Plant Reproduction Chapter 42

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

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

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

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

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

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

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

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

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

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

A Model of All the Flowering Pathways

-Phytochromes regulate CO transcription inhibition activation Repression of Floral Inhibitors Activation of Floral Meristem Identity Genes Adult meristem Floral meristem Temperature- dependent pathway Autonomous Flower- repressing genes promoting Vernalization gene expression Cold Gibberellin- Light- Gibberellin CO Light LFY AP1 ABCDE floral organ identity genes Floral organ development Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

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

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

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

Gamete Production Gametes are produced in separate, specialized structures of the flower Reproductive organs of angiosperms differ 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

Megaspore enlarges and undergoes repeated mitotic divisions Anther Microspore mother cell Pollen sac Megaspores Pollen grains (microgametophytes) Tube cell nucleus Generative cell Ovule Megaspore Surviving megaspore Antipodals Polar nuclei Degenerated megaspores Eight-nucleate embryo sac (megagametophyte) Synergids Egg cell diploid (2n) haploid (n) MEIOSIS MITOSIS Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Microspores 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

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

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

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

Pollination Self-pollinating plants usually 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

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

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

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

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

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

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

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

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

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

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

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