Sexual Reproduction in Flowering Plants: Pre-fertilisation — Notes

Flower. The site of sexual reproduction in angiosperms. The androecium (whorl of stamens) is the male reproductive organ and the gynoecium (pistil) the female.

Stamen. A long, slender filament and a terminal, usually bilobed anther. Each lobe has two thecae (dithecous), so a typical anther is tetrasporangiate: four microsporangia at the corners, which develop into pollen sacs.

Wall of the microsporangium (outside to inside):

• Epidermis — protection.
• Endothecium — fibrous thickenings help in dehiscence of the anther.
• Middle layers (two or three) — protection; degenerate later.
• Tapetum — innermost; nourishes the developing pollen grains; cells have dense cytoplasm and usually more than one nucleus.

Microsporogenesis. Cells of the sporogenous tissue act as pollen (microspore) mother cells, PMC (2n). Each divides by meiosis to form a microspore tetrad (n). As the anther matures and dehydrates, the microspores dissociate and develop into pollen grains.

Pollen grain. Usually spherical, 25–50 µm across, with two wall layers:

• Exine of sporopollenin, one of the most resistant organic materials known: it withstands high temperatures and strong acids and alkalis, and no enzyme is known to degrade it — so pollen is well preserved as fossils. Germ pores are apertures where sporopollenin is absent.
• Intine: thin, continuous, of cellulose and pectin.

The mature pollen grain contains a large vegetative cell (abundant food reserve, large irregular nucleus) and a small, spindle-shaped generative cell floating in its cytoplasm. In over 60 percent of angiosperms pollen is shed at this 2-celled stage; in the rest, the generative cell divides mitotically to give two male gametes before shedding (3-celled stage).

Viability and economic importance.

• Viability: about 30 minutes in rice and wheat; months in some members of Rosaceae, Leguminosae and Solanaceae.
• Pollen banks: pollen stored for years in liquid nitrogen (about −196 °C) for crop-breeding programmes.
• Allergy: pollen of many species (e.g. Parthenium, carrot grass, which came to India as a contaminant with imported wheat) causes chronic respiratory disorders such as asthma and bronchitis.
• Food supplements: pollen is rich in nutrients; pollen tablets and syrups are marketed.

Pistil. Stigma (landing platform for pollen), style (elongated slender part) and ovary (swollen base). Inside the ovary cavity (locule) ovules arise from the placenta. A gynoecium may be monocarpellary or multicarpellary, with carpels free (apocarpous, e.g. Michelia) or fused (syncarpous, e.g. Papaver).

Ovule (megasporangium). Attached to the placenta by the funicle; the junction of ovule and funicle is the hilum. One or two protective integuments cover the ovule except at the micropyle. Opposite the micropyle is the chalaza, the basal part. The nucellus, a mass of cells with abundant food reserve, is enclosed by the integuments; it contains the embryo sac (female gametophyte). The common angiosperm ovule is anatropous (inverted).

Megasporogenesis. One megaspore mother cell (MMC, 2n), large with dense cytoplasm and a prominent nucleus, differentiates in the micropylar region of the nucellus. It undergoes meiosis to form four megaspores (n). In most flowering plants one is functional and three degenerate. Formation of the embryo sac from a single megaspore is called monosporic development.

Female gametophyte. The nucleus of the functional megaspore divides mitotically to form two nuclei, which move to opposite poles (2-nucleate embryo sac). Two more free-nuclear divisions at each pole give 4-nucleate and then 8-nucleate stages; cell walls form only after the 8-nucleate stage.

• Micropylar end: the egg apparatus — two synergids and one egg cell. Synergids have filiform apparatus, special cellular thickenings that guide the pollen tube into a synergid.
• Chalazal end: three antipodals.
• Centre: the large central cell with two polar nuclei. So a typical mature embryo sac is 8-nucleate and 7-celled. All its nuclei are haploid.

Pollination is the transfer of pollen grains from the anther to the stigma of a pistil.

Kinds by source of pollen:

• Autogamy — to the stigma of the same flower. Complete autogamy is rare in open flowers, as it needs synchrony of pollen release and stigma receptivity and close anthers and stigma. Chasmogamous flowers have exposed anthers and stigma; cleistogamous flowers never open, so they are invariably autogamous and give assured seed set even without pollinators (e.g. Viola (common pansy), Oxalis, Commelina, which produce both kinds).
• Geitonogamy — to another flower of the same plant: functionally cross-pollination (involves an agent) but genetically similar to autogamy.
• Xenogamy — to the stigma of a different plant: brings genetically different pollen.

Agencies.

• Wind (anemophily): light, non-sticky pollen produced in large numbers; well-exposed stamens; large, often feathery stigma to trap pollen; usually a single ovule in each ovary; numerous flowers packed into an inflorescence (the corn cob’s tassels are stigmas and styles); common in grasses.
• Water (hydrophily): rare, about 30 genera, mostly monocots: Vallisneria, Hydrilla (fresh water), Zostera (marine sea grass). In Vallisneria female flowers reach the surface on long stalks and male flowers, released on the surface, are carried passively to them. In sea grasses, female flowers stay submerged and long, ribbon-like pollen grains are carried in the water; pollen of water-pollinated species has a mucilaginous covering against wetting. Not all aquatic plants use water: water hyacinth and water lily are pollinated by insects or wind.
• Animals (zoophily): bees, butterflies, flies, beetles, wasps, ants, moths, birds (sunbirds, hummingbirds), bats, some primates and reptiles. Insects, especially bees, dominate. Flowers are large, colourful, fragrant and rich in nectar; small flowers are clustered. Pollen is generally sticky. Floral rewards are nectar and pollen. Amorphophallus (tallest flower, about 6 feet) and Yucca with its moth show flowers that offer safe places to lay eggs. Pollen/nectar robbers consume rewards without pollinating.

Advantages. Self-pollination: needs no agent, seed set assured, maintains purity of line, less waste of pollen. Cross-pollination: produces genetic variation, healthier and more vigorous offspring, and adaptability; continued self-pollination results in inbreeding depression.

Outbreeding devices (contrivances that discourage self-pollination):

1. Dichogamy — pollen release and stigma receptivity are not synchronised: protandry (anthers mature first) or protogyny (stigma receptive first).
2. Herkogamy — anther and stigma are placed at different positions, so pollen cannot reach the stigma of the same flower.
3. Self-incompatibility — a genetic mechanism that prevents self-pollen (from the same flower or other flowers of the same plant) from germinating or its tube from growing in the pistil.
4. Unisexual flowers — monoecious plants (male and female flowers on the same plant: castor, maize) prevent autogamy but not geitonogamy; dioecious plants (male and female flowers on different plants: papaya) prevent both.

Pollen–pistil interaction. Pollination does not guarantee the right pollen. The pistil recognises compatible pollen of the same species and rejects the rest by preventing germination on the stigma or tube growth in the style. Recognition is mediated by chemical components of the pollen interacting with those of the pistil. Compatible pollen germinates to produce a pollen tube through one of the germ pores; the contents of the pollen grain move into the tube, which grows through the stigma and style to reach the ovary. In plants that shed 2-celled pollen, the generative cell divides into two male gametes during tube growth. The tube enters the ovule through the micropyle and then a synergid, guided by the filiform apparatus. All events from pollen deposition to tube entry are pollen–pistil interaction; understanding them helps breeders manipulate crosses, even incompatible ones.

Artificial hybridisation — crossing chosen parents in crop improvement:

• Emasculation: removing the anthers from the flower bud of a bisexual female parent before they dehisce, using forceps.
• Bagging: covering the emasculated flower with a bag of suitable size, usually butter paper, to prevent contamination by unwanted pollen.
• When the stigma becomes receptive, mature pollen from the chosen male parent is dusted on it, and the flower is rebagged to let the fruit develop.
• If the female parent bears unisexual flowers, emasculation is not needed: female buds are simply bagged before they open.