1. Plant Growth and Development
Advance Plant Physiology

2. 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?
1- Organs are generated by cell division and expansion, but they are also composed of tissues in which groups of cells have acquired specialized functions, and these tissues are arranged in specific patterns.
2- Understanding how growth, cell differentiation, and pattern formation are regulated at the cellular, biochemical, and molecular levels is the
ultimate goal of developmental biologists. Such an understanding also must include the genetic basis of development. Ultimately, development
is the unfolding of genetically encoded programs. Which genes are involved, what is their hierarchical order, and how do they bring about
developmental change?
Meristems can be considered to be cell factories in which the ongoing processes of cell division, expansion, and differentiation generate the plant body

3. 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 ovary
Gymnosperms: ovules not enclosed in ovary and seeds not enclosed in fruits
Monocots: Embryo with single cotyledons
Dicots: Embryo with two cotyledons
The outline of a mature plant!
Arabidopsis thaliana

4. 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

5. Arabidopsis thaliana: A model system for flowering plants
Advantages:
1. Life cycle
6 weeks
Small plant, easy to grow
High fecundity (10,000 seed/individual)
Self and cross-fertilization
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 : (1987)

6. Arabidopsis Genome 5 chromosomes Genome mapping project completed
Small genome composed of approximately 25,700 genes
5 chromosomes
Genome mapping
project completed
due to internationally
coordinated program
European Union
Riken US

7. Surrounds embryo and provides nutrition in the form of starch
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
Wheat endosperm?
Embryogenesis also establishes the primary meristems.Most of the structures that make up the adult plant are generated
after embryogenesis through the activity of merisstems. Although these primary meristems are established
during embryogenesis, only upon germination will they become active and begin to generate the organs and tissues of the adult
Surrounds embryo and provides nutrition in the form of starch
Small Egg

8. Embryo development in Arabidopsis
Embryo goes through divisions, generating an eight-cell (octant) embryo after 30 hrs of fertilization
Globular stage
Cell division in apical regions that later form cotyledons
Heart stage
Cell elongation throughout embryo axis and further development of cotyledons
Torpedo stage
Last stage, embryo and seed lose water to enter dormancy
Maturation stage
Seed Dormancy: growth, development and metabolic activities stop..
Why?

10. Ovule Axial Patterning
Embryogenesis and plant development:
Axial patterning
Radial patterning
Primary meristems
Shoot apical meristem
Ovule
Root apical meristem
Axial Patterning

11. 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)
Apical cell: receives more cytoplasm
Divides vertically
Generates globular (octant) embryo
Three axial regions develop before the embryo reaches the Heart stage;
Apical region: gives rise to cotyledons and shoot apical meristem
Middle Region: gives rise to hypocotyl, root and most of the root meristem
Hypophysis: gives rise to the rest of root meristem

12. Radial Patterning Visible at Globular Stage
Radially arranged three regions
Protoderm:
Cortex:
Endodermis:
Vascular tissues:
Pericycle:

13. Seed Dormancy Arrested plant growth
Survival strategy against different external threats
Controlled by biological clock that tells plant when to produce soft tissues to survive against harsh winters or other factors Interesting????
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.

14. Genes involved in Embryogenesis
Plays role in Axial Patterning
No root and cotyledons
GNOM gene
MONOPTEROS gene
No hypocotyl and root
Cotyledons are disorganized
Both take part in Radial Patterning. Hence plays role in tissue differentiation
SHORT ROOT and SCARECROW genes
The MONOPTEROS gene Although the mp mutant embryos lack a primary root when they germinate, they will form adventitious roots as
the seedlings grow into adult plants.
HOBBIT gene
Defective root meristem development
SHOOTMERISTEMLESS gene
Mutants fail to form shoot meristem

15. 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
Role of HOBBIT gene in root meristem development
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

16. 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

17. 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 roots
2-Phloem distributes food from the leaves to other parts of the plant

18. Shoot Apical Meristem Stem Leaves and lateral buds
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 apex: apical meristem+leaf primordia

19. Shoot Apical Meristem Structure
Cytohistological Zonation
Like Quiescent center in roots
internal tissues of stem
These layers are designated L1, L2, and L3, where L1 is the outermost layer (Figure 16.13). Cell divisions are anticlinal in the L1 and L2 layers; that is, the new cell wall separating the daughter cells is oriented at right angles to the meristem surface. Cell divisions tend to be less regularly oriented in the L3 layer. Each layer has its own stem cells, and all three layers contribute to the formation of the stem and lateral organs.
The center of an active meristem contains a cluster of relatively large, highly vacuolate cells called the central zone. The central zone is somewhat comparable to the quiescent center of root meristems
peripheral zone, flanks the central zone. A rib zone lies underneath the central cell zone and gives rise to the internal tissues of the stem

20. Preembryonic Meristems Postembryonic Meristems
Primary meristems
Root meristem
Shoot meristem
Secondary meristems
Axillary
Inflorescence
Floral
Intercalary
lateral
Axillary
Formed in the leaf axils
Derivative of shoot apical meristem
Produce branches
Intercalary
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 regions
Cork 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
The root and shoot apical meristems formed during embryogenesis are called primary meristems. After germination, the activity of these primary meristems generates the primary tissues and organs that constitute the primary plant body.
Axillary meristems are formed in the axils of leaves and are derived from the shoot apical meristem. The growth and development of axillary meristems produces branches from the main axis of the plant.
• Intercalary meristems are found within organs, often near their bases. The intercalary meristems of grass leaves and stems enables them to continue to grow despite mowing or grazing by cows.
• Branch root meristems have the structure of the primary root meristem, but they form from pericycle cells in mature regions of the root. Adventitious roots also can be produced from lateral root meristems that develop on stems, as when stem cuttings are rooted to propagate a plant.
• The vascular cambium (plural cambia) is a secondary meristem that differentiates along with the primary vascular tissue from the procambium within the vascular cylinder. It does not produce lateral organs, but
only the woody tissues of stems and roots. The vascular cambium contains two types of meristematic cells: fusiform stem cells and ray stem cells. Fusiform stem cells are highly elongated, vacuolate cells that divide longitudinally to regenerate themselves, and whose derivatives differentiate into the conducting cells of the secondary xylem and phloem. Ray stem cells are small cells whose derivatives include the radially oriented files of parenchyma cells within wood known as rays.
• The cork cambium is a meristematic layer that develops within mature cells of the cortex and the secondary phloem. Derivatives of the cork cambium differentiate as cork cells that make up a protective layer called the periderm, or bark. The periderm forms the protective outer surface of the secondary plant body, replacing the epidermis in woody stems and roots.

22. Fusiform Stem Cells
Highly elongated, vacuolate cells that differentiate into the conducting cells of xylem and phloem
Vascular Cambium (Lateral meristem)
It does not produce lateral organs, but only the woody tissues of stems and roots. The vascular
cambium contains two types of meristematic cells:
Ray Stem Cells
Small cells whose derivatives include the radially oriented files of parenchyma cells within wood known as Rays

23. Produce floral organs such as sepals, petals, stamens and carpals
Floral meristems
Produce floral organs such as sepals, petals, stamens and carpals
Determinate
Inflorescence meristem
Produces bracts and floral meristems in the axils of bracts
Could be determinate or indeterminate
Determinate meristems:
Genetically programmed limit to their growth
Indeterminate:
No predetermined limit to growth
Consists of one or more leaves, the node to which leaves are attached, internode and one or more axillary buds
Bracts:
A leaf from the axils of which a flower or floral axil arise
In biology and especially botany, indeterminate growth refers to growth that is not terminated in contrast to determinate growththat stops once a genetically pre-determined structure has completely formed
Could also be apical meristems provided they get the developmental potential

24. 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.
Denovo

25. 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 leaves
2- Development of suborgan domains
Primordium differentiates into specific leaf parts
Dorsiventral (abaxial-adaxial)
Proximodistal (apical-basal)
Lateral (margin-blade-midrib)
Abaxial (Dorsal) Adaxial (Ventral
3- cell and tissue differentiation
L1 layer forms epidermis
L2 layer forms photosynthetic mesophyll cells
L3 layers forms vascular elements and bundle sheath cells

26. Structural symmetry in the leaf
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.

28. 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

29. Further Readings
Growth and Development, Chapter 16, Plant Physiology by Taiz and Zeiger