10 Embryogenesis.ppt

Embryogenesis in plants

Embryogenesis…  

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Process of development of embryos May occur naturally from sexual fertilization to form a zygotic embryos- seed Plants can also form somatic embryossomatic embryogenesis 

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Without fertilization

Zygotic and somatic embryogenesis share a number of developmental stages

Zygotic embryogenesis 

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Zygote as a product of sexual fertilization Zygote goes thru mitotic divisions Embryogenesis establishes basic plant body plan only Knowledge of embryogenesis comes from Arabidopsis The stages/process inlvolved: 1) Enlargement and establishment of polarity in zygote  

Apical end Basal end

Embryogenesis… 

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2) First asymmetrical/unequal division  Small apical cell and  large basal cell The two cells follow different developmental pathway 

Apical cell gives rise to embryo proper,

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basal cell gives rise to suspensor

Embryogenesis… 

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3) Apical cell divides first longitudinally, and transversally to produce an 8 cell proembryo called an octant Divisions are precise, and simultaneous Some differentiation takes place  Upper 4 cell layers become different from lower 4 cells 4) Octant divides several times to produce a globular embryo (globular stage)

Embryogenesis… 

At globular stage, cell differentiation occurs;   

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Outer cell will become protoderm-epidermis Middle cells will become ground meristem Innermost cells will form procambial cells This is known as radial patterning of tissues

5) Globular embryo expands rapidly-by cell divisions of cells in apical end to produce a structure resembling a heartthe heart stage

Embryogenesis…  

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Uppermost part becomes cotyledons Shoot Apical meristem is established between the two cotyledons 6) Basal cell divides several times to give rise to a suspensor  uppermost cell of suspensor becomes the hypophysis Hypophysis participates in embryo formation to form part of root apical meristem

Embryogenesis… 7) Hypophysis continue to divide and establish the root apical meristem  At this stage, the Shoot –root axis/apical-basal end of plant is established 

embryogenesis 

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8) the heart-shaped embryo elongates further to form the torpedo stage embryo 9) further growth of torpedal embryo may cause it to curve  

restricted by space in ovule Embryo looks like a walking stick- this is called walking stick stage

Embryogenesis… 

Note:  Establishment of body plan of embryo occurs very early during embryogenesis  Many genes guide the development of the embryo such as the body plan and patterning  Some of these are homeotic genes

Embryogenesis… 

Hormones are involved:  Embryogenesis—  period of high cell division- IAA and cytokinins are high  IAA- involved in patterning and establishment of polarity in young embryo  GA’s- cell expansion of embryo and endosperm

Embryogenesis…

Summary- major events 

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1) establishment of polarity in zygote 2) radial patterning  Establishment of initial cells protoderm, ground meristem & procambium – @ globular stage 3) Establishment of shoot- root axis ( apical –basal pattern @end of globular/heart stage 4) Establishment of shoot-root apical meristems @ heart stage 5) initiation of cotyledons- @ heart stage