Megasporogenesis and microsporogenesismegasporocyte microsporocyte

Generates megaspore and microspores

meiosis

REVIEW

Examples of angiosperm diversity in pollen and stigma structuresA.Hydrated Lilium longiflorum pollen grain. Red stained hasseveral fissures at the single aperture . Green coating (green) forms droplets on the surfaces of the ornamented exine wallsB.Multiapertured pollen of Passiflora fanguinolentaC.Polyad pollen of Acacia retinoidesD.Several pollen typesE.A portion of stigma of Torenia fourmiereF.Stigma papillae of Arabidopsis

D

The exine protects the spore against dessication, mechanical pressure and ultraviolet radiation. Sometimes the exine layer is covered by sticky substances (pollenkitt, tryphine, elastoviscin and sporopollenin viscin threads), which are also produced by the tapetum.

Pollenkitt- an adhesive material facilitates the attachment of pollen grains to insects, and in this way also zoophilic pollination.

It also plays an important role in the adhesion of pollen grains to the female stigma and in the recognition between pollen and pistil. Also substances responsible for pollen allergy are often products originating from the tapetum.  

Pollen grains

A. Pollen tetrad from Arabidopsis mutant quartet. Four complete pollen grains connected byexine bridges

B. Fused pollen grains of Arabidopsis mutant tes/stud. Cytokinesis does not occur after meiosis resulting in multinucleate cytoplasm

c. Natural pollen tetrads from Drocera binata

The exine protects the spore against dessication, mechanical pressure and ultraviolet radiation. Sometimes the exine layer is covered by sticky substances (pollenkitt, tryphine, elastoviscin and sporopollenin viscin threads), which are also produced by the tapetum.

Pollenkitt- an adhesive material facilitates the attachment of pollen grains to insects, and in this way also zoophilic pollination.

It also plays an important role in the adhesion of pollen grains to the female stigma and in the recognition between pollen and pistil. Also substances responsible for pollen allergy are often products originating from the tapetum.  

Pollen grains

Pollen outer surface

Outer exine- multilayered , sporopollenin, interruptedby apertures

Inner intine- sometimesmultilayered with cellulose

Pollen coat- lipids, proteinspigments, aromatic compounds fill the sculptured cavities of exine.,

Pollen-stigma interaction

3 strata ( relative amountvarying between species

C

Pollen tube

Stigma- receptive part of pistil, bind pollen ,mediate tube migration into style:

Receptive stigma are of 2 types:

Wet- covered with surface cells , often lyse & release secretions. Pollen simply sinks in the viscuous fluid.

Stigma papillaefrom Arabidopsis

Dry- intact surface cells, protrusion as papillae and cells covered with cell wall, waxy cuticle and proteinaceous pellicle..

Never has copious surface secretion. Possibilility for pollen attraction and retention is adherence by chemical bonds.

Ex. A chemical bond forms where the exine contacts the pellicle or where tapetal coatings on exine flows on the stigma surface.

stigma

Pollen-stigma functions

1. Initial adhesion: exine-mediated May depend on biophysical or chemical interactions between stigma surface and exine polymers

2.Mobilization of pollen coat leads to mixing of lipids and proteins and form “foot” of contact on stigma surface.

3. Later stage: Proteins and lipids on pollen coat and proteins on stigma surface contribute to adhesion, requires protein-protein interaction

A. Pollen adhesion

D. Pollen coat has mobilized to the site of contact between pollen and the stigma forming a foot between the 2 surfaces

B. Pollen hydration: Activating metabolism. Pollen highly dessicated: 15 to 35 % water content when released from anthers. On wet stigmas water immediately surrounds pollen grainsOn dry stigmas pollen mobilize their lipid coat to form an interface between the two surfaces. This interface will promote water flow.

waternutrients, etc.

Transported rapidly into the grain from stigma exudate (wet stigma )or from stigma papillae from dry stigma

Aquaporins expression in the stigma : water channels involved in the rapid but regulated water release from stigma to pollen

Aquaporins are transport proteins that facilitate water transport across membranes

C. Pollen polarization and germination: preparing for pollen tube growth

Hydration transforms a pollen from non-polar cell to highly polarized cell. Within minutes after hydration pollen organizes its cytoplasm and cytoskeleton structures to support extension of a single tube:

Formation of filamentous cytoskeletal structures that wrap around the nuclei

Actin cytoskeleton polarization toward the site of tube emergence

Reorientation of large vegetative nucleus so that it enters the extending tube before the generative cells

Assembly of mitochondria and polysaccharide particles at the site of elongating tube tip

Selection of pollen plasma membrane for secretory vesicle targeting and deposition of callose at site of tube emergence

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

Cytoplasmic domainA Clear zone--------------------devoid of large organelles, no organized

movementB Subapical domain C. Nuclear domain D. vacuolar domain

Pollen tube elongation up to many cm thru female tissues

Pollen tubesare polarized cells.

Internal differentiation withdistinct intracellularzones

A. Tip domain- rich in Golgi vesicles B. Sub-apical domain- with metabolically active organelles: mitochondria, dictyosomes, ER, vesiclesC. Nuclear domain: large organelles and male germ unitD. Vacuolar domain. Enlarges as the tube grows.

• Secretory vesicles originate from the Golgi network and are transported over the actin cytoskeleton into the growing tip where they fuse with the tip membrane expanding the wall and the plasma membrane.

• Besides pollen tube wall proteins, these transport vesicles contain pectins as part of the primary tube tip wall.

• Total cytoplasmic vol. does not increase as pollen tube grows; bulk of cytoplasm is in close proximity to growing tip

Germinating pollen

Cytoplasm continues to move with the tip as tube elongates

Cytoskeleton continually transports organelles, the generative or sperm cells and vegetative nucleus toward growing tip

Shows “reverse fountain” cytoplasmic streaming-forward movement of organelles thru cortical region of the tube, undergoes a turnover in the subapical domain moving back centrally, away from the tip of the tube.

• Golgi/dictyosome-derived vesicles containing callose (not cellulose) transported to tip by microfilaments.

Vesicles fuse with plasma membrane at tip of tube so expands the wall

contents of vesicles expand the plasma membrane as the tube grows

vesicles

pollination

POLLEN TUBE GUIDANCE ON THE PISTIL 1. THRU PAPILLA CELL WALL OF DRY STIGMA OR INTERCELLULAR SPACES 2. BETWEEN CELLS IN SECRETORY REGIONS OF WET STIGMAS.

12hrs 24 hrs 6d

A. Porogamy (pollen directed to the micropyle by molecular signal

B. Chalazogamy ( pollen adhere and grow up the surface of chalazal region. C. Mesogamy (pollen directed to the micropyle by molecular signals from synergids)

porogamychalazogamy Mesogamy

(thru funiculus)

Entry of pollen tube in the embryo sac is under chemotropic guidance (synergids have filiform apparatus and secrete some chemicals). Once inside the cytoplasm of the synergid, pollen tube growth stops, tip ruptures, release the 2 sperm. One of the synergid cells begin to degenerate as pollen tube enters it.

Release of pollen tube contents into the synergid : A. Showing two male gametes and a degenerating vegetative nucleus near the filiform apparatus, B. Release of male gametes inside the synergid, C. Movement of male gametes towards the egg nucleus and into the central cell.

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.

Tube enters at the apex of the filiform apparatus and after growing, 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.

Degenerating synergid

filiform- specializedregion of the synergid cell wall

Synergids “ synergos”- working togetherCoined by Eduard Strasburger 19th century botanist (Vesque, 1878)

filiform apparatusIncreases surface area of plasma membrane in the region, also associated with elaborate ERSynergids begin to degenerate as pollen tube enters it or shortlyPollen tube guidance (shown by laser ablation techniques to selectively remove different cells within the female gametophyte)Mediates transport of molecules into and out of synergidsNecessary for the cessation of pollen tube growth and release of sperm cells.

MYB98 protein- functions as regulator of genes expressed in synergids required for formation of filiform apparatus, Mutants myb98 show defects in pollen tube guidance and dev of filiform apparatus

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

funiculus

chalaza- region where integuments fuse with funiculus

Pollination

Double fertilization