Plant development L1- Origin of flowering plants

Origin of flowering plants

  • The plants that coexisted with dinosaurs were very different: Conifers, ferns
  • No flowering plants at all
  • This includes grasses which our civilisation now highly relies on 
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What is Darwin's abominable mystery?

  • A long time ago there were no flowers and then suddenly the world was covered in flowering plants.
  • The mystery is that 150-200 mya, something happened and there is no fossil record indicating where flowering plants came from. 

The mystery? 

- Where did flowering plants come from and how did they take over the globe so rapidly. 
- In the Dinosaur period, gymnosperms were all that existed and now only a few hundred gynosperms left as they have almost completely been displaced by angiosperms
- There are about 400,000 angiosperms that occupy a vast range of niches
- They have done this by inventing new evolutionary innovations. 

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Angiosperms

  • Amborella is the sister plant of all other flowering plants on earth and it appears to be the oldest branching plant on earth.
  • The next one up, Nymphaeceae, includes the waterlilies.
  • There is a paper that argues that in fact the waterlilies are the oldest plant.
  • Molecular evidence suggests that it is Amborella that is the sister to all other branching plants.
  • Some developmental evidence suggesting otherwise.  
  • Also includes the monocots: These contain the grasses that support most human life. Rice, wheat and maize in particular.
  • As you move closer the present day, the development of the flower become more defined.
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Brassicales

Includes the Arabidopsis thaliana- The world's favourite model plant

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R.arnoldii

  • The world’s biggest flower
  • 1m across
  • 10kg
  • Stinking corpse lily- It very efficiently manipulates the behaviour of animals to do things that it cannot. The plant is stuck where it is living and cannot move around so it needs to coerce animals. 
  •  Looks like rotting meat and smells like it so it attracts carrying fly which pick up pollen and transfer it to another flower that smells the same.
  •   It is entirely parasitic plant, it does not have any leaves and lives entirely off plants that it parasitizes around it
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W. australiana

  • The W. australiana is the world’s smallest flower and is about 250,000 times smaller than the R. arnoldii
  • It is a little aquatic flower that has male and female organs, reproduces, sets seeds. Do not have the smallest seeds.
  • This is just to illustrate the diversity that flowering plants have
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Flowering plants manipulate animal behaviour

  • Flowering plants are largely pollinated by insects and birds
  • They can also be pollinated by nocturnal animals such as bats
  • Also, larger animals that eat the fruits of the plant and deposit the seeds of the fruit away from the mother plants in their waste
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Western Underground Orchid

  • It lives entire life underground, including flowering 
  • Only lives in Australia
  • Only discovered when farmer was ploughing a field and found that he had dug up a load of the orchids that were flowering 
  • Pollinated by underground insects
  • It is now endangered,and it is believed that this is because it is thought to have relied on a marsupial that is not extinct for its dispersal 
  • This just illustrates that these plants have come to occupy very diverse niches
  • Also, because it is entirely underground it cannot PS
  • It is another parasitic plant, but does not even make contact with host plant, it derives its nutrition from a fungus that links it to host plant. 
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Arabidopsis

  • Lots of different species- thaliana is just one of them
  • They are durable 
  • Easy to work with 
  • This was the first plant to have its genome sequences- In 2000
  • Genome is well annotated now 
  • Arabidopsis is great for genetics- it is a diploid plant with a fairly reduced genome, fairly easy to make crosses
  • There are stock centres around the world where material is provided to research the species
  • It can be transformed very easily and make transgenic Arabidopsis lines
  • This allows you to do reverse genetics- Have a gene that you feel might be important and finding out what it does. 
  • Looking at what a gene does you then study mutant in whih gene has been knoched out and look at how this effects the phenotype of the plant. 
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Why use Arabidopsis?

  • Speed- Can get 8 generations in a year, which is very fast
  • Size- A very small plant, which means that there is a lot less resources needed to grow and examine this plant compared to maize that would require and extremely large area of field with an army of people- CHEAPER
  • All of these reasons combined has led to a much larger community of Arabidopsis thaliana than other communities. 
  • This allows momentum and allows you to gain progress much quicker than in other species
  • Self's- A lot of plants, especially those that are of interest to sciene, are self-incopatible but A. thaliana is very easy to self. 
  • It does it on its own, and if you don't want it to self you have to prevent this. 
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Alternation of generations

  • Plants have 2 parts to their lifecycle:
  • A diploid sporophyte stage
  • A haploid gametophyte stage
  • The major part of most plant lifecycles is the diploid sporophyte part but there are some plants in which the gametophyte is the dominant phase (Moss etc) 
  • The diploid phase ends with meiosis to make haploid male and female gametes
  • The diploid phase starts following the fertilisation of the female haploid gamete by the male hg. 
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Haploid phase

  • Meiosis is the start of the haploid stage
  • Meiosis is obviously different in the male and female gametophytes
  • Males- Gametophyte is formed in the stamen
  • Female- Gametophute formed in the carpel
  • Male; Stamen is made up of a filament and anther
  • Anther: Where the pollen grains form 
  • Inside it there are pollen sacs which contains diploid cells called microspore mother cells
  • In the transition to the gametophyte stage the microspore mother cell undergoes meiosis to produce 4 microspores
  • Each one of these undergoes mitosis to produce a pollen grain which contains 2 haploid cell
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What about in female organs?

  • Gametophyte is formed within the ovule which is within the carpel 
  • There are lots of ovules in a carpel
  • A single cell ( the megaspore mother cell that is diploid) undergoes meiosis to produce 4 haploid megaspores
  • 3 of these megaspores die leaving one surviving megaspore
  • The surviving megaspore then goes through 3 rounds of mitosis to produce 8 nuclei without cellularisation (effectively have one big cell with 8 nuclei)
  • These 8 nuclei then get partitioned into 7 cells with one cell containing 2 nuclei (diploid) 
  • 3 at one end that are called the antipodal cells 
  • No understanding of what these do 
  • There is then a cell in the middle 
  • This is the endosperm cell which contains 2 nuclei 
  • There are also 3 cells at the other end of the embryo sac.
  • The 2 on the outside are called synergids and the central cell is the egg cell
  • The egg cell will be fertilised to produce zygote
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Diploid phase

  • You have the embryo which is within a seed 
  • This undergoes germination 
  • In this is goes through primary development which is sometimes known as vegetative development 
  • Then you will go through secondary development which is known as reproductive development in which it will flower (angiosperms) 
  • Sometimes the vegetative phase is split into juvenile and adult phase
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Secondary fertilisation

  • The pollen grain lands on the top of the carpel on the stigmatic papillae
  • The pollen grain then germinates and produces a pollen tube which grows down transmitting tissues to the bottom of the ovule through the micropyle and releases the 2 nuclei
  • One of them fertilises the egg cell which will become the diploid zygote and the other one fertilises the diploid endosperm cell to make a triploid tissue (endosperm) whcih provides nutrition for the developing embryo 
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