Reproduction in Flowering Plants

1. 38 Reproduction in Flowering Plants

2. In angiosperms, flowers contain sex organs Nearly all reproduce sexually, many reproduce asexually as well Sexual reproduction produces new gene combinations and diverse phenotypes Asexual reproduction produces clones of genetically identical individuals Tubers, stolons, grafts, runners, bulbs, rhizomes This dune plant is reproducing asexually by setting out runners

3. In agriculture, both modes are important Many annual crops are grown from seeds (e.g., grains [grasses], soybean, etc.) Other crops such as strawberries, potatoes, and bananas, are produced asexually Navel oranges are seedless, and must be propagated asexually. All descend from one mutant tree Seedless navel orange Stawberry plant with stolen www.biology.iastate.edu/Courses/202L/New%20Site%20S05/27AngioReprod/reprod www.hort.cornell.edu/4hplants/Fruits/Images/

4. A complete flower has four concentric groups of organs arising from modified leaves:

5. Perfectflowers have both stamens and carpels Imperfectflowers have only stamens or only carpels Monoecious (“one house”) Plants that bear both male and female flowers on an individual plant Dioecious Plants that bear either male-only or female-only flowers on an individual plant

6. Flowers A complete flower has four groups of organs (modified leaves): Carpels Stamens Petals Sepals

7. Flowers - Carpels Carpels are female sex organs Pistil – composed of one or more carpels Style – stalk of pistil, with stigma at the end Ovary – at base of pistil containing ovules, each of which contain a megasporangium Female gametophyte develops in the megasporangium

8. Flowers - Stamens Stamens are male sex organs Each has a filament with two-lobed anther, containing four fused microsporangia. Male gametophyte develops in microsporangia.

9. Flowers - Petals Petals make up corolla Often colored to attract pollinators Sepals make up the calyx All flower parts attached to stem tip or receptacle Nipplewort (Lapsana communis) http://oregonstate.edu/dept/nursery-weeds/weedspeciespage/lapsana

10. Alternation of generations multicellular diploid generation alternates with a multicellular haploid generation. In angiosperms, 2nsporophyte generation is larger and more conspicuous Sporophyte produces flowers 1ngametophytes develop from 1n spores in sporangia within flower Pink and white lady’s slipper (Cypripedium reginae) http://www.sos.state.mn.us/student/artwork/State-Flower.gif

11. The two haploid cells within pollen grain are: • generative cell (mitosis  2 sperm cells) • tube cell • Further development is delayed until pollen arrives at a stigma Lilium pollen grain http://botit.botany.wisc.edu/Interface/weblessons/Diversity/plants/Vascular%20Plants/Magnoliophyta/images

12. Pollination: transfer of pollen from anther to stigma Photo 38.8 Stigma of Hibiscus sp.; pollen grains adhere to sticky stigma. SEM.

13. Evolution of pollen made it possible for male gametes to reach female gametophyte without water This selective advantage allowed pollen-bearing plants to colonize land Many mechanisms have evolved for pollen transport In many species, wind transports pollen The flowers have sticky or featherlike stigmas, and pollen grains are produced in great numbers Most angiosperms rely on animals (insects, birds, and bats) to transfer pollen Flowers are pigmented, shaped, and scented to attract the pollinating animal The animals receive nutrition from the flowers (nectar and pollen) The relationship is mutually beneficial

14. Preventing Self-fertilization In some (e.g. peas), self pollination occurs before flower opens, resulting in self-fertilization  leads to homozygosity, which reduces genetic diversity Most plants have mechanisms to prevent self-fertilization Separation of male and female flowers on monoecious plants Recognition of self pollen Self-pollinated flower of a peanut plant is produced on a slender stalk (pedicel) near the base of the plant. Following pollination and fertilization, the flower stalk curves downward and pushes into the soil. Below ground, the ovary develops into a 2-seeded pod. http://waynesword.palomar.edu/images

15. Preventing self-fertilization Geneticself-incompatibility When plants reject pollen from their own flowers  promotes outcrossing and increases heterozygosity must determine whether pollen is genetically similar or not S gene is responsible for self-incompatibility S genelocus encodes proteins in pollen and style that interact during recognition process This locus has many alleles When pollen carries an allele that matches an allele of the recipient pistil, the pollen is rejected

16. Germination of pollen grain involves growth of pollen tube in stigma Pollen tube grows through style to reach ovule Mature pollen grains consist of two cells The larger tube cell encloses the generative cell Tube cell nucleus directs growth of pollen tube through style tissue to embryo sac Generative cell undergoes mitotic division to produce 2 sperm cells (1n) 1 will fertilize the egg 1 will join with 2 polar nuclei in large central cell to form endosperm Double fertilization

17. Fertilization initiates growth and development of the embryo, endosperm, integuments, and carpel Integuments are the tissues surrounding megasporangium  develop into seed coat Carpel becomes wall of the fruit that surrounds seed

18. After double fertilization First mitotic division of zygote results in asymmetric division of cytoplasm One daughter cell becomes embryo, other becomes a supporting structure suspensor Asymmetric division establishes polarity and longitudinal axis of new plant

19. In eudicots, embryo develops into heart stage as cotyledons begin to form Capsella (Shepherd's-Purse) “heart” embryo (dark purple structure) with suspensor www.texarkanacollege.edu/~mstorey/1407

20. Further elongation gives rise to torpedo stage Tissues begin to differentiate Shoot apex forms between cotyledons Root apex forms at other end Both contain meristematic cells

21. In later stages of development, seed loses water and becomes dormant Integuments – tissues surrounding megasporangium develop into seed coat to protect embryo Mature corn “seed”. This fruit type, a "grain“, is characterized by an ovary wall fused to the seed coat. Monocot seeds usually have lots of starch filled endosperm. http://www.lima.ohio-state.edu/academics/biology/images

22. Fruit Develops from ovary wall  carpel becomes wall of fruit surrounding seed May consist only of mature ovary and seeds, or include other structures Fruits protect seed from damage by animals and infection by microbes An apple, is a pome type fruit. The fleshy part one eats is the receptacle; the mature ovary is actually just the core. www.csdl.tamu.edu/FLORA/tfp

23. Fruits also facilitate seed dispersal Some fruits (dandelion) have a “parachute” for wind dispersal. Some fruits float on water (coconuts). Some cling to animals (burrs). http://buytaert.net/cache/images-miscellaneous-2005-dandelion-700x700.jpg Fruits of sandbur or sandspur http://fortdesoto.com/images/pool/

24. Other seeds are eaten by animals and pass through the digestive tract before germinating Some species produce fleshy, edible fruits, such as tomatoes Fruit-eating iguana http://www.richard-seaman.com/Reptiles/BelizeAndGuatemala/IguanaEatingFruit.jpg

25. Flowering is a major event in a plant’s life When a plant is old enough, it can respond to internal or external signals (such as light or temperature) to start reproduction. Plants fall into three categories in terms of maturation and flowering: Annuals complete life cycle in one year and have little or no secondary growth. After flowering, most of their energy is used to develop seeds and fruits Biennials take two years to complete life cycle. Produce vegetative growth during first year and store carbohydrates in underground roots (carrots) or stems (onion) In second year, stored carbohydrates are used to produce flowers and seeds Perennials live three or more years. Typically flower every year, and keep growing for another season In some species the reproductive cycle repeats each year; others grow vegetatively for many years, flower once, and die (Agave)

26. Environmental cues can trigger transition to the flowering state Photoperiodism is control of flowering and other responses by length of day or night First studied in 1920s in mutant tobacco plants Plants would not flower until day length was shorter than 14 hours — critical day length Wild type and gigantea (gi) mutantArabidopsis plants. Mutation delays flowering, allowing for large growth. http://3e.plantphys.net/images/ch24/wt2405a.jpg

27. Asexual reproduction can involve vegetative parts: tubers, stolons, runners Asexual production of seeds is called apomixis (dandelions, citrus, blackberries) Seeds are genetically identical to parent Sometimes requires pollination Photo 38.26 Dandelion heads, with asexually produced seeds ready for dissemination on the wind.

28. Vegetative reproduction is used extensively in agriculture Stem cuttings inserted into soil will often grow into a new plant, especially if treated with auxin (“rooting hormone”) Row of Dahlia cuttings http://www.gardenbuddies.com/forum/messages/60521/1099136.jpg

29. Grafting Process of attaching bud or piece of stem from one plant onto root or stem (with roots) of another plant Root-bearing plant is stock, the part grafted onto it is scion Vascular cambia of each must grow together so water and minerals can be transported to scion. Usually closely related species are used. Most fruit marketed in U.S. from grafted trees Grafted pink lemon tree on root stock http://www.beyondgutenberg.com/Peek/Garden/LemonTree/PinkLemonadeGraft.jpg

30. Researchers are investigating ways to reproduce plants by tissue culture. Many plant cells are totipotent — undifferentiated cells can give rise to entire plants Culturing tiny bits of apical meristem can eliminate plant viruses Cattleya orchid www.ppm4plant-tc.com/ http://members.tripod.com/~rsauleda/

31. Treatment with hormones can cause apical meristems to produce millions of plants in a year Used for strawberries and potatoes to produce virus-free plants Tissue culture of commercial strawberries http://www.noursefarms.com/images/staticPages/tour/tissueCulture.jpg

32. 38.3 How Do Angiosperms Reproduce Asexually? Recombinant DNA also used with tissue culture to produce plants with disease resistance, increased nutritive value, etc. Golden Rice (right) is bioengineered to produce and accumulates pro-vitamin A (β-carotene) in the grain, often in short supply in developing countries. www.goldenrice.org/image/