The evolution of auxin transport and its contribution to plant morphology

Abstract

Directional transport of the phytohormone auxin causes differential auxin distribution within tissues. This gradient is essential for various aspects of plant morphogenesis, such as embryogenesis, tissue patterning, and tropism. The auxin efflux carrier PIN FORMED (PIN) is responsible for such transport by being localized to a specific side of the plasma membrane. PIN exists in organisms throughout the Streptophyta, and polar auxin transport is detected in the Charophyceae algae Chara braunii. This suggests that the importance of auxin transport is conserved among land plants and charophytes. However, the morphology of land plants and charophytes is highly diversified, from complex angiosperms to simple algae without any differentiated cells. Therefore, the role of PIN and auxin transport may be different between these plants. Here, we review the current findings on the function of PIN and auxin transport reported in Charophyte, Bryophyte, and Fern. Based on these reports, we introduce three different types of auxin transport, “cell-to-environment auxin transport”, “non-canalized intercellular auxin transport”, and “canalized auxin transport”. Each of these is assumed to contribute to plant development differently. Finally, we discuss how such types of auxin transport are involved in plant morphogenesis and their relation to plant evolution.