Auxin and other signals on the move in plants.

As multicellular organisms, plants, like animals, use endogenous signaling molecules to coordinate their own physiology and development. To compensate for the absence of a cardiovascular system, plants have evolved specialized transport pathways to distribute signals and nutrients. The main transport streams include the xylem flow of the nutrients from the root to the shoot and the phloem flow of materials from the photosynthetic active tissues. These long-distance transport processes are complemented by several intercellular transport mechanisms (apoplastic, symplastic and transcellular transport). A prominent example of transcellular flow is transport of the phytohormone auxin within tissues. The process is mediated by influx and efflux carriers, whose polar localization in the plasma membrane determines the directionality of the flow. This polar auxin transport generates auxin maxima and gradients within tissues that are instrumental in the diverse regulation of various plant developmental processes, including embryogenesis, organogenesis, vascular tissue formation and tropisms.