Phase Change, Branching, and Domestication

Question 1

What is phytomer and what do they do?

Answer 1

A unit of a plant that makes a particular organ.

Phytomer identity and organisation is dependent upon the growth phase.
Phytomers make same organs over and over again.
Meristems can change identity eg. VM (vegetative meristem) to IM (inflorescence meristem). Inflorescence is production of flowers- plant goes from vegetative to reproductive.

Question 2

What are the two stages of the vegetative stage.

Answer 2

Juvenile- Insensitive to flowering cues, Juvenile plants don’t flower- not mature enough.

Adult- Often shows a change in leaf shape, Competent to respond to flowering cues.

Question 3

How do leaves change in Arabidopsis through the stages?

Answer 3

Leaves are arranged in a rosette. Still make phytomers, but compressed into rosette
.
Changes in leaf shape, trichomes (hairs on leaf top surfaces)- (only in adult vegetative state).

Leaf shape depends on growth phase or plant “age”.

Adult leaves in Arabidopsis have more serrations along the leaf margin.

Question 4

What are microRNAs?

What does microRNA156 do?

Answer 4

microRNAs are 20-21 nucleotide long single-strand RNA molecules with complimentary to specific, target mRNAs. MiRNAs downregulate their targets.

microRNA156 (miR156) levels were associated with phase change.
Expressed more in juvenile stage, but reduces over time.
35S- constitutive promoter- overexpress 156- stays in juvenile stage for longer.
Knock down 156- early flowering.

Changes in phase timing by altering miR156 leads to dramatic changes in plant body plan.
Just keep making leaves if you overexpress 156.

Question 5

How does plant growth change when phase time is altered?

Answer 5

Drastic change in final plant architecture due to altered phase timing.

35S::miR156
very slow to flower and then kept growing for longer.

Live longer in overexpression. Greater change in architecture.

Question 6

How does microRNA156 control phase change?

How does this work at the genetic level?

Answer 6

Took mutants involved in phase change. Look at gene expression changes, see which ones were most common- SPL (squamosa promoter binding-like family).
SPL increases with age, as 156 decreases with age.

156 binding site at 3’ end of SPL.
Micro RNA- tiny RNAs, ~20 nucleotides long.
Influential regulators of gene expression in eukaryotes.
156 has complementary sequence to SPL- correlation.
MiRNAs prevent translation of their targets RNAs.

Question 7

How important is miR156-regulation for SPL3 function?

Answer 7

Make SPL3 miR156-resistant- rSPL3.

Can change one of the sequences in the 3’ site.
Sequence of protein is identical, protein doesn’t change, but RNA changes.

miRNA156 can’t bind to SPL site any more.

Question 8

What is an axillary bud/meristem?

Answer 8

Normally arrested after growth
Axillary bud- dormant meristem but could be initiated.

Axillary meristems can form branches.
Activation of axillary meristems leads to more shoots.

Question 9

What is apical dominance?

Answer 9

Apical dominance describes a plant form where the central stem is dominant over axillary shoots.

The more branched it is, the less apically dominant it is.

Question 10

What causes apical dominance?

Answer 10

Apical dominance is due to an inhibitory signal from the primary shoot tip.

Shoot inhibitory signal is Auxin
But, auxin does NOT actually enter the axillary meristem.

Shoot branching is a great example of plasticity- development adjusts due to external event.
Add auxin to plants that have had shoot meristem removed- nothing happens.
Block transport of auxin- stops growth.
Shows auxin is involved, but other things happen too.

Question 11

What is the other plant hormone, apart from auxin, that is involved i branching?

Answer 11

Strigalactone (SL).

The branch-controlling signal can be made in either tissue, and can move from root to shoot.

SL secreted by roots, known for stimulating branching of fungi to associate with plant roots. Symbiotic relationship.
Striga (witchweed)- parasitic plant, grows on other plants and kill them eg. crops.
Knows that its near plants cause seeds recognise strigalactone which causes them to germinate.

Question 12

What do SLs do?

Answer 12

SLs inhibit shoot branching.

Across plant species, plants unable to make SLs show a loss of apical dominance (many more shoot branches grow out).

Both auxin and SL inhibit shoot branching.

Question 13

What does cytokinin do?

Answer 13

Unlike Auxin and SL, cytokinin is associated with promoting axillary shoot outgrowth.

Locally elevated cytokinin induces bud outgrowth.

Has positive effect.

Question 14

How do auxin, SLs and cytokinin interact?

Answer 14

All hormones work together to control shoot branching.

Auxin downregulates cytokinin synthesis (IPT is a cytokinin biosynthesis gene.)

Auxin upregulates strigolactone synthesis (CCD7 and CCD8 are strigalactone biosynthesis genes.)

Question 15

What was key for domestication?

Answer 15

Architectural variation within species.

Question 16

What are some traits common in domesticated plants?

Answer 16

Loss of grain shattering or brittle-ness.
Loss of dormancy.
Compact growth habit – reduced stature and less branching.
Selfing rather than outcrossing.
Larger seeds – (polyploid implications here).
Faster flowering.
Loss of toxic compounds.

Domestication involved drastic changes in architecture.

Question 17

What cause apical dominance in maize?

Answer 17

Apical dominance in maize due to upregulation of a single gene, Tseosinte branched 1 (Tb1), a TCP transcription factor.

Mutant is interfertile.
All of the differences explained by around 5 different gene loci.
No difference in TF sequence. Instead, TF is upregulated- cause of transposon found ~60 bases upstream in wild plant.
Major loci 1.

Question 18

What causes the liberation of grain from the husk in maize?

Answer 18

Important part of domestication of maize was the liberation of the grain (corn) from the tough husk (glume) which encases the tseosinte grain.

Tseosinte Glume Architecture 1 (TGA1) = encodes an SPL.

Amino acid differences between maize and tseosinte TGA1 gene.

Major loci 2.
Glume- conatin silica, are lignified.
Length of glume, amount of silica/lignin- controlled by TF.
Genes have large effects rather than a large number of genes.

Question 19

What was lost from domesticated rice plants to prevent grain shattering?

Answer 19

Loss of grain shattering was pivotal to early domestication –
Ease of harvest.

Wild rice relative –
spontaneously shatters.

Domesticated rice –
Grain stays on plant.

sh4, a Myb transcription factor:

• expressed at a higher level in wild relative.
• directs the formation of the abscission zone – a layer of cell detachment enabling seed dispersal.

Question 20

What is the rice ideal plant architecture?

Answer 20

Low tiller (axillary branch) number.
More grains.
Thick and sturdy stems.

IPA= More resources to producing seeds rather than leaves.
All of the difference explained by mutation in SQUAMOUS TF.