Rapid redistribution of auxin-regulated RNAs during gravitropism.

Gravitropism, the bending of plants in response to gravity, is caused by differential growth rates on two sides of a responding organ. The general belief, although somewhat controversial, is that auxins play a major role in gravitropism by controlling the rate of cell extension. The tissue print technique was used to ascertain the distribution of auxin-regulated RNAs during the gravitropic response of soybean hypocotyls. In vertically oriented seedlings, auxin-regulated RNAs are symmetrically distributed in the elongating region of the hypocotyl. In horizontally orientated seedlings the distribution becomes asymmetrical within 20 minutes and the greatest asymmetry coincides with the onset of rapid bending. The results provide a clear correlation between the dynamic expression of genes under auxin control and a morphogenetic phenomenon traditionally known as an auxin response.