Transmembrane auxin carrier systems - dynamic regulators of polar auxin transport

Recent investigations of the biochemistry, physiology and molecular genetic s of polar auxin transport have greatly advanced our understanding of the p rocess and of the part it plays in the regulation of development and in the responses of cells, tissues and organs to internal and external stimuli. T he molecular and physiological characterization of mutants which exhibit le sions in polar auxin transport has led to the isolation and sequencing of g enes which encode putative components of auxin carrier systems, or proteins which directly or indirectly regulate these systems. This work has reveale d that specific auxin uptake and efflux carriers are coded not by single ge nes, but by whole families of genes, the expression of which is tissue or s timulus specific. Furthermore, evidence is accumulating rapidly that at lea st the auxin efflux carrier is a multi-component system consisting of both catalytic and regulatory subunits, including a separate phytotropin-binding protein. Other genes have been tentatively identified which code proteins that regulate the expression of genes coding auxin carrier components, or w hich regulate the intracellular traffic or activity of auxin carriers. Inve stigations of the turn-over and Golgi-mediated trafficking of auxin carrier proteins have revealed that essential components of at least the efflux ca rrier have a very short half-life in the plasma membrane and are replaced w ithout the need for concurrent protein synthesis, leading to speculation th at they might cycle between internal stores and the plasma membrane. The wa y is now clear for the development of specific molecular probes with which to investigate the intracellular transport and targeting of auxin carrier p roteins.