Plant development Chapter 7

Question 1

Did multicellularity and development evolve concurrently in the plant and animal kingdom?

Answer 1

You know the answer… No. It did not. The plant and animal kingdoms developed a system for development totally separately.
So if we compare plants and animals we can get a look at what might be essential, and what might just be the way it turned out.

Question 2

Has multicellularity evolved multiple times?

Answer 2

Multicellularity has evolved at least a dozen times in different lineages. It seems to me that development would have to evolve with each instance of multicellularity, though it could build of of gene homologs.

Question 3

Plants like to be different:

Answer 3

Plant cells do not seem to ever be truly determined
Plasmodesmota allows the utilization of transcription factor morphogens.
Environmental factors do much of the shaping of the final forms of plants. This is relatively obvious.

Question 4

Cell lineage does not matter as much in the determination of fate.

Answer 4

Which relates to cells not being determined. Position therefor will be important.

Question 5

Plants are also in a continuous state of development

Answer 5

There meristems do not grind to a halt.

Question 6

What are some apparent universal rules of development (seen in plants and animals):

Answer 6

Cells must communicate frequently
Developmental control is necessary and must gradually unfold
Certain protein usages are in both systems, which makes sense as both systems require

Question 7

What is our model

Answer 7

arabidopsis, a small plant

Question 8

What is the embryo of a plant.

Answer 8

A seed, a seed is an entire embryo in the plant.

Question 9

Why is Arabidopsis used as a model?

Answer 9

It is diploid (classical genetics work well)
It is small
Short GI
and it doesn’t take up much space.

Question 10

Plants are said not to have a dedicated germline, give an example which illustrates this.

Answer 10

Plants utilize apical meristems which makes shoots. These meristems can become inflorescent meristems.

Question 11

What does Dr. Podgorski mean when he says that all plant meristems are continuous embryos?

Answer 11

It means that the developmental processes that make adult plants are continuous in the meristem. The plants are developing in many ways the same structures over and over again.

Question 12

Cotyledons is a funny word, what does it mean?

Answer 12

Cotyledons are the two strange earlike leaves which flank the apical meristem.

Question 13

Plants do not have a dedicated germline, why would it be unrealistic for them to have one?

Answer 13

Their cells are not motile. If you cannot mobilize your germline, where are you going to keep it? On the outside where a predator can eat it? Inside where it cannot gain access to pollen? Or distribute new seed embryos. The current conversion of apical meristems into inflourescent meristems is key.

Question 14

Plant morphology:

- What is a the basal cell

Answer 14

The basal cell is equivalent to the placenta. The basal cell will give nutrients to the to the embryo proper. The basal cell attaches to the seed, and can access nutrients from the seed to grow the embryo.

Question 15

The first division of the zygote in the embryo.

Answer 15

The first division will form the basal cell and the apical cell. The division will be asynchronous, forming a much smaller apical cell. This apical cell will give rise to the embryo proper. The basal cell will continue to support the growth of the apical cell.

Question 16

The basal cell supports the embryo proper, which is the apical cell.

Answer 16

The apical cell will continue to divide. A suspensor will connect it to the basal cell. As the apical cell divides it will form larger and larger structures. It will form a blob, then a heart as the cotyledons form (the ears). The apical meristem will form at the point where the cotyledons meet. The root meristem will form towards the bottom of the plant, though strangely not the exact bottom. Throughout this whole process the embryo is within the seed. The basal cell has fueled all growth through the suspensor. Cotyledons ; P

Question 17

Is a plant limited to a single meristem?

Answer 17

No silly. ;)

Question 18

Germination define

Answer 18

Germination is the when the embryo breaks out of the seed and begins proper growth. Embryo development will often arrest until the right conditions are met for germination, after these conditions are met the plant will begin to grow, root leaving first. Gravity determination must be possible in the plant.

Question 19

What are the three radial layers in the established by the early embryo

Answer 19

Vascular tissue is in the center
Ground tissue is between the vascular tissue and dermal tissue
The dermal tissue is on the outside, touching only the ground tissue.

Question 20

Dermal tissue is similar to ectoderm
Ground tissue is similar to mesoderm
And…

Answer 20

Vascular tissue is similar to endoderm.

Question 21

Periclinal divisions are:

Answer 21

Parallel to the dermal tissue. This means they are parallel to the circle of the dermal tissue, not the length of the dermal tissue. A periclinal is parallel to the circumference

Question 22

Radial symmetry not bilateral

Answer 22

outside -> inner layer
Dermal tissue
Ground tissue
Vascular tissue

Question 23

Periclinal:

Answer 23

Are parallel to the circle of the of plant

Question 24

Anticlinal:

Answer 24

Anticlinal are oppose the circumference of the plant. They are perpendicular

Question 25

Three tissue types

Answer 25

center to outside
vascular tissue
Ground tissue
Dermal tissue

Question 26

Periclinal is…

Answer 26

parallel to the dermal tissue

Question 27

Anticlinal is…

Answer 27

perpendicular to the dermal tissue.

Question 28

What dictates cell fate in a plant?

Answer 28

Cell fate is dictated by position of the cells in the apical to basal structure.

Question 29

How do I know that cell fate is not dictated by the division cleavages, and that cytoplasmic determinants are dictating cell fate instead of position along apical to basal axis?

Answer 29

An experiment was performed, in this experiment a gene which regulates the orientation of the planes of cleavage was mutated. Even with planes of cleavage in disarray and some ugly looking plants, the plant still organized cell fate by position. Which argues strongly against cytoplasmic determinants

Question 30

3 radial layers

Answer 30

outside in
dermal
ground
vascular
tissue

Question 31

Periclinal

Answer 31

parallel with the circle of the dermal tissue

Question 32

Anticlinal

Answer 32

Perpendicular to the circle of the dermal tissue.

Question 33

If a mutant with disorganized cleavage planes produced a normal fate map, then what is dictating fate.

Answer 33

Cell to cell communication.

Question 34

How long before all embryonic pattern elements are set by the plant?

Answer 34

They do so within the seed, at stage known only as the torpedo stage.

Question 35

What is the root shoot axis?

Answer 35

The root shoot axis is an axis through the roots and shoots. Exactly what you would think. This is seen by the first division, when the fertilized egg divides into the basal and apical cell. The basal cell is the root axis. The apical cell is the shoot axis.

Question 36

In the early embryo where is auxin transported?

Answer 36

Auxin is moved up the suspensor into the apical cell, and then back and forth between apical cells. Auxin is moved by PIN proteins. PIN proteins must be unidirectional.

Question 37

Auxin is a small molecule :

Answer 37

Auxin is not very large, it is a modified form of tryptophan. Notably, auxin is not large enough to act as a transcription factor. so after auxin is localized by PIN proteins auxin must have another way of activating gene expression.

Question 38

What is the mechanism of action of auxin?

Answer 38

Auxin binds to AUX/IAA. AUX/IAA is an inhibitor of transcription. By binding to AUX/IAA auxin allows polyubiquination of AUX/IAA marking it for destruction

Question 39

Why you may ask does auxin mediated binding to AUX/IAA and the subsequent destruction of AUX/IAA after AUX/IAA is polyubiquitinized matter?

Answer 39

Because AUX/IAA is a party pooper. That’s right. AUX/IAA is an inhibitor of transcription at multiple sites. So Auxin enters the seen. Binds to AUX/IAA, AUX/IAA is marked for polyubiquination, AUX/IAA is then destroyed, stopping AUX/IAA from acting as an inhibitor of transcription.

Question 40

Why do we care about PIN proteins?

Answer 40

PIN proteins are auxin efflux proteins, so if we want auxin to relieve inhibition of AUX/IAA in a specific site we must move it there using PIN efflux proteins.

Question 41

After movement of Auxin to the apical cells and movement of auxin back and forth in the apical cells, where is auxin moved too next?

Answer 41

Auxin is brought back into the lower part of the cell. The apical (stem axis) portion of the embryo is now producing auxin.

Question 42

What is occurring in the globular stage of the plant embryo?

Answer 42

The globular embryo produces auxin. PIN7 is sending auxin down as it is being produced into the suspensor.

Question 43

An example of genetic approaches in understanding plant biology includes shoot meristemless. What does the name of this gene imply?

Answer 43

Well, when the gene shoot meristemless was knocked out the plant was… shoot meristemless… It formed no apical meristem is what this likely means.
the protein SHOOT MERISTEMLESS is a transcription factor. SHOOT MERISTEMLESS is confined to the apical meristem (shoot meristem) and is important in order to have a shoot meristem.

Question 44

Monopterous, is an ARF enconding gene. (ADP ridopsinating factor)

Answer 44

Loss of monopterous results in what? The lack of a hypocotyl.
Monopterous and hypcotyl.

Question 45

The knockout of SHOOT MERISTEMLESS and MONOPTEROUS are used to show what in broad terms?

Answer 45

They show that traditional genetics are still useful in plant biology. It also shows that specific genes are still necessary for specific regions to form, such as the shoot meristem and the hypocotyl in this case.

Question 46

AGAMOUS is a gene despite using the wrong nomenclature to specify it.

Answer 46

Agamous is important do you know why?

Question 47

Yes I know why agamous is important. Agamous demonstrates the presence of homeotic genes in plants. The knock out of agamous results in?

Answer 47

The knockout of agamous results in the formatino of petals where we would normally see sepals. This means agamous will show up later when we do floral patterning.

Question 48

Agamous is important in specification of the sepal identity.

Answer 48

Auxin is a protein transported by PIN proteins. Auxin bind to AUX/IAA in the cell. I’m going to say AUX/IAA is indeed a transcription factor, even though it may only be inhibitory. The movement of auxin is controlled by PIN proteins. Auxin binds to AUX/IAA and enables it to be marked for proteolysis (poly-ubiquinated) This enables transcription of genes previously being inhibited by AUX/IAA

Question 49

Auxillary meristems?

Answer 49

Are beneath the apical meristem, and are likely some back up system? more information soon.

Question 50

In a simple diagram of the apical meristem, three zones are labelled. What are these zones?

Answer 50

The three zones of the apical meristem, are the central zone where stem cells reside, the organizing center which is under the central zone and the central zones signal enables the maintenence of the stem cells in the central zone. The Peripheral zone is a rapidly dividing region of cells which have just left the central zone. They ‘move’ outward into the leaf primordium

Question 51

A stem cell niche is created by…

Answer 51

The organizing center.

Question 52

Let’s talk about WUS. WUS is a transcription factor that has a homeodomain, how does that make you feel?

Answer 52

Great. The presence of a homeodomain on WUS shows that the gene for the homeodomain (a 60 amino acid 3 alpha helix section which binds to DNA directily, and is contained in all hox gene (mostly)), is present in plants. It also implies that homeodomains existed prior to the evolution of development in plants and animals. This is likely because the two homeodomains are likely orthologs, and development had not yet evolved back when plants and animals last shared a common ancestor.

Question 53

What does WUS do?

Answer 53

WUS, a homeodomain transcription factor produced by the organizing center of the meristem ascends in the plant. WUS rises to the central zone above, and it is the action of the WUS, the homeodomain transcription factor which maintains a population of stemcells above the organizing center.

Question 54

Show me a sweet negative feedback loop and tie it back to plant development. Do so now.

Answer 54

WUS -> activates CLV3 expression (CLV3 is a ligand which diffuses throughout the central zone and into the peripheral zone. -> CLV3 activates CLV1/2 which causes a feedback inhibition of WUS.

Question 55

Why is WUS an elegant system?

Answer 55

WUS rises, and through binding and utilization of a homeobox domain induces stem cells, it also makes these cells produce the ligand CLV3. CLV3 is an inhibitor of the genes which make the organizer. This means that if the stem cells start to grow out of control, they will overproduce the CLV3 ligand, it will greatly inhibit WUS expression, and there will be a significant reduction in the amount of stem cells, as their genes are no longer being activated. CLV3 of course acts by binding to CLV1/2

Question 56

What happens when you induce ectopic expression of WUS?

Answer 56

Ectopic expression of WUS results in the formation of a shoot meristem. This tells us that WUS likely WUS alone is capable of inducing and maintaining a meristem. Which is really quite powerful. It also tells us that there is nothing special about the cells above the organizer. They are not some special lineage passed down through the generations. WUS expression induces meristem formation just about anywhere. What causes WUS expression is an interesting question.

Question 57

You are a growing meristem. Explain the significance of L1, L2, and L3 to you.

Answer 57

Let L stand for layer, as that is likely what L is standing for… L1 is the first layer, the top layer of the meristem. The top layer of the meristem is fated to become dermal tissue. The majority of divisions the cells will undergo are anticlinal. anticlinal divisions will ‘push’ cells away from the apical meristem. This is what allows the maintenance of the L1 layer. If a periclinal division has occurred then the plant effectively is growing up not just moving cells out to create width. A periclinal division will cause one of the daughter cells to take on an L2 identity. An L2 identity. This fact, the fact that if a cell is no longer on the outer layer, even if it arose from an outer layer cell, it will become the a part of the cell layer it is now in, shows that position is more important in identity determination then ancestry. POSITION!

Question 58

L1
L2
L3

Answer 58

L1 is outermost layer 1st layer
L2 is cortical or vascular tissue
L3 is cortical or vascular tissue

Question 59

What else does the presence of L1, L2, and L3 tell you that you did not know?

Answer 59

It also tells me that the organizer through WUS -> CLV3 -> CLV1/2 -| WUS is creating a stem cell region several cells in depth from an apical approach.

Question 60

Many different expression domains are necessary for the formation of an embryo. Expression domains are restricted to specific regions during specific points in development.

Answer 60

Genes expressed during the globular embryo stage may not be expressed during the traditional apical meristem growth. This shows that the plant is also using complicated means of regulating the location of multiple genes. It is a dynamic system, and is using as complex of methodologies as is seen in animal cell to cell communication in order to confer specific location and temporal expression to the correct genes.

Question 61

How are plants similar to the syncytial blastoderm of drosophila?

Answer 61

Plants are similar because they can impose transcription factor gradients, the presence of plasmodesmata enables this.

Question 62

What is a difference between plasmodesmata and gap junctions?

Answer 62

Plasmodesmata can be notably larger then gap junctions. They can also be more specific in terms of the proteins which they allow through (does not stop viral entry via plasmodesmata)

Question 63

Describe LEAFY:

Answer 63

LEAFY so named because when you knock it out you see only leaves… is an important transcription factor in an inflorescent meristem.

Question 64

Explain the difference between mRNA localization of LEAFY and the localization of the protein transcription of factor of leafy.

Answer 64

mRNA from leafy transcription is seen in the apical most cells of the meristem in a very tight domain. If we look at the distribution of the leafy protein, we that it is not tightly restricted to the apical domain. leafy is a transcription factor, so this illustrates the ability of transcription factors in plants to travel from cell to cell utilizing plasmodesmata

Question 65

Auxin let’s talk science and get real.

Answer 65

To be honest, Auxin appears to travel by moving to whatever place it hasn’t been before. Let me explain why I am begining to think this in terms of formation of leaf primordia. Leaf primordia creation is signalled by the dynamic distribution of auxin (in large part), now hear my words. Leaf primordia are also spaced to appear as far as possible from each other, since high auxin leads to primordia initiation it makes sense to have auxin flow away from where it just was. We therefor see primordia formation in a rotational pattern, after the formation and growth of 1 and two, which will appear opposite each other. Afterwards we will have a spiral pattern in the plant, with primordia popping up in an appropriate pattern.

Question 66

Shoot modularity. Plants repeat a particular pattern as the grow, essentially they create themselves in repetitive modules. These modules are similar to reiterative animal segmentation, but kind of not that similar truth be told…

Answer 66

We will have internode -> node (stem -> leaf) -> internode -> node (stem -> leaf) -> internode -> node (stem -> leaf)

an auxiliary bud appears at each node. This bud is a dormant meristem.

Question 67

Shoot modularity continued, what determines this?

Answer 67

It does not like the mechanism has a very complete story. Shoot modularity is controlled by the pattern utilized for primordia formation. Get a drink and repeat that, and then say that this is an incomplete answer because the current flow of auxin initiating primordia formation does not get adequately explained by this model.

Question 68

So we see the modular design of plant development (internode -> node ( -> leaf) -> internode), this design is patterned by something, explain what is behind, in part, the patterning of the modularity (what maintains this modular growth).

Answer 68

The modular growth and repeating pattern is controlled partially through by regulating the initiation of organ primordia from the shoot meristem. Organ primordia is also good terminology. Note: the modular pattern is likely not the same for all plants.

Question 69

Define abaxial and adaxial:

Answer 69

abaxial faces away from the center of the leaf, towards the bottom, abaxial is specified by FIL
Adaxial is the upper portion of the leaf, analogous to dorsal, dorsal has a d, adaxial has a d. Three transcription factors are found only in the adaxial portion of the leaf.

Question 70

FIL encodes a protein specifying what?

Answer 70

The gene FIL filamentous flower, specifies the abaxial portion of the plant, it encodes for a transcription factor which specifies for abaxial development, if FIL is ubiquitously expressed, then the entire leaf will be abaxial in nature.

Question 71

Three transcription factors PHAB, PHAV, and REV are restricted to only the adaxial portion of the leaf.

Answer 71

What restricts PHAB, PHAV and REV to the adaxial portion of the leaf? The three transcription factors are restricted do to miRNA expression located in the abaxial portion of the leaf. There is a sharp boundary maintained between the abaxial adaxial border maintained by this border.

Question 72

What we don’t know about abaxial adaxial specification…

Answer 72

Abaxial and adaxial specification maintained by miRNA restricting access of of adaxial localized transcription factors (by stopping them from being translated), there are three adaxial transcription factors it stops, PHAV, PHAB, and REV. It is not known how the miRNA which maintains this gradient is maintained in the leaf.

Question 73

How miRNA works, a look into the little mechanism which helps maintain the abaxial portion of the leaf by restricting axis to adaxial transcription factors and making them leave FIL alone.

Answer 73

miRNA are made when a stem loop structure is formed, generally in an intron. A protein known as dicer comes and cuts the miRNA out of the stem loop structure, into little 20 nucleotide long sections which are now single stranded. These associate with RISC and bind the mRNA that their sequence matches too. miRNA can either bind mRNA and stop them from being translated or they can mark mRNA for destruction, either way miRNA is doing its job.

Question 74

What is an inflorescent meristem?

Answer 74

An inflorescent meristem gives rise to a floral meristem. The floral meristem will become a flower.

Question 75

Shoot apical meristem growth is indeterminate, growth of the inflorescent meristem is either determinate or indeterminate (depends on the species), and the growth of the floral meristem is determinant.

Answer 75

Indeterminate growth of the shoot apical meristem is logical, this is the default state of the plant, and in absence of an environmental signal showing that flowering is appropriate (aka in absence of the signal which shows that now is a good time to put significant resources into reproduction) the shoot apical meristem will just grow and grow and grow.

Question 76

It’s different once an environmental signal comes along… First we will see transition to an inflorescence meristem, depending on the species it will either give rise to a floral meristem in a set number of cellular divisions (essentially its determined, though it is probably not as deterministic as I just made it sound), or growth will still continue in indeterminate matter until more signals arrive.

Answer 76

If the floral meristem develops… it’s game time baby. That’s right! A floral meristem always has determinant growth. It will become something specific, it cannot keep growing forever. The plant has committed to reproductive effort by transitioning to the floral meristem. Though depending of the species an inflorescent meristem is also a commitment to reproduction (if growth of the inflorescent meristem is indeterminate)

Question 77

What is LEAFY.

Answer 77

Oh LEAFY, my old friend… LEAFY is a transcription factor. We know that LEAFY is a transcription factor because LEAFY was used as the model to show transcription factor gradients and the ability of transcription factors to travel through plasmodesmata. LEAFY is the moderator of the shoot meristem to inflorescent meristem transition.

Question 78

If you knock out LEAFY what do you get? (hint: it is really freakin obvious what you will get when you knock out LEAFY)

Answer 78

In the absence of LEAFY the plant will never produce a inflorescent meristem and will therefore never flower. It will however, be leafy, because all it will do is produce leaves and grow in the indeterminate step.

Question 79

meristem identity gene are what?

Answer 79

A meristem identity gene is damn useful terminology. Pardon the language. A meristem identity gene is any gene, such as LEAFY, which causes the transition from a shoot meristem to an apical meristem. I repeat… A meristem identity gene specifies the transition to inflorescent meristem growth.

Question 80

LEAFY is a…

Answer 80

Meristem identity gene.

Question 81

Suppose I want to stop growing as a shoot apical meristem, and maybe try an inflorescent transition and a little bit of development into a floral meristem, what type of gene would I use?

Answer 81

Oh … a Meristem Identity GENE

Question 82

Meristem Identity Gene.

Answer 82

LEAFY is one…

Question 83

What transition does leafy or any meristem identity gene determine.

Answer 83

A meristem identity gene such as leafy marks a transition from growth to reproductive effort. A meristem identity gene helps to transition between… meristem identities. Leafy is expressed in the apical portion of the inflorescent meristem.

Question 84

CONSTANS do what…

Answer 84

They are transcription factors. The CONSTANS transcription factors have a name implying CONSTANS are related to time, this is correct. Indeed a circadian clock is essentially made in plants using CONSTANS.

Question 85

Where are CONSTANS produced?

Answer 85

CONSTANS are produced in leaves, their production peaks at late afternoon, which implies that CONSTANS is stable in sunlight.

Question 86

Explain why CONSTANS is actually a rather brilliant system.

Answer 86

CONSTANS production peaks during the afternoon. During the dark however CONSTANS is targeted for destruction. So if the days are short, then by late afternoon it is already getting dark, and we see little survival of CONSTANS over the long night. If however CONSTANS is produced during days with long enough daylight, then late afternoon will not be during night, survival of constans will be seen. This build up in constans will occur in the leaves and tell the plant when it is daylight.

Question 87

Flowering Locus T is a transcription factor. It is also a meristem identity gene. CONSTANS will activate a series of genes which will in turn activate Flowering Locus T. Flowering Locus T will be transported up the shoot into the meristem where it will act as an meristem identity gene should and change the meristem identity.

Answer 87

Note: constans is a transcription factor already, so why bother will production of flowering locus t. This is a moderately good question. But constans is already tied to a degredation timer, it is likely better to send up a more stable transcription factor to change meristem identity. Therefor = flowering locus T.

Question 88

Sepal:

Answer 88

A sepal is the outer most whorl. A sepal is the green leafy coloring which surround the flowers in the bud, the sepals will also support the petals.

Question 89

Petals are therefor the next layer, if the sepals are outside supporting the petals (sePALS) pals support!!!! FREAK YA! If the sepals are supporting the petals, the petals should be next.

Answer 89

This is truth. sePAL.

Question 90

Stamen. Stamen rhymes with semen. Semen is what my testis produce. Therefore the stamen is the male reproductive organ. The male reproductive organ is a tad more external then the female reproductive organ, which is also the way it works in anatomy.

Answer 90

The Carpel is the central organ, it begins with the same letter as a very rude way to describe the female genitalia. It is also central as it will hold the seed.

Question 91

Outside to inside we have…

Answer 91

Sepals, Petals, Stamens, Carpels

Question 92

SEPAL PETAL STAMEN and CARPEL

Answer 92

so Sepal, Petal, Stamen, Carpel or Se, Pe, St, Ca

Question 93

There are three genes, A, B, and C in the abc model. mutations of these are homeotic, why?

Answer 93

Ok… A has mutual inhibition with C class genes, therefor their expression domains do not overlap. Beneath I will make a sweet model.

A AB BC C BC BA A
Se, Pe, St, Ca, St, Pe, Se

Question 94

So the phenotypes to genotypes are as follows

Answer 94

A -> sepal
AB -> petal
BC -> Stamen
C -> Carpel
C -> Carpel
BC -> Stamen
BA -> petal
A -> Sepal

Question 95

C
AB
BC
A
BC
AB

Answer 95

C    Carpel
AB  Petal
BC  stamen
A     Sepal
BC  Stamen
AB  Petal

Question 96

What allows actual floral patterning? We have a series of transcription factors… great… how do they interact:

Answer 96

Well the answer is they form complexes. WUS Is still present in the floral meristem, UFO (unusual floral organs) is also present.
WUS, UFO, and LEAFY work together to activate the appropriate ABC genes, these transcription factors form complexes with each other. These transcription factor complexes are capable of creating expression and specific phenotypes we are used to seeing.

so gene A transcription factors complex with unknown genes to activate the genes for sepals
Gene A, B, and SEP3 (another transcription factor complex to form petals)
Stamens are produced through a SEP3, A, and C complex
Carpels are produced through a SEP3, and gene C complex.