ABSTRACT
Polar transport of the phytohormone auxin and the establishment of localized auxin maxima regulate embryonic
development, stem cell maintenance, root and shoot architecture, and tropic growth responses. The past decade
has been marked by dramatic progress in efforts to elucidate the complex mechanisms by which auxin transport regulates
plant growth. As the understanding of auxin transport regulation has been increasingly elaborated, it has become clear
that this process is involved in almost all plant growth and environmental responses in some way. However, we still lack
information about some basic aspects of this fundamental regulatory mechanism. In this review, we present what we
know (or what we think we know) and what we do not know about seven auxin-regulated processes. We discuss the
role of auxin transport in gravitropism in primary and lateral roots, phototropism, shoot branching, leaf expansion,
and venation. We also discuss the auxin reflux/fountain model at the root tip, flavonoid modulation of auxin transport
processes, and outstanding aspects of post-translational regulation of auxin transporters. This discussion is not meant to
be exhaustive, but highlights areas in which generally held assumptions require more substantive validation.
Key words: Auxin transport; ABCB; AUX1; PIN; phototropism; gravitropism; fountain model; shoot branching; leaf expansion;
venation; flavonoids.