Plant Growth and Development Plant Physiology 3(2-1)

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?

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 endosperm?

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 Arabidopsis thaliana

Embryogenesis and plant development:  Axial patterning  Radial patterning  Primary meristems Axial Patterning Shoot apical meristem Root apical meristem Cytoplasmic Large vacuole

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 meristem Middle Region: gives rise to hypocotyl, root and most of the root meristem Hypophysis: gives rise to the rest of root meristem

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

Primary meristems ProtodermProcambiumGround 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

Embryo development in Arabidopsis Globular stage Heart stage Torpedo stage Maturation stage Cell division in apical regions that later form cotyledons Cell 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?

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 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 factros Intersting????

Genes involved in Embryogenesis GNOM gene MONOPTEROS gene SHORT ROOT and SCARECROW genes HOBBIT gene SHOOTMERISTEMLESS gene Plays role in Axial Patterning No root and cotyledons No hypocotyl and root Both take part in Radial Patterning 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

Acknowledgement Plant Physiology by Taiz and Zeiger Wikipedia