Chiral and non-chiral nutations in Arabidopsis roots grown on the random positioning machine

Abstract

Arabidopsis thaliana roots grown on a vertically set plate do not elongate straight down the gravitational vector, but by making waves and coils, and by conspicuously slanting towards the right-hand. This behaviour, in a previous paper, was ascribed to the simultaneous effect of three processes: circumnutation, positive gravitropism and negative thigmotropism. However, when the plants are grown on the Random Positioning Machine (RPM), in conditions that are believed to simulate space microgravitational conditions closely, the roots do not show the usual pattern. In the wild type, the roots make large loops to the right-hand side, whereas in the gravitropic and auxinic mutants aux1, eir1, rha1, they just move randomly around the initial direction. Therefore, if the movements made on the RPM are those produced by the exclusion of gravitropism and negative thigmotropism, as is apparent, the conclusion is that Arabidopsis roots are animated by a form of chiral circumnutation, that is lacking in the auxinic and gravitropic mutants aux1, eir1 and rha1. In addition, the 1 g condition appears to reduce the scatter among the circumnutating tracks produced by the roots of the wild types, but not among those of the mutants. Because there is a scarcity of literature regarding circumnutation in roots, it is not known how widely root chiral circumnutation is spread, but it is known that, in some previously studied species, just random nutations are observed. Two kinds of nutating movements seem to exist in plant roots and, whereas the random process does not seem to be connected with auxin physiology and transport, the chiral process appears to be connected in the same way as gravitropism is.