sugars and other nutrients absorbedfrom the endospermby the scutellum (cotyledon) are consumed during growth of the embryo into a seedling.


2 The aleurone responds by synthesizing and secreting digestive enzymes thathydrolyze stored nutrients inthe endosperm. One exampleis -amylase, which hydrolyzesstarch. (A similar enzyme inour saliva helps in digestingbread and other starchy foods.)

Aleurone

Endosperm

Water


GA

GA

-amylase

Radicle

Sugar

2 1 After a seedimbibes water, theembryo releasesgibberellin (GA)as a signal to thealeurone, the thinouter layer of theendosperm.

3 Sugars and other nutrients absorbedfrom the endospermby the scutellum (cotyledon) are consumed during growth of the embryo into a seedling.


Abscisic Acid

• Two of the many effects of abscisic acid (ABA) are

– Seed dormancy

– Drought tolerance

• Seed dormancy has great survival value

– Because it ensures that the seed will germinate only when there are optimal conditions

• ABA is the primary internal signal

– That enables plants to withstand drought (signals stomata to close)


Ethylene

• Plants produce ethylene (gas)

– In response to stresses such as drought, flooding, mechanical pressure, injury, and infection

• A burst of ethylene

– Is associated with the programmed destruction of cells, organs, or whole plants (Apoptosis: Programmed Cell Death)

• A burst of ethylene production in the fruit

– Triggers the ripening process (ethylene triggers ripening and ripening triggers ethylene = positive feedback, one bad apple spoils the bunch)


Leaf Abscission

• A change in the balance of auxin and ethylene controls leaf abscission

– The process that occurs in autumn when a leaf falls

Figure 39.16

0.5 mm

Protective layer Abscission layer

Stem Petiole


Photomorphogenesis- plant response to light

• Plants not only detect the presence of light

– But also its direction, intensity, and wavelength (color)

Wavelength (nm)

1.0

0.8

0.6

0.2

0450 500 550 600 650 700

Light

Time = 0 min.

Time = 90 min.

0.4

400Pho

totr

opic

eff

ectiv

enes

s re

lativ

e to

436

nm

A graph called an action spectrum

Depicts the relative response of a process to different wavelengths of light


• Research on action spectra and absorption spectra of pigments

– Led to the identification of two major classes of light receptors: blue-light photoreceptors and phytochromes (red light)

• Various blue-light photoreceptors

– Control hypocotyl elongation, stomatal opening, and phototropism

• Phytochromes

– Regulate many of a plant’s responses to light throughout its life


• A phytochrome

– Is the photoreceptor responsible for the opposing effects of red and far-red light

A phytochrome consists of two identical proteins joined to formone functional molecule. Each of these proteins has two domains.

Chromophore

Photoreceptor activity. One domain,which functions as the photoreceptor,is covalently bonded to a nonproteinpigment, or chromophore.

Kinase activity. The other domainhas protein kinase activity. Thephotoreceptor domains interact with the kinase domains to link light reception to cellular responses triggered by the kinase.

Figure 39.19


• Phytochromes exist in two photoreversible states

– With conversion of Pr to Pfr triggering many developmental responses

Figure 39.20

Synthesis

Far-redlight

Red light

Slow conversionin darkness(some plants)

Responses:seed germination,control offlowering, etc.

Enzymaticdestruction

PfrPr


Phytochromes and Shade Avoidance

• The phytochrome system

– Also provides the plant with information about the quality of light

• In the “shade avoidance” response of a tree

– The phytochrome ratio shifts in favor of Pr when a tree is shaded


Biological Clocks and Circadian Rhythms• Many plant processes

– Oscillate during the day

• Many legumes

– Lower their leaves in the evening and raise them in the morning

Noon Midnight


• Cyclical responses to environmental stimuli are called circadian rhythms

– And are approximately 24 hours long

– Can be entrained to exactly 24 hours by the day/night cycle

• Phytochrome conversion marks sunrise and sunset

– Providing the biological clock with environmental cues


Photoperiodism and Responses to Seasons

• Photoperiod, the relative lengths of night and day

– Is the environmental stimulus plants use most often to detect the time of year

• Photoperiodism

– Is a physiological response to photoperiod

• Some developmental processes, including flowering in many species

– Requires a certain photoperiod


Flowering times

• Short-day plants – require a period of continuous darkness in order to flower. Short-day plants are actually long-night plants. These plants flower in early spring or fall.

• Long-day plants – flower only if a period of continuous darkness is shorter than the critical period. Flower in late spring or early summer.

• Day-neutral plants can flower in days of any length.


Gravity

• Response to gravity

– Is known as gravitropism

• Roots show positive gravitropism

• Stems show negative gravitropism


• Plants may detect gravity by the settling of statoliths

– Specialized plastids containing dense starch grains

Figure 39.25a, b

Statoliths20 m

(a) (b)


Mechanical Stimuli

• Growth in response to touch

– Is called thigmotropism

– Occurs in vines and other climbing plants

• Rubbing the stems of young plants a couple of times daily

– Results in plants that are shorter than controls


• Rapid leaf movements in response to mechanical stimulation

– Are examples of transmission of electrical impulses called action potentials

Figure 39.27a–c

(a) Unstimulated (b) Stimulated

Side of pulvinus withflaccid cells

Side of pulvinus withturgid cells

Vein

0.5 m(c) Motor organs

Leafletsafterstimulation

Pulvinus(motororgan)


Environmental Stresses

• Environmental stresses

– Have a potentially adverse effect on a plant’s survival, growth, and reproduction

– Can have a devastating impact on crop yields in agriculture


Drought

• During drought

– Plants respond to water deficit by reducing transpiration

– Deeper roots continue to grow


Flooding

• Enzymatic destruction of cells

– Creates air tubes that help plants survive oxygen deprivation during flooding

Figure 39.28a, b

Vascularcylinder

Air tubes

Epidermis

100 m 100 m(a) Control root (aerated) (b) Experimental root (nonaerated)


Salt Stress

• Plants respond to salt stress by producing solutes tolerated at high concentrations

– Keeping the water potential of cells more negative than that of the soil solution

• Heat-shock proteins

– Help plants survive heat stress

• Altering lipid composition of membranes

– Is a response to cold stress


Defenses Against Herbivores

• Herbivory, animals eating plants

– Is a stress that plants face in any ecosystem

• Plants counter excessive herbivory

– With physical defenses such as thorns

– With chemical defenses such as distasteful or toxic compounds


Recruitment ofparasitoid waspsthat lay their eggswithin caterpillars

4

3 Synthesis andrelease ofvolatile attractants

1 Chemicalin saliva

1 Wounding

2 Signal transductionpathway

• Some plants even “recruit” predatory animals

– That help defend the plant against specific herbivores

Figure 39.29


Defenses Against Pathogens

• A plant’s first line of defense against infection

– Is the physical barrier of the plant’s “skin,” the epidermis and the periderm

• Once a pathogen invades a plant

– The plant mounts a chemical attack as a second line of defense that kills the pathogen and prevents its spread


• The second defense system

– Is enhanced by the plant’s inherited ability to recognize certain pathogens

• A virulent pathogen

– Is one that a plant has little specific defense against

• An avirulent pathogen

– Is one that may harm but not kill the host plant


• Gene-for-gene recognition is a widespread form of plant disease resistance

– That involves recognition of pathogen-derived molecules by the protein products of specific plant disease resistance (R) genes


Figure 39.30a

Receptor coded by R allele

(a) If an Avr allele in the pathogen corresponds to an R allelein the host plant, the host plant will have resistance,making the pathogen avirulent. R alleles probably code forreceptors in the plasma membranes of host plant cells. Avr allelesproduce compounds that can act as ligands, binding to receptorsin host plant cells.

• A pathogen is avirulent

– If it has a specific Avr gene corresponding to a particular R allele in the host plant

Signal molecule (ligand)from Avr gene product

Avr allele

Plant cell is resistantAvirulent pathogen

R


• If the plant host lacks the R gene that counteracts the pathogen’s Avr gene

– Then the pathogen can invade and kill the plant

Figure 39.30b

No Avr allele;virulent pathogen

Plant cell becomes diseased

Avr allele

No R allele;plant cell becomes diseasedVirulent pathogen

Virulent pathogen

No R allele;plant cell becomes diseased

(b) If there is no gene-for-gene recognition because of one ofthe above three conditions, the pathogen will be virulent,causing disease to develop.

R


3 In a hypersensitiveresponse (HR), plantcells produce anti-microbial molecules,seal off infectedareas by modifyingtheir walls, andthen destroythemselves. Thislocalized responseproduces lesionsand protects otherparts of an infectedleaf.

4 Before they die,infected cellsrelease a chemicalsignal, probablysalicylic acid.

6 In cells remote fromthe infection site,the chemicalinitiates a signaltransductionpathway.

5 The signal is distributed to the rest of the plant.

2 This identification step triggers a signal transduction pathway.

1 Specific resistance is based on the binding of ligands from the pathogen to receptors in plant cells.

7 Systemic acquiredresistance isactivated: theproduction ofmolecules that helpprotect the cellagainst a diversityof pathogens forseveral days.

Signal

7

6

54

3

2

1

Avirulentpathogen

Signal transductionpathway

Hypersensitiveresponse

Signaltransduction

pathway

Acquiredresistance

R-Avr recognition andhypersensitive response

Systemic acquiredresistanceFigure 39.31

Plant Responses to Pathogen Invasions

• A hypersensitive response against an avirulent pathogen

– Seals off the infection and kills both pathogen and host cells in the region of the infection


Systemic Acquired Resistance

• Systemic acquired resistance (SAR)

– Is a set of generalized defense responses in organs distant from the original site of infection

– Is triggered by the signal molecule salicylic acid

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