Plant Growth and Development Advance Plant Physiology.
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Transcript of Plant Growth and Development Advance Plant Physiology.
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Plant Growth and Development
Advance Plant Physiology
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From germination to senescence!!
Zygote Embryo Seedling
How do new plant structures arise from preexisting structures?
How do plant tissues grow in a particular pattern?
What are the basic principles that govern plant growth and development?
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The outline of a mature plant!
Angiosperms: Flowering plants whose ovules are produced within ovary and whose seeds occur within a fruit that develops from the ovaryGymnosperms: ovules not enclosed in ovary and seeds not enclosed in fruitsMonocots: Embryo with single cotyledonsDicots: Embryo with two cotyledons
Arabidopsis thaliana
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Plant Genetic Model Systems
• Crop plants– Rice – Alfalfa– Tomato– Tobacco– Maize
Considerations•Genome size•Polyploidy•Translation to crop plants
• Model systems– Mosses – Algae – Poplar– Arabidopsis
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Arabidopsis thaliana: A model system for flowering plantsAdvantages:
1. Life cycle • 6 weeks• Small plant, easy to grow• High fecundity (10,000
seed/individual)• Self and cross-fertilization
2. Genome• Diploid• 125 Mb, smallest known in
plant kingdom• Little repetitive DNANo
immediate agricultural importance and is not thought to cure any disease
Arabidopsis is a member of the mustard (Brassicaceae) family, which includes cultivated species such as cabbage and radish.Meyerowitz. Ann. Rev. Genet. 21 : 93-111(1987)
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Arabidopsis Genome
Small genome composed of approximately 25,700 genes
5 chromosomes Genome mapping project completed due to internationally coordinated program•European Union•Riken•US
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Embryogenesis
Sperm+Egg Zygote
During embryogenesis:•Single-celled zygote is transformed into multicellular, microscopic plant (embryo) that has the complete body plan of a mature plant present in a rudimentary form•It occurs within the Embryo sac of the ovule•Ovule and Endosperm are parts of a seed
Small
Egg
Surrounds embryo and provides nutrition in the form of starch
Wheat endosper
m?
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Embryo development in Arabidopsis
Globular stage
Heart stage
Torpedo stage
Maturation stage
Cell division in apical regions that later form
cotyledonsCell elongation throughout embryo axis and further
development of cotyledons
Last stage, embryo and
seed lose water to enter
dormancy
Embryo goes through divisions, generating an eight-cell (octant) embryo after 30 hrs of fertilization
Seed Dormancy: growth, development and metabolic activities stop..
Why?
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Embryogenesis and plant development:Axial patterningRadial patterningPrimary meristems
Axial Patternin
g
Shoot apical meristem
Root apical meristem
Ovule
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Apical cell: receives more cytoplasm•Divides vertically•Generates globular (octant) embryo
First division of zygote
Basal cell: receives large vacuole•Horizontal division•Suspensor cells 6-9 cells that attach the embryo to the vascular system•Hypophysis derivative of basal cell that contributes to embryo development and forms Columella (central part of root cap)
Three axial regions develop before the embryo reaches the Heart stage;Apical region: gives rise to cotyledons and shoot apical meristemMiddle Region: gives rise to hypocotyl, root and most of the root meristemHypophysis: gives rise to the rest of root meristem
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Radial Patterning
•Visible at Globular Stage•Radially arranged three regions
• Protoderm:• Cortex:• Endodermis:• Vascular tissues:• Pericycle:
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When a mature seed is placed under favorable conditions and fails to germinate, it is said to be dormant. Seed dormancy is referred to as embryo dormancy or internal dormancy and is caused by endogenous characteristics of the embryo that prevent germination. The oldest seed that has been germinated into a viable plant was an approximately 1,300-year-old lotus fruit recovered from a dry lakebed in northeastern China. Seed Coat Dormancy: External dormancy or hardseededness, which is caused by the presence of a hard seed covering or seed coat that prevents water and oxygen from reaching and activating the embryo. It is a physical barrier to germination, not a true form of dormancy.
Seed Dormancy
Arrested plant growthSurvival strategy against different external threatsControlled by biological clock that tells plant when to produce soft tissues to survive against harsh winters or other factors------ Interesting????
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Genes involved in Embryogenesis
GNOM gene
MONOPTEROS gene
SHORT ROOT and SCARECROW genes
HOBBIT gene
SHOOTMERISTEMLESS gene
Plays role in Axial PatterningNo root and cotyledons
No hypocotyl and rootCotyledons are disorganized Both take part in Radial
Patterning. Hence plays role in tissue differentiation
Defective root meristem development
Mutants fail to form shoot meristem
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HOBBIT gene
Role of HOBBIT gene in root meristem development
Columella (COL):Lateral Root Cap (LRC):Quiscent Center (QC): Slowly dividing root meristematic cells that regulate the differentiation of neighboring cells
•Marker of root meristem identity•hbt mutant shows abnormality in two- or four-cell stage•Hypophyseal precursor divides vertically instead of horizontally•Root without Hypophysis fails to form Quiescent Center and Columella•Consequently hbt mutants are unable to form lateral roots
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Meristems in Plant Development
•Small isodiametric cells with embryonic characteristics•Retain their embryonic character indefinitely•Some differentiate while others retain capacity for cell division
Stem cells: cells that retain their capacity for cell division indefinitely
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Primary meristems
Protoderm
Procambium
Ground meristem
Epidermis Primary vascular tissues and vascular cambium
Cortex and endodermis
Vascular Tissues: The tissue in vascular plants that circulates fluid and nutrients. Comprise of;1- Xylem conducts water and nutrients up from the roots2-Phloem distributes food from the leaves to other parts of the plant
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Shoot Apical Meristem
Stem Leaves and lateral buds
Shoot apex: apical meristem+leaf primordia
•Shoot apical meristem can contain a few hundred to a thousand cells but Arabidopsis SAM has about 60 cells•Small thin-walled cells, dense cytoplasm, lacks large central vacuole•Grows rapidly in spring-slow growth during summer-dormant in winter
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Shoot Apical Meristem Structure
internal tissues of stem
Like Quiesce
nt center in roots
Cytohistological Zonation
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Postembryonic Meristems
Primary meristems1. Root meristem 2. Shoot meristem
Secondary meristems1. Axillary2. Inflorescence 3. Floral4. Intercalary5. lateral
Preembryonic Meristems
Axillary •Formed in the leaf axils•Derivative of shoot apical meristem•Produce branchesIntercalary•Found within organs, near their bases•Enables grasses to continue to grow despite mowing or grazing
Branch root•Formed from pericycle cells in mature root regionsCork Cambium (Lateral meristem)•Develops within mature cortex cells and secondary phloem•Periderm or Bark are its derivative layers that form outer protective surface in woody trees
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Vascular Cambium (Lateral meristem)It does not produce lateral organs, but only the woody tissues of stems and roots. The vascularcambium contains two types of meristematic cells:
Fusiform Stem CellsHighly elongated, vacuolate cells that differentiate into the conducting cells of xylem and phloem
Ray Stem CellsSmall cells whose derivatives include the radially oriented files of parenchyma cells within wood known as Rays
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Floral meristems•Produce floral organs such as sepals, petals, stamens and carpals•DeterminateInflorescence meristem•Produces bracts and floral meristems in the axils of bracts•Could be determinate or indeterminate
Determinate meristems:Genetically programmed limit to their growthIndeterminate:No predetermined limit to growthBracts:A leaf from the axils of which a flower or floral axil arise
Consists of one or more leaves, the node to which leaves are attached, internode and one or more axillary buds
Could also be apical meristems provided they get the developmental potential
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Leaf Development
Axil Development• leaves are lateral organs.• leaves display consistent orientation and
polarity relative to the shoot i.e. axial information in the leaf does not arise de novo but depends on existing axial information.
• Angiosperm leaf is almost always a determinate organ.
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Stages of leaf development
1- Organogenesis:•Leaf founder cells formed by L1 and L2 layers of apical meristem, produce leaf primordium that ultimately develops into leaves2- Development of suborgan domains•Primordium differentiates into specific leaf parts
• Dorsiventral (abaxial-adaxial)• Proximodistal (apical-basal)• Lateral (margin-blade-midrib)
3- cell and tissue differentiation•L1 layer forms epidermis•L2 layer forms photosynthetic mesophyll cells•L3 layers forms vascular elements and bundle sheath cells
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Simple leaves have three axes of symmetry.
• proximodistal axis from base of the leaf to the tip.
• adaxial-abaxial axis from the upper to the lower epidermis.
• centrolateral axis from the midrib to the margin.
Structural symmetry in the leaf
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Leaf Primordia Arrangement
Phyllotaxy: The arrangement of leaves around the stem
Single leaf
Paired leaf
Opposite leaves per node at right angle to each other
More than two leaves per node
Spiral arrangement of leaves
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Further Readings
• Growth and Development, Chapter 16, Plant Physiology by Taiz and Zeiger