adaxial-abaxial leaf polarity

Question 1

What are the characteristics of the adaxial side of the leaf?

Answer 1

• Adaxial refers to the upper side of the leaf, adjacent to the axis.
• Responsible for light harvesting and photosynthesis.
• Contains photosynthesizing cells and trichomes (hair-like structures).

Question 2

What are the characteristics of the abaxial side of the leaf?

Answer 2

• Abaxial refers to the lower side of the leaf.
• Responsible for gas exchange and transpiration.
• Contains stomata (pores for gas exchange).

Question 3

How is the adaxial-abaxial boundary formed in leaf development?

Answer 3

• Leaf patterning specifies distinct cell fates for the adaxial, abaxial, and middle domains.
• This leads to the formation of a developmental boundary.
• The leaf lamina flattens as a result of the parallel development of the adaxial and abaxial cells.

Question 4

How are genes involved in leaf patterning?

Answer 4

• Genetic screens for mutant plants reveal genes encoding transcription factors for the adaxial, abaxial, and middle domains.
• These transcription factors regulate the cell fate of each domain and guide proper leaf development.

Question 5

How is adaxial identity specified in leaves?

Answer 5

• PHANTASTICA (PHAN): A key gene expressed at the adaxial side of the leaf primordium in snapdragon, encoding a transcription factor.
• Loss-of-function phan mutants produce rod-like, abaxialized leaves, confirming PHAN’s role in specifying adaxial identity.
• In Arabidopsis, the genes PHABULOSA (PHB), PHAVOLUTA (PHV), and REVOLUTA (REV) also regulate adaxial identity.
• These genes encode transcription factors that specify adaxial identity at the primordium’s adaxial side.

Question 6

How is abaxial identity specified in leaves?

Answer 6

• In Arabidopsis, KANADI (KAN) and YABBY (YAB) gene families specify abaxial identity.
• These genes encode transcription factors expressed at the abaxial side of the leaf primordium.
• Loss-of-function mutants lacking KAN or YAB produce leaves with an adaxialized phenotype.

Question 7

What role does the middle domain play in leaf development?

Answer 7

• The middle domain is essential for blade outgrowth and is marked by the expression of WUSCHEL-RELATED HOMEOBOX (WOX) transcription factors.
• WOX is expressed between the adaxial and abaxial domains after primordium emergence.
• Loss-of-function wox mutants exhibit a narrow leaf phenotype, indicating that WOX genes are required for middle domain formation.

Question 8

How are transcription factors coordinated in leaf development?

Answer 8

• The expression of adaxial and abaxial identity genes is mutually exclusive.
• There is also mutual repression between genes specifying adaxial and abaxial fates, ensuring proper polarity and development of leaf structures.

Question 9

What are the key events in leaf development?

Answer 9

• Recruitment: Some cells in the peripheral zone are recruited to form the leaf primordium.
• Acquisition of Identity: Cells acquire the new identity of a leaf.
• Polarity Formation: The leaf acquires its polarity (adaxial vs. abaxial).
• Growth: The leaf grows to its final shape and size.
• Differentiation: Specialized cells differentiate to form the fully functional leaf.