Plants require certain chemical elements to complete their life cycle
Plants derive most of their organic mass from the CO 2 of air
But they also depend on soil nutrients such as water and minerals
Figure 37.2 CO 2 , the source of carbon for Photosynthesis, diffuses into leaves from the air through stomata. Through stomata, leaves expel H 2 O and O 2 . H 2 O O 2 CO 2 Roots take in O 2 and expel CO 2 . The plant uses O 2 for cellular respiration but is a net O 2 producer. O 2 CO 2 H 2 O Roots absorb H 2 O and minerals from the soil. Minerals
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: Solution containing all minerals Experimental: Solution without 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.
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 on this pea plant root are nodules containing Rhizobium bacteria. The bacteria fix nitrogen and obtain photosynthetic products supplied by the plant. Nodules Roots
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 from a root nodule of soybean is filled with bacteroids in vesicles. The cells on the left are uninfected. 5 m Bacteroids within vesicle
Figure 37.11 Infection thread Rhizobium bacteria Dividing cells in 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 Developing root nodule Dividing cells in pericycle Infected root hair 1 2 3 Nodule vascular tissue 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
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 (fungal sheath) Endodermis Fungal hyphae between cortical cells (colorized SEM) 100 m (a)
Figure 37.12b Epidermis Cortex Fungal hyphae Root hair 10 m (LM, stained specimen) Cortical cells Endodermis Vesicle Casparian strip 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)
Figure 38.2a, b Anther at tip of stamen Filament Anther Stamen Pollen tube Germinated pollen grain ( n ) (male gametophyte) on stigma of carpel Ovary (base of carpel) Ovule Embryo sac ( n ) (female gametophyte) FERTILIZATION Egg ( n ) Sperm ( n ) Petal Receptacle Sepal Style Ovary Key Haploid ( n ) Diploid (2 n ) (a) An idealized flower. (b) Simplified angiosperm life cycle. See Figure 30.10 for a more detailed version of the life cycle, including meiosis. Mature sporophyte plant ( 2n ) with flowers Seed (develops from ovule) Zygote (2 n ) Embryo (2 n ) (sporophyte) Simple fruit (develops from ovary) Germinating seed Seed Carpel Stigma
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. (See Figure 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 4 microspores Generative cell (will form 2 sperm) Male Gametophyte (pollen grain) Nucleus of tube cell Each one of the microsporangia contains diploid microsporocytes (microspore mother cells). 1 75 m 20 m Ragweed pollen grain Figure 38.4a MEIOSIS MITOSIS KEY to labels Haploid ( 2n ) Diploid ( 2n )
Key to labels MITOSIS MEIOSIS Ovule Ovule Integuments Embryo sac Mega- sporangium Mega- sporocyte Integuments Micropyle Surviving megaspore Antipodel Cells (3) Polar Nuclei (2) Egg (1) Synergids (2) Development of a female gametophyte (embryo sac) (b) Within the ovule’s megasporangium is a large diploid cell called the megasporocyte (megaspore mother 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 four haploid cells, but in most species only one of these survives as the megaspore. 2
Growth of the pollen tube and double fertilization
Stigma Polar nuclei Egg Pollen grain Pollen tube 2 sperm Style Ovary Ovule (containing female gametophyte, or embryo sac) Micropyle Ovule Polar nuclei Egg Two sperm about to be discharged Endosperm nucleus (3 n ) (2 polar nuclei plus sperm) Zygote (2 n ) (egg plus sperm) Figure 38.6 If a pollen grain germinates, a pollen tube grows down the style toward the ovary. 1 The pollen tube discharges two sperm into the female gametophyte (embryo sac) within an ovule. 2 One sperm fertilizes the egg, forming the zygote. The other sperm combines with the two polar nuclei of the embryo sac’s large central cell, forming a triploid cell that develops into the nutritive tissue called endosperm. 3
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
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.
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 fused with seed coat Endosperm Epicotyl Hypocotyl Radicle
Figure 38.9a–c Pineapple fruit Raspberry fruit Pea fruit Stamen Carpel (fruitlet) Stigma Ovary Raspberry flower Each segment develops from the carpel of one flower Pineapple inflorescence Stamen Carpels Flower Ovary Stigma Stamen Ovule Pea flower Seed 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)
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
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)
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. A dark-grown potato has tall, spindly stems and nonexpanded leaves—morphological adaptations that enable the shoots to penetrate the soil. The roots are short, but there is little need for water absorption because little water is lost by the shoots.
The plant undergoes profound changes called de-etiolation , in which shoots and roots grow normally
Figure 39.2b (b) After a week’s exposure to natural daylight. The potato plant begins to resemble a typical plant with broad green leaves, short sturdy stems, and long roots. This transformation begins with the reception of light by a specific pigment, phytochrome.
Figure 39.4 1 Reception 2 Transduction 3 Response CYTOPLASM Plasma membrane Phytochrome activated by light Cell wall Light cGMP Second messenger produced Specific protein kinase 1 activated Transcription factor 1 NUCLEUS P P Transcription Translation De-etiolation (greening) response proteins Ca 2+ Ca 2+ channel opened Specific protein kinase 2 activated Transcription factor 2 1 The light signal is detected by the phytochrome receptor, which then activates at least two signal transduction pathways. 2 One pathway uses cGMP as a second messenger that activates a specific protein kinase.The other pathway involves an increase in cytoplasmic Ca 2+ that activates another specific protein kinase. 3 Both pathways lead to expression of genes for proteins that function in the de-etiolation (greening) response.
Figure 39.8 Expansin CELL WALL Cell wall enzymes Cross-linking cell wall polysaccharides Microfibril H + H + H + H + H + H + H + H + H + ATP Plasma membrane Plasma membrane Cell wall Nucleus Vacuole Cytoplasm H 2 O Cytoplasm 1 Auxin increases the activity of proton pumps. 4 The enzymatic cleaving of the cross-linking polysaccharides allows the microfibrils to slide. The extensibility of the cell wall is increased. Turgor causes the cell to expand. 2 The cell wall becomes more acidic. 5 With the cellulose loosened, the cell can elongate. 3 Wedge-shaped expansins, activated by low pH, separate cellulose microfibrils from cross-linking polysaccharides. The exposed cross-linking polysaccharides are now more accessible to cell wall enzymes.
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 that hydrolyze stored nutrients in the endosperm. One example is -amylase, which hydrolyzes starch. (A similar enzyme in our saliva helps in digesting bread and other starchy foods.) Aleurone Endosperm Water Scutellum (cotyledon) GA GA -amylase Radicle Sugar 1 After a seed imbibes water, the embryo releases gibberellin (GA) as a signal to the aleurone, the thin outer layer of the endosperm. 3 Sugars and other nutrients absorbed from the endosperm by the scutellum (cotyledon) are consumed during growth of the embryo into a seedling.
2 The aleurone responds by synthesizing and secreting digestive enzymes that hydrolyze stored nutrients in the endosperm. One example is -amylase, which hydrolyzes starch. (A similar enzyme in our saliva helps in digesting bread and other starchy foods.) Aleurone Endosperm Water Scutellum (cotyledon) GA GA -amylase Radicle Sugar 2 1 After a seed imbibes water, the embryo releases gibberellin (GA) as a signal to the aleurone, the thin outer layer of the endosperm. 3 Sugars and other nutrients absorbed from the endosperm by the scutellum (cotyledon) are consumed during growth of the embryo into a seedling.
Is the photoreceptor responsible for the opposing effects of red and far-red light
A phytochrome consists of two identical proteins joined to form one functional molecule. Each of these proteins has two domains. Chromophore Photoreceptor activity. One domain, which functions as the photoreceptor, is covalently bonded to a nonprotein pigment, or chromophore. Kinase activity. The other domain has protein kinase activity. The photoreceptor domains interact with the kinase domains to link light reception to cellular responses triggered by the kinase. Figure 39.19
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 resistant Avirulent pathogen R Figure 39.30a
Receptor coded by R allele
(a) If an Av r allele in the pathogen corresponds to an R allele in the host plant, the host plant will have resistance, making the pathogen avirulent. R alleles probably code for receptors in the plasma membranes of host plant cells. Avr alleles produce compounds that can act as ligands, binding to receptors in host plant cells.
If the plant host lacks the R gene that counteracts the pathogen’s Avr gene
Then the pathogen can invade and kill the plant
R Figure 39.30b No Avr allele; virulent pathogen Plant cell becomes diseased Avr allele No R allele; plant cell becomes diseased Virulent pathogen Virulent pathogen No R allele; plant cell becomes diseased (b) If there is no gene-for-gene recognition because of one of the above three conditions, the pathogen will be virulent, causing disease to develop.
A hypersensitive response against an avirulent pathogen
Seals off the infection and kills both pathogen and host cells in the region of the infection
3 In a hypersensitive response (HR), plant cells produce anti- microbial molecules, seal off infected areas by modifying their walls, and then destroy themselves. This localized response produces lesions and protects other parts of an infected leaf. 4 Before they die, infected cells release a chemical signal, probably salicylic acid. 6 In cells remote from the infection site, the chemical initiates a signal transduction pathway. 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 acquired resistance is activated: the production of molecules that help protect the cell against a diversity of pathogens for several days. Signal 7 6 5 4 3 2 1 Avirulent pathogen Signal transduction pathway Hypersensitive response Signal transduction pathway Acquired resistance R-Avr recognition and hypersensitive response Systemic acquired resistance Figure 39.31