Polar auxin transport , which takes place with the carrier protein , is a unique way to distribute auxin in plant enabling the formation in particular cells or tissues phytohormone concentration gradient . The auxin transport protein involved belong to three families : media PIN (PIN -formed ) , the pump PGP / ABCb ( P-glycoprotein/ATP binding cassette ) and permease AUX1/LAX ( auxina RESISTANT1/LIKE-AUX1 ) . The results of recent studies have confirmed that five of the eight PIN proteins in A. thaliana , functioning as carriers transporting auxin out of the cell .

Polar auxin transport is essential for normal plant growth and development. Auxin moves between plant cells through a combination of membrane diffusion and carrier-mediated transport. Recent studies have identified several classes of membrane proteins involved in auxin transport, and have started to uncover a system that regulates auxin flux through plant tissues via the subcellular asymmetric locali- zation of these proteins.