Plant Reproduction

1. Plant Reproduction Chapter 42

2. Reproductive Development Angiosperms represent an evolutionary innovation with their production of flowers and fruits Plants go through developmental changes leading to reproductive maturity by adding structures to existing ones with meristems -A germinating seed becomes a vegetative plant through morphogenesis

3. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gamete production and pollination n 2n n 2n 2n Maturation and flowering Fertilization CHAPTER 42 2n CHAPTER 36 Zygote 2n Embryo development CHAPTER 37 Development of plant body 2n 2n 2n Dispersal and germination Fruit and seed maturation

4. Reproductive Development Flowering is the default state In Arabidopsis, the gene embryonic flower (EMF) prevents early flowering -emf mutants lacking a functional EMF protein flower immediately

5. Reproductive Development The juvenile-to-adult transition can be induced by overexpressing a flowering gene -LEAFY (LFY) was cloned in Arabidopsis -Overexpression of LFY in aspen, causes flowering to occur in weeks instead of years

6. Flower Production Four genetically regulated pathways to flowering have been identified 1. The light-dependent pathway 2. The temperature-dependent pathway 3. The gibberellin-dependent pathway 4. The autonomous pathway Plants can rely primarily on one pathway, but all four pathways can be present

7. Let’s Take a Look at the Autonomous Pathway in Some Detail First

8. Autonomous Pathway The autonomous pathway does not depend on external cues except for basic nutrition It allows day-neutral plants to “count” nodes and “remember” node location

9. -Tobacco plants produce a uniform number of nodes before flowering-Upper axillary buds of flowering tobacco remember their position if rooted or grafted Upper Axillary Bud Released from Apical Dominance Lower Axillary Bud Released from Apical Dominance 13 nodes* replaced 13 nodes* removed 5 nodes* replaced 5 nodes* removed Shoot removed here Shoot removed here Intact plant Shoot removed Replacement shoot Intact plant Shoot removed Replacement shoot *nodes = leaf bearing node Autonomous Pathway--Plants Can Count

10. Shoot Florally Determined Shoot Not Florally Determined Shoot removed here Shoot removed here Shoot removed Shoot removed Flowering rooted shoot Flowering rooted shoot Intact plant Rooted shoot Intact plant Rooted shoot a. b. Autonomous Pathway--Plants Can Remember Not-Florally Determined Plants are said not to remember...Florally Determined plants are said to remember

11. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Experimental plant: Lower leaves were continually removed Experimental plant: pot-on-pot treatment Control plant: no treatment Roots Inhibit Flowering

12. A Model of All the Flowering Pathways

13. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Repression of Floral Inhibitors Cold Temperature- dependent pathway Vernalization Flower- repressing genes Autonomous pathway Autonomous gene expression Flower- promoting genes Gibberellin- dependent pathway Gibberellin LFY ABCDE floral organ identity genes Floral organ development Light- dependent pathway AP1 CO Light Adult meristem Floral meristem inhibition Activation of Floral Meristem Identity Genes activation Gibberellin binds to the promoter of LFY CO is a transcription factor that turns on other genes, resulting in the expression of LFY -Phytochromes regulate CO transcription

14. Flower Structure Floral organs are thought to have evolved from leaves A complete flowerhas four whorls -Calyx, corolla, androecium, and gynoecium An incomplete flower lacks one or more of these whorls

15. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Stamen Malestructure Anther Carpel Femalestructure Filament Stigma Style Petal Ovary Ovule Sepal Receptacle all stamens = androecium all carpels = gynoecium all petals = corolla all sepals = calyx

16. Gamete Production Plant sexual life cycles are characterized by an alternation of generations -Diploid sporophyte  haploid gametophyte In angiosperms, the gametophyte generation is very small and is completely enclosed within the tissues of the parent sporophyte -Male gametophyte = Pollen grains -Female gametophyte = Embryo sac

17. Gamete Production Gametes are produced in separate, specialized structures of the flower Reproductive organs of angiospermsdiffer from those of animals in two ways: 1. Both male and female structures usually occur together in the same individual 2. Reproductive structures are not permanent parts of the adult individual

18. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Anther Microspore mother cell Pollen sac MEIOSIS Microspores Ovule MEIOSIS diploid (2n) Megaspore mother cell haploid (n) Megaspores MITOSIS Generative cell Surviving megaspore Antipodals Tube cell nucleus Polar nuclei Egg cell MITOSIS Pollen grains (microgametophytes) Degenerated megaspores Synergids Eight-nucleate embryo sac (megagametophyte) Generative Cells go on to make 2 sperm Cells Megaspore enlarges and undergoes repeated mitotic divisions to produce eight haploid nuclei...Egg cell is enclosed within a seven-celled embryo sac

19. Pollination Pollination is the process by which pollen is placed on the stigma -Self-pollination = Pollen from a flower’s anther pollinates stigma of the same flower -Cross-pollination = Pollen from anther of one flower pollinates another flower’s stigma -Also termed outcrossing

20. Pollination Successful pollination in many angiosperms depends on regular attraction of pollinators Flowers & animal pollinators have coevolved resulting in specialized relationships -Bees are the most common insect pollinators

21. Pollination Flowers that are visited regularly by birds often have a red color -Usually inconspicuous to insects Hummingbirds obtain nectar from flowers that match the length and shape of their beaks

22. Pollination Self-pollinating plantsusually have small, relatively inconspicuous flowers that shed pollen directly into the stigma Self-pollination is favored in stable environments 1. Plants do not need to be visited by animals to produce seed 2. Offspring are more uniform and probably better adapted to their environment

23. Pollination Several evolutionary strategies promote outcrossing 1. Separation of male and female structures in space -Dioecious plants produce only ovule or only pollen -Monoecious plants produce male and female flowers on the same plant

24. Pollination 2. Separation of male and female structures in time -Even if functional stamens and pistils are both found in the same flower, they may reach maturity at different times -Plants in which this occurs are called dichogamous

25. Pollination 3. Self-incompatibility -Pollen and stigma recognize each other as self and so the pollen tube is blocked -Controlled by alleles at the S locus -Gametophytic self-incompatibility -Block is after pollen tube germination -Sporophytic self-incompatibility -The pollen tube fails to germinate

26. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gametophytic Self-Incompatibility Sporophytic Self-Incompatibility S2 S2 S1 S1 S2 S2 S2 S1 X X X S2S3 stigma of pollen recipient S2S3 stigma of pollen recipient S1S2 pollen parent S1S2 pollen parent a. b. Pollination

27. Fertilization Angiosperms undergo a unique process called double fertilization -A pollen grain that lands on a stigma forms a pollen tube that pierces the style -While the pollen tube is growing, the generative cell divides to form 2 sperm cells -When pollen tube reaches the ovule, it enters one of the synergids and releases the two sperm cells

28. Fertilization -Then double-fertilization occurs -One sperm cell nucleus fuses with the egg cell to form the diploid (2n) zygote -Other sperm cell nucleus fuses with the two polar nuclei to form the triploid (3n) endosperm nucleus -Eventually develops into the endosperm that nourishes embryo

29. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Pollen grain Tube cell Stigma Generative cell Sperm cells Tube cell Style Carpel Tube cell nucleus Ovary Embryo sac Ovule Pollination

30. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Endosperm nucleus (3n) Zygote (2n) Pollen tube Antipodals Egg cell Polar nuclei Synergids Release of sperm cells Double fertilization Growth of pollen tube

31. Plant Life Spans Once established, plants live for variable periods of time, depending on the species Woody plants, which have extensive secondary growth, typically live longer than herbaceous plants, which don’t -Bristlecone pine, for example, can live upward of 4,000 years

32. Plant Life Spans Perennial plants are able to flower and produce seeds and fruit for an indefinite number of growing seasons -May be herbaceous or woody -In deciduous plants all the leaves fall, and the tree is bare, at a particular time of year -In evergreen plants, the leaves drop throughout the year, and so the plant is never completely bare

33. Plant Life Spans Annual plants grow, flower, and form fruits and seeds, and typically die within one growing season -Are usually herbaceous -The process that leads to the death of the plant is called senescence Biennial plants have two-year life cycles -They store energy the first year and flower the second year