Abstract
In 1880, Charles Darwin and his son published a book, “The power of movement in plants”, in which they described plant tropic behavior. Through their detailed observations of the tropic curvature of monocot coleoptiles, they concluded that “when seedlings are freely exposed to a lateral light, some influence is transmitted from the upper to the lower part, causing the latter to bend”. This observation is the first suggestion of the importance of “some influence” transmitted from the tip to basal growing parts, at least in the bending of monocot coleoptiles after lateral light stimulus. Following their suggestion, much research was conducted on plant tropic curvature, and indicated that the “influence” is a substance, auxin (indole-3-acetic acid; IAA), the first plant hormone to be identified. Tropic responses are generally explained by the Cholodny–Went Hypothesis; that is, they occur via differential growth on the two sides of the elongating shoot, which results from asymmetrical IAA distribution. However, their theory did not include the role of the apical tip region as the site of production of IAA, which is subsequently supplied to the lower growing region. A number of studies have shown the importance of the apical tip region of monocot coleoptiles and dicot seedlings for the perception of photo- and gravi-stimuli, as well as tropic curvature. However, the exact origin of the tip-localized IAA and the mechanisms regulating the amount of IAA remain unclear. In this mini-review, we summarize our research as a story from tip-specific IAA biosynthesis to gravitropic curvature in maize (Zea mays) coleoptiles.
