Plant Reproduction Chapter 42 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 2 3 Before flowers can form, plants must undergo a phase change to prepare a plant to respond to internal and external signals Phase change can be morphologically obvious or very subtle -In oak trees, lower branches (juvenile phase) cling to their leaves in the fall -Juvenile ivy makes adventitious roots and has alternating leaf phyllotaxy -Mature ivy lacks adventitious roots, has spiral phyllotaxy, and can make flowers 4 5 6 Flower Production Flowering is the default state In Arabidopsis, the gene embryonic flower (EMF) prevents early flowering -emf mutants lacking a functional EMF protein flower immediately 7 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 8 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 9 1
Light-Dependent Pathway Also termed the photoperiodic pathway -Sensitive to the amount of darkness a plant receives in each 24-hour period -Short-day plants flower when daylight becomes shorter than a critical length -Long-day plants flower when daylight becomes longer -Day-neutral plants flower when mature regardless of day length Light-Dependent Pathway In obligate long- or short-day plants there is a sharp distinction between short and long nights, respectively In facultative long- or short-day plants, the photoperiodic requirement is not absolute -Flowering occurs more rapidly or slowly depending on the length of day 10 11 12 Light-Dependent Pathway Light-Dependent Pathway Light-Dependent Pathway Using light as cue allows plants to flower when environmental conditions are favorable -Manipulation of photoperiod in greenhouses ensures that shortday poinsettias flower in time for the winter holidays 13 Flowering is regulated by phytochromes (redlight receptors) and cryptochrome (blue light receptor) via the gene CONSTANS (CO) -Phytochromes regulate CO transcription -CO mrna is low at night and increase at daybreak -Cryptochrome modulates CO protein level -Stabilizes CO and protects it from CO is a transcription factor that turns on other genes, resulting in the expression of LFY -LFY is a key gene that tells a meristem to switch over to flowering One intriguing possibility is that CO is the elusive flowering hormone -Or that it affects such a flowering signal proteasome degradation in the day 14 15 Temperature-Dependent Pathway Some plants require a period of chilling before flowering called vernalization -It is necessary for some seeds or plants in later stages of development Analysis of plant mutants reveals that vernalization is a separate flowering pathway Gibberellin-Dependent Pathway Gibberellin binds to the promoter of LFY -Enhances its expression, thereby promoting flowering In Arabidopsis and other species, decreased levels of gibberellins delay flowering 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 -Tobacco plants produce a uniform number of nodes before flowering -Upper axillary buds of flowering tobacco remember their position if rooted or grafted 16 17 18 2
Autonomous Pathway Autonomous Pathway Autonomous Pathway How do shoots count and remember? -Experiments using bottomless pots have shown that it is the addition of roots, and not the loss of leaves, that inhibits flowering A balance between floral promoting and inhibiting signals may regulate flowering 19 20 21 Model for Flowering The four flowering pathways lead to an adult meristem becoming a floral meristem -They activate or repress the inhibition of floral meristem identity genes -Key genes: LFY and AP1 (APETALA1) -These two genes turn on floral organ identity genes -Define the four concentric whorls 22 23 24 26 27 Model for Flowering The ABC model proposes that three organ identity gene classes specify the four whorls 1. Class A genes alone Sepals 2. Class A and B genes together Petals 3. Class B and C genes together Stamens 4. Class C genes alone Carpels When any one class is missing, aberrant floral organs occur in predictable positions 25 3
Model for Flowering Recently, two other classes were identified -Class D genes are essential for carpel formation -Class E genes (SEPALATA) -SEP proteins interact with class A, B and C proteins that are needed for the development of floral organs Thus, a modified ABC model was proposed 28 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 29 30 Flower Structure Male structure Female Trends in Floral Evolution structure Calyx = Consists of flattened sepals Corolla = Consists of fused petals Androecium = Collective term for stamens -A stamen consists of a filament and an anther Gynoecium = Collective term for carpels -A carpel consists of an ovule, ovary, style, and stigma Floral specialization 1. Separate floral parts have been grouped 2. Floral parts have been lost or reduced -Wild geranium 31 32 33 Trends in Floral Evolution Gamete Production Gamete Production Floral symmetry -Primitive flowers are radially symmetrical -Advanced flowers are bilaterally symmetrical -Orchid 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 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 34 35 36 4
Pollen Formation Embryo Sac Formation Anthers contain four microsporangia which produce microspore mother cells (2n) -Each microspore mother cell produces four haploid (n) microspores through meiosis -Each microspore develops by mitosis into a pollen grain (microgametophyte) -The generative cell in the pollen grain will later divide to form two sperm cells Within each ovule, a diploid microspore mother cell undergoes meiosis to produce four haploid megaspores -Usually only one survives -Enlarges and undergoes repeated mitotic divisions to produce eight haploid nuclei -Enclosed within a seven-celled embryo sac 37 38 is the process by which pollen is placed on the stigma Successful pollination in many angiosperms depends on regular attraction of pollinators Flowers & animal pollinators have coevolved resulting in specialized relationships -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 40 42 Flowers that are visited regularly by butterflies often have flat landing platforms Flowers that are visited regularly by moths are often white, or pale in color 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 -They also tend to be heavily scented 43 -Bees are the most common insect pollinators 41 Bees typically visit yellow or blue flowers -Yellow flowers are marked in distinctive ways that are normally invisible to us -Bull s eye or landing strip 39 44 45 5
Other animals, including bats and small rodents, may aid in pollination -The signals here are also species-specific Monkeys are attracted to orange and yellow -They can thus disperse fruits of this color in their habitat Some angiosperms are wind-pollinated -A characteristic of early seed plants Flowers of these plants are small, green, and odorless, with reduced or absent corollas -Often grouped and hanging down in tassels Stamen- and carpel-containing flowers are usually separated between individuals -Strategy that greatly promotes outcrossing 46 47 48 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 49 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 50 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 51 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 52 53 Determined by the genotype of the haploid pollen itself Determined by the genotype of the diploid pollen parent 54 6
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 55 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 56 57 Asexual Reproduction Produces genetically identical individuals because only mitosis occurs -Far more common in harsh environments where there is little leeway for variation It is used in agriculture and horticulture to propagate a particularly desirable plant Apoximis refers to the asexual development of a diploid embryo in the ovule 58 59 Asexual Reproduction In vegetative reproduction new plant individuals are cloned from parts of adults -Comes in many and varied forms -Runners or stolons -Rhizomes -Suckers -Adventitious plantlets 60 Asexual Reproduction Asexual Reproduction Asexual Reproduction (Cont.) Whole plants can be cloned by regenerating plant cells or tissues on nutrient medium A protoplast is a plant cell enclosed only by a plasma membrane -When single plant protoplasts are cultured, cell wall regeneration takes place -Cell division follows to form a callus, an undifferentiated mass of cells -Whole plants are then produced 61 62 63 7
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 64 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 65 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 66 Plant Life Spans 67 8