Plant Growth and Development Plant Physiology 3(2-1) 15-02-2012
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 Wheat endosperm? Surrounds embryo and provides nutrition in the form of starch Small Egg
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
Embryogenesis and plant development: • Axial patterning • Radial patterning • Primary meristems Shoot apical meristem Cytoplasmic Large vacuole Root apical meristem Axial Patterning
First division of zygote • Apical cell: receives more cytoplasm • Divides vertically • Generates globular (octant) embryo • Basal cell: receives large vacuole • Horizontal division • Suspensor cells 6-9 cells that attach the embryo to the vascular system • Hypophysisderivative 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 Protoderm Procambium Ground meristem Epidermis Cortex and endodermis Primary vascular tissues and vascular cambium Vascular Tissues: The tissue in vascular plants that circulates fluid and nutrients. Comprise of; 1- Xylemconducts water and nutrients up from the roots 2-Phloemdistributes food from the leaves to other parts of the plant
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?
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???? 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 dormancyor 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.
Genes involved in Embryogenesis Plays role in Axial Patterning No root and cotyledons GNOM gene MONOPTEROS gene No hypocotyl and root SHORT ROOT and SCARECROW genes Both take part in Radial Patterning HOBBIT gene Defective root meristem development SHOOTMERISTEMLESS gene Mutants fail to form shoot meristem
HOBBIT gene 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 • hbtmutant 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