Reproduction

Plant reproduction

Plant phylogeny

Alternation of generationsAll sexually reproducing organisms have an alternation of generations

– the organism passes through a haploid and a diploid stage in their life cycle – in humans, for example, the diploid state is us; the haploid state is the gamete (sperm

and egg). All the machinery for producing the gametes is contained within the individual.

In plants, however, the machinery for producing gametes is not always contained within the individual.

– Haploid - gametophyte, produces gametes (then syngamy) – Diploid - sporophyte, produces spores (meiosis)

Primitively, the haploid state is the dominant stage – the diploid is only briefly seen

However, as plants evolved, the diploid state became more dominant

Plants with dominant haploid stagesChlorophyta - green algae. Diverse morphology (solitary, colonial, multicellular) • Aquatic (usually fresh water) • Even though the chlorophytes are more the

sister taxa to the terrestrial plants, they are often placed in the Kingdom Protista. This is incorrect in a phylogenetic context.

Bryophta - mosses, liverworts, hornworts. "Amphibians of the plant world." • No vascular tissue. • Only gametophyte is photosynthetic (free-

living). • Sporophyte is a parasite • How do they deal with life on land?

– They are small, low to the ground, only grow where moist.

• What are some limitations to this life-style? – Reduced environmental tolerance. Poor genetics

(haploid dominant)

Plants with dominant diploid stages

Lycophyta (lycopods) • free-living sporophyte and gametophytes • leaves and roots different from other land plants • in the geologic past (ca 300 MYA), the lycopods were the

dominant floral elements with large, expansive swamp-forests of lycopods. Much of the coal and oil found today were formed from the extensive growth of these plants

Psilophyta (wiskferns) free-living sporophyte and gametophyte • no leaves or roots • three-lobes spornangia • one known genus survives today, but they were much more

diverse in the pastNOTE even though they are called "wiskferns," they are not true ferns

Sphenophyta (horsetails) free-living sporophyte and gametophyte • circular, ribbed stems with whorled leaves

again, much more diverse in the pastPteridophyta (true ferns) free-living sporophyte and gametophyte • vascular tissue • by far the most diverse non-seed land plant today

Plants with dominant diploid stages: Gymnosperms

Cycadophyta - cycads (look like palms) • Pollination by wind and insects. • Seeds eaten by birds - poisonous to humans (causes slow,

degenerative brain disease)Coniferophyta - conifers (pines, firs, spruces, hemlocks) • Wind pollination• Produce cones or ovules - two year cycle • Taxus and others can produce fleshy arils which are edible

by animals – dispersal strategy similar to angiosperms with fruits

Ginkgophyta - ginkgo. Only one species kept alive in monasteries • Very resistant to disease and pollution - often planted in

cities • Stinky seeds

The evolution of pollen and seeds

• Pollen - male gametophyte, protected by a spore wall (protects from harsh environment) • Gain - protection •Loss - need some mechanism to carry pollen (air, insects)

•Ovule - contains the female gametophyte (the egg sac), protected by sporophyte tissue (the integument - this becomes the seed coat) • Gain - protection •Loss - extra care for sporophyte

•Seeds - baby plant in a box with its lunch The seed is one of the most distinct adaptations in the plant world •The young sporophyte no longer needs to grow and photosynthesize immediately (can go dormant). Can be dispersed (air, animal vector) •The plant produces food for each ovule, in the hopes that it will be fertilized • Gain - protection, dispersal, dormancy •Loss - great energy cost to plant

Adaptations to reduce the vulnerability of the gametophyte - gametophyte stage is no longer free-living. • Gains - protection of gametophyte • Losses - more stress on the sporophyte

Plants with dominant diploid stages: Angiosperms (the flowering plants)

Double fertilization– Allows for diversification – Acts as a signal to produce food only when fertilized - no more baby

food before the baby! – Pollen sperm with two nuclei - one for fertilization of egg, one for polar

bodies – Much more rapid life cycle

• Coevolution with pollinators - insects and plants (pollen dispersal) – Possess flowers to attract pollinators

• Coevolution with larger animals - mammals/birds and plants (seed dispersal) – Possess edible fruits and seeds to attract dispersal organisms

Double fertilization

Flower structure• Sepals, • Petals, • Stamens, • Carpels

Fruits • A fruit is a ripened ovary - it contains the seeds • Often the fruit is edible - why would this be advantageous to a plant? • There is a wide variety of fruit types - but all fruits are basically formed from a ripened ovary Seeds • A seed is a ripened or mature ovule • Often the seed is edible - this serves the same function as do edible fruits • There is a wide variety of seed types Vegetables • A vegetable is an edible part of a plant that is not part of the reproductive organs (edible roots, stems, leaves, etc.)

Types of angiosperms

The AntherThe anther is deeply bilobed. Within each lobe there are two microsporangia. At this stage of anther development there are numerous diploid microsporocytes(pollen mother cells) in the centre of each microsporangium. A single layer of columnar shaped young tapetum cells surrounds the central microsporocytes. The diploid tapetum cells transfer nutrients and certain cell wall materials to the developing pollen. As anther development proceeds, the microsporocytes divide by meiosis to produce a spherical tetrad of four microspores.

The microspores separate from the tetrad, enlarge and form a thick two layered wall (exine and intine). Within the microspore (endosporic development) the nucleus divides by mitosis. A cell membrane develops around one of the nuclei. This is the elliptical shaped generative cell. The other nucleus is the tube cell nucleus. These two cells constitute the early microgametophyte (pollen grain).

The AntherThe anther splits longitudinally (microsporangium dehiscence) and the pollen grains are released. They are transferred through the process of pollination to the stigma of the pistil (carpel). A pollen tube develops from an outgrowth of the inner (intine) wall of the pollen grain and begins to digest its way through the tissue of the stigma and style of the pistil (carpel). Unlike Pine, growth of the pollen tube in angiosperms is fairly rapid. The time interval between pollen tube initiation and the tube reaching an unfertilized ovule is quite variable, depending on the species. It is generally from a few hours to one to two days. During pollen tube growth in Lilium the generative cell divides once by mitosis to produce two sperm cells. The mature microgametophyte consists of the tube cell nucleus and two sperm.

The Anther

The OvuleDepending on the species, there may be from one to several hundred ovules produced in the ovary of each pistil (carpel). The ovule primordium is a small dome-shaped swelling that arises from the inner wall of the ovary. Two envelopes of tissue, the integuments, develop from the base of the primordium and completely enclose the megasporangium (nucellus). A micropyle is present at one end of the ovule. In the majority of angiosperms a single cell of the megasporangium, the megasporocyte (megaspore mother cell), enlarges and divides by meiosis to produce four haploid megaspore cells, three of which degenerate. The surviving megaspore enlarges and by means of three successive mitotic divisions, within the megaspore, gives rise to an eight nucleate megagametophyte (embryo-sac). Thin walls develop around six of the nuclei. At the micropyle end of the megagametophyte an egg cell is flanked by two synergids. At the opposite end of the megagametophyte are three antipodals. The large central region of the megagametophyte is referred to as the central cell and contains two polar nuclei.

Embryo sacEach ovary contains one or more ovules. The green structure at the top of the diagram is the ovule. The integuments are the 2 walls of the ovule. There is a small opening in the walls called a micropyle. This is where the pollen tube will enter. (Will be discussed later.) The nucellus is cells that provide nutrition for the growth of the ovule. The embryo sac, also known as the megaspore, divides by meiosis to form 4 haploid cells. Three of these cells degenerate and one remains. Only one megaspore survives in each ovule. This becomes the embryo sac. The haploid nucleus of the surviving megaspore undergoes three mitotic divisions. Eight haploid nuclei are now present. Within the swollen ‘megaspore cell’ six haploid cells and two ‘polar nuclei’ are formed. The entire structure is called the embryo sac. One of the cells near to the micropyle end of the ovule is the haploid female gamete (egg cell).

PollinationThe transfer of pollen from the anther to the female stigma is termed pollination. This is accomplished by a variety of methods. • Entomophyly is the transfer of pollen by an

insect. • Anemophyly is the transfer of pollen by

wind. Other pollinators include birds, bats, water, and humans. Some flowers (for example garden peas) develop in such a way as to pollinate themselves. Others have mechanisms to ensure pollination with another flower.

Flower color is thought to indicate the nature of pollinator: red petals are thought to attract birds, yellow for bees, and white for moths. Wind pollinated flowers have reduced petals, such as oaks and grasses.

Pollination

GynoeciumThe gynoecium consists of the stigma, style, and ovary containing one or more ovules. These three structures are often termed a pistil or carpel. In many plants, the pistils will fuse for all or part of their length.

The Stigma and Style

The stigma functions as a receptive surface on which pollen lands and germinates its pollen tube. Corn silk is part stigma, part style. The style serves to move the stigma some distance from the ovary. This distance is species specific.

The Ovary

The ovary contains one or more ovules, which in turn contain one female gametophyte, also referred to in angiosperms as the embryo sac. Some plants, such as cherry, have only a single ovary which produces two ovules. Only one ovule will develop into a seed.

Gynoecium fusion

A syncarpous gynoecium is composed of two or more connate carpels. You can often tell that carpels are connate when several stigmas are present. The term "compound pistil" is equivalent to a "syncarpous gynoecium".

Gynoecium fusion

When there are two or more distinct carpels per flower, the flower is apocarpous.

Gynoecium fusion

Flowers that have only one carpel are said to have a monocarpous gynoecium.

PlacentationThe arrangement and attachment of ovules within the ovary is called placentation.

• The placentae is the region or the line(s) along which the ovules are attached.

• A locule is a chamber within the ovary.

• In monocarpous and apocarpous gynoecia (i.e. carpels distinct), the ovules are arranged along the suture of the carpel. There is one locule per carpel, no septum. This is called marginal placentation.

PlacentationA syncarpous gynoecium is composed of two or more connate carpels.In a syncarpous gynoecium, there can be one or more locules, and various possible types of placentation. This can be observed on cross- and lateral sections of the ovary.

A septum (= "wall") is an interior wall which separates the locules when two or more chambers occur. The presence of septa is characteristic of axile placentation.

The placentation is axile when there are septa which divide the ovary into two or more locules. The ovules are attached along the central axis, in the inner angle formed by the septa.

Parietal placentation: there is no septum, so that the ovary is unilocular. The ovules are borne on the inner surface of the ovary walls (or extensions of the walls).

Free-central placentation : the ovules are borne on a central column which is not connected to the walls of the ovary by septa, i.e. there is only one locule, and the column does NOT reach the top of the ovary .

PlacentationPlacentation can be also deduced (generally more easily) from the observation of older ovaries, and fruits (while stamens have to be observed on fresh or even non-open flowers). But sometimes, septa tend to secondarily disappear in ovaries with axile placentation. Thus when a cross-section of an ovary or a fruit show only one locule, it is necessary to also observe a longitudinal section of the ovary to say whether the placentation is free-central (the column does not reach the top of the ovary) or axile (the column reaches the top of the ovary).

Placentation

Placentation

FertilizationThe pollen tube grows up the micropyle of the ovule, through the megasporangium and penetrates the synergid of the megagametophyte. The tip of the tube bursts and the sperm are released.

A unique feature of the angiosperms is the process of "double fertilization". One sperm fuses with the egg to produce the zygote, the other sperm migrates to the central cell and fuses with the two polar nuclei (fusing nucleus) to produce the primary endosperm nucleus.

With double fertilization a number of processes are initiated:

1. The zygote develops into an embryo 2. The integuments develop into a seed coat 3. The ovary develops into a fruit 4. The primary endosperm nucleus divides to form endosperm

The endosperm functions as the nutritive tissue for the developing embryo and, in many cases, as the food reserve for the mature embryo during seed germination

Seed FormationThe fertilized becomes the seed. The integuments become the wall of the seed called the testa. The micropyle closes. The endosperm nucleus leads to the formation of triploid endosperm, a food tissue. The diploid zygote, by mitosis, develops into a plant embryo. The developing embryo draws nourishment from the endosperm. The embryo ceases development and goes dormant. The ovule becomes a seed, which contains a dormant plant embryo, food reserve, and the protective coat called the testa.

The EmbryoThe embryo is made up of the radicle or future root and the plumule or future shoot. The endosperm cells divide many times and absorb the nucellus. This is the nutrition (mainly fats, oils and starch) for the embryo.

There are 2 types of seeds. Some are endospermic while others are non-endospermic. In endospermic seeds the food reserve is the endosperm, which is outside the plant embryo. Examples of this type of seed are maize and wheat. Non-endospermic seeds have food reserve within the cotyledon(s) of the plant embryo. This occurs in broad beans.

GerminationThe embryo will germinate from the seed if the proper environmental conditions are present. When this occurs the embryo resumes its growth. In order for germination to occur the following conditions must be present:

• Water which allows the seed to swell and enzymes to function.

• Oxygen must be present in the soil for intensive metabolism.

• The temperature must be suitable for the species of plant. Suitable temperatures are usually between 5-30 degrees Celsius depending on the species.

The dormancy period must be complete.Some seeds need light and others need darkness.

Events of Germination

When germination begins the first thing that happens is water is absorbed by the seed through the micropyle and through the testa. Enzymes in the soil now digest the foods stored in the seeds:· Oils become fatty acids and glycerol· Starch becomes glucose· Protein becomes amino acids· These foods now are absorbed by the embryo. · The glucose and amino acids make new structures such as cell walls and enzymes. · The fats and glucose are used in cellular respiration to produce energy.· The stored food of the seed is being used up as the embryo grows larger.· The radicle grows larger and breaks through the testa. It becomes the roots of the new plant.· The plumule grows larger and emerges above the ground. · Leaves form.

Germination in dicots

Germination in monocots

Sources

Big thanks to http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect18.htm