Journal of Experimental Botany, Vol. 54, No. 389, pp. 1909-1918,
August 1, 2003
© 2003 Oxford University Press
Chiral and non-chiral nutations in Arabidopsis roots grown on the random positioning machine
Received 17 December 2002; Accepted 2 May 2003
1 Institute of Agroenvironmental Biology and Forestry, Consiglio Nazionale delle Ricerche, Via Salaria Km 29.300, 00016 Monterotondo, Rome, Italy
2 Department of Biology, University of Sassari, Italy
* To whom correspondence should be sent. Fax: +39 06 9064492. E-mail: fernando.migliaccio{at}ibaf.cnr.it 
The direction toward which the Arabidopsis roots slant during elongation in the wild type is considered to be the right-hand because, when the plant is seen from above the shoot apex, the root appears to move forward making loops in the clockwise sense. This movement, as is known in Physics, is considered right-handed, and this definition is applied to the whole universe (Gardner, 1990). However, it needs to be remembered that Linnaeus and others scientists (Hart, 1990) considered the above movement left-handed, because they imagined seeing the helix from its interior, in which case the view, logically, is reversed. 
Recently, the Hashimoto group produced some significant papers (Furutani et al., 2000; Hashimoto, 2002) about two mutants called spir1 and spir2, which are studied mainly for the strong right-handed torsion seen in their shoots, but also for the right-handed torsion seen in the roots. The roots of these mutants, however, as seen from the pictures in their papers, appear to wave as they normally do in the wild type, where the torsion is alternatively to the left and to the right (Okada and Shimura, 1990). This effect is surely present also in the spir mutants or at least a reduction of the right-handed torsion should be seen when the wave is moving toward the side that is considered the right-hand (in which case the torsion is left-handed in the wild type). However, if just the roots are considered, spir1 and spir2 are left-handed mutants, like 1-6C, because they slant to the side that is considered the left-hand (as explained in ).
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.
Key words: Arabidopsis, auxin, circumnutation, gravitropism, microgravity, roots.
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