Leaf development

Question 1

What is a leaf?

Answer 1

Leaves are:

• Lateral outgrowths of a
  plant shoot
• Usually determinate
  (finite growth)
• Vascularized
• Initiated by the shoot apical meristem
• Usually photosynthetic
• hvae disctinct upper and lower sides with different fuctions: Upper- light absorption and lower- gas exchange
• they sequester carbon dioxide and produce oxygen, act as heat sinks, and circulate water. -> maintaining a hospitable climate

Question 2

Where do leaves come from?

Answer 2

Leaves are formed post embryonically, after germination by the shoot apical meristem

Question 3

What determines where a leaf forms?

Answer 3

Phyllotaxy:
* Alternate
* Opposite
* Whorled
* Spiral

The NEXT leaf to form is called the Incipient primordium (I 1).

And the older primordia control the placement of the incipient primordia, via auxin maxima where the organ initiates.
**The polarity of PIN1 proein froms an Auxin maximum at I1. (direction of transport). **

For the next organ: A subsequent reversal in PIN1 polarity changes the position of the auxin maximum, specifying the site of the next primordium.
Direct oxin flow into the develoig midvein, after primordium initiation.

(AHP6 protein, a cytokinin signaling repressor, also plays a role in the process. It moves away from existing primordia and makes te cell less sensitive to auxin signal -> correct temporal sequence of initiation evens)

Question 4

How do cell know where to place their PIN proteins?

Answer 4

Two hypotheses:
1) “up the gradient”: towards the neighbouring cell with the highest auxin concentration

2) “with the flux”: in the direction of the strongest net auxin flow (cf. canalisation hypothesis for leaf vein formation)

Question 5

What else plays a role in organ initiation except auxin?

Answer 5

Biomechanics may also play a role, particularly in larger meristems like sunflowers.

Ectopic expression of expansin (cell wall
loosening protein) induces ectopic primordium initiation.
-> Ectopic primordia can reverse phyllotaxis

Question 6

The role of transcription factors in leaf identity

Answer 6

A leaf acquires identity by turning OFF meristem genes and turning ON leaf genes.

• KNOX-1 (meristem- specific gene), ARP (Primordium- SPecific gene) and boundary genes encode transcriptional regulators that control expression of other genes.
• Precise control of cell fates involves tight control of transcription by developmentally regulated activators and repressors

Question 7

How does a leaf acquire polarity?

Answer 7

The central side is
adaxial, and peripheral is abaxial.

• A signal from the meristem moves
  through the epidermis into the incipient
  primordium
• The signal conveys the ad axial positional information.
• The nature of the signal (Sussex signal) is not known, but there is maybe a low-auxin zine on the adaxial side of leaf primordia
• (Forcing auxin to the adaxial primoridum side causes abaxialization. Loss of ad axial or ab axial fate causes radial leaves)

Question 8

WHat are important adaxial and abaxial genes?

Answer 8

Leaf polarity requires an unknown signal from the meristem and the domain specific expression of adaxial and abaxial specific transcription factors.

Adaxial genes:
* PHAN
* PHB -> regulated by miRNA, AGO1 mutant have radial leaves
* REV & PHV

ABaxial genes:
* KANADI 1, 2, 3
* YABBY