PIN2s

PIN2s are members of the PIN-FORMED (PIN) family of auxin efflux transporters, with the PIN2 protein from Arabidopsis thaliana providing a well-characterized example. PIN2 proteins function to export the plant hormone auxin from cells and contribute to the directional, polar flow of auxin within root tissues. As with other PIN family members, PIN2 proteins are polarly localized at the plasma membrane, which enables vectorial transport of auxin between adjacent cells.

Distribution and localization: In Arabidopsis, PIN2 is expressed predominantly in the root epidermis and cortex, and homologs are found in many plant species. PIN2 proteins display lateralized localization on the cell membrane and are subject to continuous cycling between the plasma membrane and internal compartments. This trafficking is essential for adjusting the direction and rate of auxin transport in response to developmental cues and environmental signals.

Role in development: PIN2 plays a central role in root gravitropism and patterning by directing auxin flow along the root axis. When roots are reoriented relative to gravity, asymmetric PIN2 localization leads to asymmetric auxin distribution, which inhibits growth on the lower side and promotes bending toward gravity. PIN2 also participates in other aspects of root development, including elongation and lateral root initiation, through its control of local auxin accumulation.

Regulation and mechanism: PIN2 trafficking and activity are regulated by phosphorylation, endocytosis and recycling processes, and interaction with trafficking regulators such as GNOM and PINOID kinases. Environmental factors such as light, nutrient availability and stress conditions influence PIN2 abundance and localization, enabling the root system to adapt to changing conditions.

Evolution and significance: PIN2 proteins are conserved across many flowering plants, though sequence and regulatory differences exist between species. Studying PIN2s enhances understanding of plant architecture, adaptive growth, and responses to gravity, with potential implications for crop breeding aimed at modifying root systems.