Plant Growth and Development

Advance Plant Physiology

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?

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

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

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)

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

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?

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?

Embryogenesis and plant development:Axial patterningRadial patterningPrimary meristems

Axial Patternin

g

Shoot apical meristem

Root apical meristem

Ovule

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

Radial Patterning

•Visible at Globular Stage•Radially arranged three regions

• Protoderm:• Cortex:• Endodermis:• Vascular tissues:• Pericycle:

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????

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

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

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

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

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

Shoot Apical Meristem Structure

internal tissues of stem

Like Quiesce

nt center in roots

Cytohistological Zonation

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

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

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

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.

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

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

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

Further Readings

• Growth and Development, Chapter 16, Plant Physiology by Taiz and Zeiger