The growth and gravitropic responses of wild-type and auxinresistant mutants of Arabidopsis thaliana

Growth and gravitropism have been studied in three mutant strains of Arabidopsis thaliana L, that are resistant to auxin-herbicide. Two of the mutations are allelic and recessive ( aux-1 and aux-2 ) and are unlinked to a dominant mutation, Dwf , which confers a very high level of auxin-resistance and is apparently lethal when homozygous. The aux-1 and Dwf strains have altered response to gravity whereas aux-2 appears to be gravitropically normal.
After 96 h in the normal, vertical position only minor differences in elongation were observed between roots of wild-type, aux-1 and aux-2 , but the hypocotyls of aux-1 were significantly retarded compared with the gravitropically normal aux-2 and wild-type. In the progeny of selfed Dwf plants, where both normal ( dwf ) and agravitropic ( Dwf ) seedlings are present, the Dwf seedlings had much longer roots and shorter hypocotyls than dwf ⁺. During 22 h of continuous stimulation the optimum angle for gravitropism in wild-type roots and hypocotyls was 135° (i.e. the organ points obliquely upwards), with decreasing responses in the order 90° and 45°. The agravitropic nature of the roots of aux-1 was confirmed as no significant response was obtained at any of the stimulation angles. In marked contrast, the negative gravitropic response of aux-1 hypocotyls was greater than the wild-type response in terms of the final angle attained at 22 h, but between 6 and 22 h the elongation rate was lower in aux-1 . After varying stimulation periods in the horizontal position, the curvature which had developed, decreased rapidly and almost disappeared during ensuing rotation on clinostats (2 and 4 rpm). Rotation on the clinostats had no effect on the agravitropic behaviour of aux-1 .