adaxial-abaxial leaf polarity

• 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).

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

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.