Plant Reproduction More Exciting Than You Think
Ever seen Silence of the Lambs?? Fava beans anyone? Check this. Now for Chapter 4 of Survival of the Sickest.
Fig. 38.1
2. Flowers are specialized leaves of the angiosperm sporophyte Flowers are typically composed of four whorls of highly modified leaves called floral organs. The four kinds of floral organs are the sepals, petals, stamens, and carpels (or pistils). Their site of attachment to the stem is the receptacle.
Flower parts Fig. 38.2
The stamens and carpels of flowers contain sporangia, within which spores are made by??? What develop from the spores???? The male gametophytes are sperm-producing structures called pollen grains, which form within anthers. The female gametophytes are egg-producing structures called embryo sacs, which form within the ovules in ovaries. Notice that these gametophytes are VERY reduced (small and dependant).
There are complete flowers, those having all four organs, and incomplete flowers, those lacking one or more of the four floral parts. A bisexual flower (in older terminology a perfect flower) is equipped with both stamens and carpels. A unisexual flower is missing either stamens (therefore, a carpellate flower) or carpels (therefore, a staminate flower).
A monoecious plant has staminate and carpellate flowers at separate locations on the same individual plant. For example, maize and other corn varieties have ears derived from clusters of carpellate flowers, while the tassels consist of staminate flowers. A dioecious species has staminate flowers and carpellate flowers on separate plants. For example, date palms have carpellate individuals that produce dates and staminate individuals that produce pollen. Holly and marijuana are also dioecious.
The development of angiosperm gametophytes involves meiosis and mitosis. How about a review of that Alternation of Generations thing? Fig. 38.4
The male gametophyte begins its development within the sporangia (pollen sacs) of the anther. Within the sporangia are microsporocytes, each of which will form four haploid microspores through meiosis. Each microspore can eventually give rise to a pollen grain.
A microspore divides once by mitosis and produces a generative cell and a tube cell. The generative cell will eventually form 2 sperm cells. The tube cell, enclosing the generative cell, produces the pollen tube, which delivers sperm to the egg. This two-celled structure is encased in a thick, ornate, distinctive, and resistant wall. This is a pollen grain, an immature male gametophyte. Fig. 38.5
A pollen grain becomes a mature gametophyte when the generative cell divides by mitosis to form two sperm cells. In most species, this occurs after the pollen grain lands on the stigma of the carpel and the pollen tube begins to form.
Ovules form within the chambers of the ovary. One cell in each ovule, the megasporocyte, goes through meiosis, producing four haploid megaspores. In many angiosperms, only one megaspore survives. This megaspore divides by mitosis three times, resulting in one cell with eight haploid nuclei. Membranes partition this mass into a multicellular female gametophyte - the embryo sac.
Two synergid cells flank the egg cell. The synergids function in the attraction and guidance of the pollen tube. At the other end of the egg sac are three antipodal cells of unknown function. The other two nuclei, the polar nuclei, share the cytoplasm of the large central cell of the embryo sac. The ovule now consists of the embryo sac and the surrounding integuments (from the sporophyte). This will eventually form a SEED.
Pollination is next. Some plants, such as grasses and many trees, use the wind. They make LOTS of pollen. Most angiosperms interact with insects or other animals that transfer pollen directly between flowers, one of the classic examples of co-evolution. Let s watch some video: hammertime!
Other examples of pseudocopulatory orchids
4. Plants have various mechanisms that prevent self-fertilization Some flowers self-fertilize, but most angiosperms have mechanisms that make this difficult. Why do you figure these mechanisms evolved?
For example, in some species stamens and carpels mature at different times. Alternatively, they may be arranged in such a way that it is unlikely that an animal pollinator could transfer pollen from the anthers to the stigma of the same flower. Fig. 38.6
The most common anti-selfing mechanism is selfincompatibility, the ability of a plant to reject its own pollen and that of closely related individuals. If a pollen grain from an anther happens to land on a stigma of a flower on the same plant, a biochemical block prevents the pollen from completing its development and fertilizing an egg. The self-incompatibility systems in plants are analogous to immune responses of animals. The key difference is that the animal immune system rejects nonself, but self-incompatibility in plants is a rejection of self.
5. Double fertilization gives rise to the zygote and endosperm The generative cells divides by mitosis to produce two sperm. Directed by a chemical attractant, possibly calcium, the tip of the pollen tube enters the ovary, probes through the micropyle (a gap in the integuments of the ovule), and discharges two sperm within the embryo sac. This is a good example of CELL SIGNALING. Let s watch the pollen do its thing.
Here s why they call it double : One sperm fertilizes the egg to form the zygote. The other sperm combines with the two polar nuclei to form a triploid nucleus in the central cell. This large cell will give rise to the endosperm, a food-storing tissue of the seed. Watch here
Fig. 38.9
Double fertilization is also present in a few gymnosperms, probably via independent evolution. Double fertilization ensures that the endosperm will develop only in ovules where the egg has been fertilized. This prevents angiosperms from squandering nutrients in eggs that lack an embryo. Good adaptation, eh?
So, summarizing this confusing stuff When a plant has babies, they look very different. An oak tree, for instance, has pollen grains and embryo sacs for babies. They ARE the next generation. Pollen grains and embryo sacs have oak trees for babies. Confusing?
The ovule develops into a seed containing an embryo and a supply of nutrients The ovule develops into a seed, and the ovary develops into a fruit enclosing the seed(s). The embryo inside a seed needs food, because it can t make its own yet. It gets it from two places, but not until getting a little from mom
The first mitotic division of the zygote is transverse, splitting the fertilized egg into a basal cell, and a terminal cell which gives rise to most of the embryo. The basal cell continues to divide transversely, producing a thread of cells, the suspensor, which anchors the embryo to its parent. Thus mom gives nutrients to the embryo for a while. Fig. 38.10
During the last stages of maturation, a seed dehydrates until its water content is only about 5-15% of its weight. The embryo stops growing until the seed germinates. The embryo and its food supply are enclosed by a protective seed coat formed by the integuments of the ovule.
Check out these seed parts. Fig. 38.11a
7. The ovary develops into a fruit adapted for seed dispersal The ovary of the flower is developing into a fruit, which protects the enclosed seeds and aids in their dispersal by wind or animals. Watch this Idea for a lab? Or Vol. 1 Branching Out of The Private Life of Plants series, 12:25. Pollination triggers hormonal changes that cause the ovary to begin its transformation into a fruit. If a flower has not been pollinated, fruit usually does not develop, and the entire flower withers and falls away.
What do you eat that is fruit????? The apples, oranges, and other fruits in grocery stores are exaggerated versions of much smaller natural varieties of fleshy fruits. The staple foods for humans are the dry, wind-dispersed fruits of grasses, which are harvested while still on the parent plant. The cereal grains of wheat, rice, maize, and other grasses are easily mistaken for seeds, but each is actually a fruit with a dry pericarp that adheres tightly to the seed coat of the single seed within. WHOLE GRAIN foods contain all these parts.
Evolutionary adaptations of seed germination contribute to seedling survival As a seed matures, it dehydrates and enters a dormancy phase, a condition of extremely low metabolic rate and a suspension of growth and development. Some seeds germinate as soon as they are in a suitable environment. Others remains dormant until some specific environmental cue causes them to break dormancy. Another example of responses to environmental stimuli.
Seed dormancy increases the chances that germination will occur at a time and place most advantageous to the seedling. For example, seeds of many desert plant germinate only after a substantial rainfall, ensuring enough water. Where natural fires are common, many seeds require intense heat to break dormancy, taking advantage of new opportunities and open space. Where winters are harsh, seeds may require extended exposure to cold, leading to a long growing season. Other seeds require a chemical attack or physical abrasion as they pass through an animal s digestive tract before they can germinate.
Germination of seeds 1. Imbibe water 2.This triggers the embryo to release giberellic acid, 3. which in turn stimulates the release of amylase. 4. Amylase begins digesting the amylose of endosperm or cotyledons, and the maltose formed is transferred to the growing regions of the embryo. Fig. 38.13
The radicle, the embryonic root comes out first. Watch here Next, the shoot tip must break through the soil surface. What adaptations protect it, you ask? In garden beans and many other dicots, a hook forms in the hypocotyl, and growth pushes it aboveground. Stimulated by light, the hypocotyl straightens, raising the cotyledons and epicotyl. Fig. 38.14a
Peas, though in the same family as beans, have a different style of germinating. A hook forms in the epicotyl rather than the hypocotyl, and the shoot tip is lifted gently out of the soil by elongation of the epicotyl and straightening the hook. Pea cotyledons, unlike those of beans, remain behind underground. Fig. 38.14b
Corn and other grasses, which are monocots, use yet a different method for breaking ground when they germinate. Some convergent evolution here? A sheath called the coleoptile, pushes upward through the soil and into the air. The shoot tip then grows straight up through the tubular coleoptile. Fig. 38.14c
CHAPTER 38 PLANT REPRODUCTION AND BIOTECHNOLOGY Section B: Asexual Reproduction 1. Many plants clone themselves by asexual reproduction 2. Sexual and asexual reproduction are complementary in the life histories of many plants 3. Vegetative propagation of plants is common in agriculture
3. Vegetative propagation of plants is common in agriculture Various methods have been developed for the asexual propagation of crop plants, orchards, and ornamental plants. These can be reproduced asexually from plant fragments called cuttings. These are typically pieces of shoots or stems.
At the cut end, a mass of dividing undifferentiated cells, called the callus, forms, and then adventitious roots develop from the callus. If the shoot fragment includes a node, then adventitious roots form without a callus stage. Some plants, including African violets, can be propagated from single leaves. In others, specialized storage stems can be cut into pieces and develop into clones. For example, a piece of a potato including an eye can regenerate a whole plant.
Biotechnologists have adopted in vitro ways to create and clone novel plants varieties. Whole plants are cultured from small tissue pieces or even single parenchyma cells, on an artificial medium containing nutrients and hormones. Through manipulations of the hormonal balance, the callus that forms can be induced to develop shoots and roots with fully differentiated cells. Fig. 38.16
Once the roots and shoots have developed, the testtube plantlets can be transferred to soil where they continue their growth. This test-tube cloning can be used to clone a single plant into thousands of copies by subdividing calluses as they grow. This technique is used to propagate orchids and for cloning pine trees that deposit wood at an unusually fast rate.
Plant tissue culture facilitates genetic engineering of plants. Test-tube culture makes it possible to regenerate genetically modified (transgenic) plants from a single cell into which foreign DNA has been incorporated. For example, to improve the protein quality of sunflower seeds, researchers have transferred a gene for bean protein into cultured cells from a sunflower plant.
One method that researchers use to insert foreign DNA into plant cells is by firing DNA-coated pellets into cultured plant cells. These penetrate cell walls and membranes, introducing foreign DNA into the nuclei. A cell that integrates this DNA into its genome can produce a plantlet which can be cloned. Fig. 38.17
Another approach uses protoplast fusion to invent new plant varieties. Protoplasts are plant cells that have had their cell walls removed enzymatically by cellulases and pectinases. It is possible in some cases to fuse two protoplasts from different plant species that would otherwise be incompatible. The hybrids can regenerate a wall, be cultured, produce a plantlet. and hybrid Fig. 38.18
One success of this technique has been the development of a hybrid between a potato and a wild relative called black nightshade. The nightshade is resistant to an herbicide that is commonly used to kill weeds. The hybrids are also resistant, enabling a farmer to weed a potato field with a herbicide without killing the potato plants.
Unlike traditional plant breeders, modern plant biotechnologists, using the techniques of genetic engineering, are not limited to transferring genes between closely related species or varieties of the same species. Instead, genes can be transferred between distantly related plant species to create transgenic plants, organisms that have been genetically engineered to express a foreign gene from another species.
2. Biotechnology is transforming agriculture Whatever the social and demographic causes of human starvation around the world, increasing food production seems like a humane objective. Because land and water are the most limiting resources for food production, the best option will be to increase yields on available lands. Based on conservative estimates of population growth, the world s farmers will have to produce 40% more grain per hectare to feed the human population in 2020.
The commercial adoption by farmers of transgenic crops has been one of the most rapid cases of technology transfer in the history of agriculture. Between 1996 and 1999, the areas planted commercially with transgenic crops increased from 1.7 to 39.9 million hectares. These include cotton, maize, and potatoes that contain genes from Bacillus thuringiensis. These transgenes encode for a protein (Bt toxin) that effectively controls several insect pests. This has reduced the need for application of chemical insecticides.
3. Plant biotechnology has incited much public debate Many people, including some scientists, are concerned about the unknown risks associated with the release of GM organisms into the environment. Much of the animosity regarding GM organisms is political, economic, or ethical in nature, but there are also biological concerns about GM crops. The most fundamental debate centers on the extent to which GM organisms are an unknown risk that could potentially cause harm to human health or to the environment.
There are concerns that GM crops might have unforeseen effects of nontarget organisms. One recent study indicated that the caterpillars of monarch butterflies responded adversely after eating milkweed leaves heavily dusted with pollen from transgenic maize that produced Bt toxin. The Bt toxin normally is toxic to pests closely related to monarch butterflies. In the field, the transgenic pollen appears to be abundant primarily in or very close to the fields. Also, the alternative to transgenic maize, spraying chemical insecticides, may be even more harmful to nearby monarch populations.
Probably the most serious concern that some scientists raise is the possibility that introduced genes may escape from a transgenic crop into related weeds through crop-to-weed hybridization. This spontaneous hybridization may lead to a superweed which may be more difficult to control. A recent type of golf course grass has been developed to be resistant to Round Up, a common weed killer. It is feared that its spread cannot be controlled and it may spread the resistance to weeds. Here s my idea of a golfer-friendly hole.