Plant Reproduction

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Plant Reproduction

• Level 1 Biological Diversity
• Jim Provan

Campbell Chapter 38
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Alternation of generations a review

• Angiosperm life cycle includes alternation of
  generations haploid gametophyte generations
  alternate with diploid sporophyte generation
• Sporophyte is recognisable plant - produces
  haploid spores by meiosis in sporangia
• Spores undergo mitotic division and develop into
  multicellular male or female gametophyte
• Gametophytes produce gametes (sperm and eggs) by
  mitosis gametes fuse to form zygote which
  develops into multicellular sporophyte
• Sporophyte is dominant in angiosperm life cycle
  gametophyte stage is reduced and is totally
  dependent on sporophyte

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Alternation of generations a review
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Variations on the basic flower structure

• Complete has sepals, petals, stamens and carpels
• Incomplete missing one or more organs (e.g.
  grasses)
• Perfect has both stamens and carpels
• Imperfect either staminate or carpellate -
  unisex
• Monoecious has both staminate and carpellate
  flowers on same plant
• Dioecious has staminate and carpellate flowers
  on separate individual plants

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Floral diversity
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Development of male gametophyte (pollen)
Within sporangial chamber of anther, diploid
microsporocytes undergo meiosis to form four
haploid microspores
Haploid microspore nucleus undergoes mitotic
division to give rise to a generative cell and a
tube cell
Wall of microspore thickens
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Development of female gametophyte (embryo sac)
Megasporocyte in sporangium of each ovule grows
and goes through meiosis to form four haploid
megaspores (only one usually survives)
Remaining megaspore grows and its
nucleus undergoes three mitotic divisions,
forming one large cell with eight haploid nucleii
Membranes partition this into a
multicellular embryo sac
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Pollination brings male and female gametophytes
together

• Pollination the placement of pollen onto the
  stigma of a carpel
• Some plants use wind to disperse pollen
• Others interact with animals that transfer pollen
  directly
• Some plants self pollinate, but most
  cross-pollinate
• Most monoecious angiosperms have mechanisms to
  prevent selfing - maximises genetic variation
• Stamens and carpels may mature at different times
• Structural arrangement of flower reduces chance
  that pollinator will transfer pollen between
  anthers and stigma of same plant
• Some plants are self-incompatible

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Genetic basis of self-incompatibility

• Based on S genes
• Many alleles in plant population gene pool
• Pollen landing on stigma with same allele at
  S-locus is self-incompatible
• Pollen grain will not initiate or complete
  formation of pollen tube
• Prevents fertilisation between plants with
  similar S-alleles

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Multiple mechanisms at S-loci

• Mechanism underlying inhibition of pollen tube
  varies
• Block occurs in pollen grain (gametophyte
  self-incompatibility) RNAses from carpel enter
  pollen and destroy RNA
• Block occurs in stigma (sporophyte
  self-incompatibility) e.g. signal transduction
  systems in mustards

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Double fertilisation gives rise to the zygote and
the endosperm

• Double fertilisation union of two sperm cells
  with two cells of the embryo sac
• Pollen grain germinates and extends pollen tube
• Generative cell undergoes mitosis, forming two
  sperm
• Pollen tube enters through micropyle and
  discharges sperm
• One sperm unites with egg
• Other sperm unites with polar nuclei forming
  endosperm (3n)

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Endosperm development

• Begins before embryo development
• Triploid nucleus divides to form multinucleate
  supercell
• This undergoes cytokinesis, forming cell
  membranes and walls and thus becoming
  multicellular
• Endosperm is rich in nutrients, which it provides
  to the developing embryo
• In most monocots, endosperm stocks nutrients that
  can be used by the seedling after germination
• In many dicots, food reserves of the endosperm
  are exported to the cotyledons

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Embryo development

• First mitotic division transverse
• Large basal cell forms suspensor
• Terminal cell divides several times to form
  spherical proembryo
• Cotyledons appear at either side of apical
  meristem
• Suspensor attaches at apex of embryonic root and
  meristem
• After germination, apical and root meristems
  sustain growth

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Structure of the mature seed

• In dicot seeds
• Hypocotyl terminates in the radicle (embryonic
  root)
• Epicotyl terminates in the plumule (shoot tip)
• Monocot seeds have a special cotyledon called a
  scutellum
• Large surface area - absorbs nutrients from
  endosperm during germination
• Embryo enclosed in sheath
• Coleoptile protects the shoot
• Coleorhiza protects the root

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The ovary develops into a fruit adapted for seed
dispersal

• A true fruit is a ripened ovary
• Fruits can be classified by their origin
• Simple fruits derived from a single ovary e.g.
  cherry
• Aggregate fruits derived from a single flower
  with several carpels e.g. blackberry
• Multiple fruits develop from an inflorescence

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Seed dormancy

• Prevents germination when conditions for seedling
  growth are unfavourable
• Conditions for breaking dormancy vary depending
  on type of environment plant occupies
• Seeds of desert plants will not germinate until
  there has been a heavy rainfall and not after a
  light shower
• In chaparral regions where bushfires are common,
  seeds may not germinate until exposed to heat of
  fire which clears away older, competing
  vegetation
• Other seeds require cold, sunlight or passage
  through an animals digestive system
• Viability ranges from a few days to decades

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Seed germination

• Imbibition causes seed to swell, rupturing seed
  coat
• Metabolic changes restart growth of the embryo
• Storage materials are digested by enzymes and
  nutrients transferred to growing parts of embryo
• Radicle (embryonic root) emerges from seed

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Seed germination (continued)

• Next stage involves shoot tip breaking through
  soil surface
• In many dicots, a hook forms in the hypocotyl
• Light stimulates the hypocotyl to straighten,
  raising the cotyledons
• Other plant species follow different germination
  methods
• In peas, hook forms in epicotyl which straightens
  and leaves cotyledons in ground
• In monocots, shoot grows straight up through
  coleoptile tube

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Many plants can clone themselves by asexual
reproduction

• Asexual reproduction production of offspring
  from a single parent without recombination ?
  clones
• Two natural mechanisms of vegetative
  reproduction
• Fragmentation separation of parent plant into
  parts that reform whole plants
• Most common form of vegetative reproduction
• Some species of dicots develop adventitious
  shoots that become separate shoot systems
• Apomixis production of seeds without meiosis and
  fertilisation
• Diploid cell in ovule gives rise to an embryo
• Ovules mature into seeds which are dispersed
  (e.g. dandelions)

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Sexual and asexual reproduction are complementary
in many plants

• Both have had featured roles in adaptation of
  plant populations to their environments
• Benefits of sexual reproduction
• Generates genetic variation
• Produces seeds, which can disperse to new
  locations
• Benefits of asexual reproduction
• In a stable environment, plants can clone many
  copies of themselves in a short period
• Progeny are mature fragments of the parental
  plant, as opposed to small, fragile seedlings
  produced by sexual reproduction