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?
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 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
Embryogenesis and plant development: • Axial patterning • Radial patterning • Primary meristems Shoot apical meristem Ovule Root apical meristem Axial Patterning
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?
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:
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.
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
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 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
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+leafprimordia
Shoot Apical Meristem Structure CytohistologicalZonation Like Quiescent center in roots internal tissues of stem
Preembryonic Meristems Postembryonic Meristems Primary meristems Root meristem Shoot meristem Secondary meristems Axillary Inflorescence Floral Intercalary lateral • 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 • Axillary • Formed in the leaf axils • Derivative of shoot apical merstem • Produce branches • Intercalary • Found within organs, near their bases • Enables grasses to continue to grow despite mowing or grazing
Fusiform Stem Cells Highly elongated, vacuolate cells that differentiate into the conducting cells of xylem and phloem Vascular Cambium (Lateral meristem) Woody tissues of stems and roots Ray Stem Cells Small 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 • 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 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 leaves • 2- 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
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.
Leaf Primordia Arrangement Phyllotaxy: The arrangement of leaves around the stem Opposite leaves per node at right angle to each other Single leaf Paired leaf More than two leaves per node Spiral arrangement of leaves
Further Readings • Growth and Development, Chapter 16, Plant Physiology by Taiz and Zeiger