1. Level 1 Biological Diversity Jim Provan
Plant Reproduction
Level 1 Biological Diversity
Jim Provan
Campbell: Chapter 38

2. 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

3. Alternation of generations: a review

4. 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

5. Floral diversity

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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)

12. 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

13. 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

14. 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

15. 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

16. 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 animal’s digestive system
Viability ranges from a few days to decades

17. 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

18. 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

19. 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)

20. 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