Microsporogenesis- formation of spores called microspores

Microgametogenesis-development of microspore into the microgametophyte or the pollen grain containing sperm cells

Anthers surrounding a central ovary in Lily

microspores

Pollen development and maturation

The end of meiosis in the microsporocyte or microspore mother cell marks a turning point in microsporogenesis .

Results in the production of 4 microspores, each with its own callose envelope. A candidate gene for separation of microspores from the tetrad in Arabidopsis anthers is designated as QUARTET (QRT).Outcome of microsporogenesis affected by this mutation is release of microspore in tetrads Failure of microspore separation in qrt mutants Traced to the fusion of exine layer of adjacent microspores Failure of protein degradation

Pollen development and maturation

The end of meiosis in the microsporocyte or microspore mother cell marks a turning point in microsporogenesis .

Results in the production of 4 microspores, each with its own callose envelope. A candidate gene for separation of microspores from the tetrad in Arabidopsis anthers is designated as QUARTET (QRT).Outcome of microsporogenesis affected by this mutation is release of microspore in tetrads Failure of microspore separation in qrt mutants Traced to the fusion of exine layer of adjacent microspores Failure of protein degradation

Pectin is absent in primary wall of wild type microspores at the time of release from tetrad Pectin remains as integral part of the microspore wall of mutant

QRT gene functions in degradation of pectin in order to separate the microspore from tetrad.

Model of primary cell wall

Pollen grain has two cells: from first mitotic division

Vegetative cell- develops into pollen tube. Contains most cytoplasmic organelles

Generative cell- small, produces the sperm-cytoplasm partitioned unequally during mitotic division of microspore -lacks mitochondria and chloroplast -at some point in pollen dev., divides by

mitosis, each daughter cell differentiates into sperm cells, will lack also chloroplasts and mitochondria. This is the basis of for the maternal inheritance of chloroplast and mitochondrial genomes which occurs in ca 90% of all angiosperm species.

Parts of ovule

1.nucellus- central body with vegetative cells enclosing theSporogenous cells2. 1 or 2 integuments (unitegmic or bitegmic) enclosing the Nucellus3. funiculus-stalk connecting ovule with the placenta.4. chalaza-where nucellus, integuments and the funiculus merge

X-section of an ovary Ovary contains a cavitylined with an epidermal layer. Ovules develop from the epidermal cells and are contained within the cavity of the ovary, attached to its inner surface by a short stalk- funiculus

Ovary bears ovules on a ridge on the ovary wall called placenta

Developing ovules of Lilium. Ovule emerges from the placenta as conical protuberance with the first sporogenous cell,called archesporial cell. Integuments formed by periclinal div. of epidermis

Chalazal region

A, D. (bel mutant) exposed nucellus and a single integument

F. Larger mutant ovuleOuter integument has many cells.

Megaspore mother celldifferentiates from surrounding nucellar tissue and undergoes meiosis.

Inner integ.

Outer integ

Begin as ridges of tissue early inOvule dev.

C, E,F differential growth of ovule causes them to curve so micropyle is bent around funiculus and placenta (C).

Development of embryo sac and female gamete (in an anatropous ovule)

A hypodermal cell of the nucellus enlarges and becomes differentiated into a megaspore mother cell or megasporocyte. This diploid megaspore mother

cell increases in size and undergoes meiosis to form a linear tetrad of 4 haploid megaspores, 3 of which

degenerate and the 4th becomes the functional megaspore in monosporic types, all 4 become

functional in tetrasporic typesFemale Gametophyte

The nucleus of the megaspore undergoes three successive mitotic divisions forming eight nuclei. The

megaspore enlarges into an oval shaped structure called the embryo sac. The eight nuclei of the embryo sac arrange themselves in 3 groups.

Micropyle

Inner integument

Outer integument

placenta

funuculus

Outer and inner integument completely overgrow the nucellusExcept for the micropyle.

--Begins with elongation of the functional megaspore, usually at chalazal end.-- initially megaspore is non-vacuolate but later small vacuoles appear which may fuse to form large vacuole.

Development of embryo sac

A. First megaspore mitosis yields binucleate embryo sac. Spindle of first nuclear div oriented along the long axis of the cell. Wall formation Does Not follow the nuclear division. Both nuclei divide 2x, forming 4 in B then 8 in C

B. Large vacuole appears between the two daughter nuclei. As cell expands, nuclei are pushed toward opposite poles of the cell. Both nuclei from each pole divide twice

D. The 8 nuclei arrange themselves in two clusters of 4 nuclei one at each opposite ends. One nucleus from each end migrates towards the middle, called polar nuclei (named for where they came from, not where they end up).

C.8-nucleate state . All 8 nuclei are present in a common cytoplasm, they move around probably from remnants of spindle fibers from earlier divisions.

Chalazal trio called antipodals( Latin “against the foot”) at opposite end of the egg and antipodals

Egg apparatus consists of larger egg flanked by two smaller cells called synergids (greek for “helpers” or cooperators

The large binucleate

nucellus

funiculus

Megaspore mother cell devs. from surrounding nucellar tissue and undergoes meiotic division to form megaspore. Nucellus considered as a megasporangium

funiculus

nucellus

chalaza- region where integuments fuse with funiculus

megasporogenesis

Megagameto-genesis

8-nuclei stage of embryo sac

micropyle

funiculus

Mutants in ovule determination

1. bell (bel1)- ovule lacks inner integument

2. Aberrant testa shape (ats)- no clear distinction between inner and outer integument

3. Extreme types of integument mutations:

aintegumenta (ant)

huellenhos (hll)Do not develop integument and embryo is disrupted.

Megasporogenesis Differential growth causesovules to curve so the micropyle is bent around to the funiculus and the placenta

Embryo sac cells1.Egg- highly vacuolate, strongly polarized. In Arabidopsis, a large vacuole aligned toward micropylar end and an aggregation of cytoplasmic organelles and nucleus atchalazal end. Ultrastructural simplicity of cytoplasmcharacterize egg cells. amount of cytoplasm is limited cytoplasm spread as a thin layer surrounding vacuole cytoplasm contains very little ER, limited no. of plastids

mitochondria, dictyosomes but high ribosomes which are randomly distributed rather than aggregated as polyribosomes cell wall does not extend over the entire cell but wall

shows various attenuation toward chalazal pole

2 Synergids- limited life span, wilt after fertilization. Probably involved in nutrition of egg. has extensive wall ingrowth at micropylar region

called filiform apparatus metabolically active

3 Antipodals-transient existence , minimal cytoplasmic organelle show nuclear abnormalities like

endoreplication

2 polar nuclei-metabolically active, extensive ER, numerous plastids, mitochondria, dictyosomes and polysomes, has large quantities of starch, proteins and lipids

Landing on the stigma

Pollen tubes

Germination of pollen tube

Pollen tubes extend up to sev. cm to reach embryo sac. Cell wall lacks cellulose but has another polysaccharide- callose- , a glucan

Callose –synthesized by Golgi and transported to the extreme tip of pollen tube by Golgi-derived vesicles .Fusion of vesicles with plasma membrane expand the cell membrane of

elongating tubeContent of vesicles expand the wall of elongating tube

In angiosperms, to effect fertilization, the pollen grainsgerminate on the stigma by putting forth tubes (pollen tubes) which grow thru the style and find their way into the ovules where they discharge the sperms in thevicinity of the egg.

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Tube enters at the apex of the filiform apparatus and after growing thru it arrives in the cytoplasm of the synergid. The penetrated synergid starts degenerating before the arrival of the Pollen tube, but after pollination. The process of discharge takes place in seconds.

In cotton, the contents of the tube are discharged thru a subterminal pore which faces the chalaza.

Pollen tube discharge: includes 2 sperms, the veg, nucleus and a fair amount of cytoplasm. A portion of cytoplasm is retained in the pollen tube.No mixing between cytoplasm released by the pollen tube and that of the synergid. They remain as two separate entities.