New methods to analyse auxin-induced growth I: Classical auxinology goes Arabidopsis

During recent years, genetic approaches have become invaluable tools to analyse signalling chains in plants. The first step to understand auxin signalling is the isolation of signal transduction mutants. In Arabidopsis, screening for auxin resistant plants has identified a number of mutants. However, it is difficult to link the mutated genes to the rapid growth response to auxin. As yet, there is no published method to measure auxin-induced growth at a sufficient temporal resolution. A novel auxanometer described here will close that gap. Hypocotyl segments are immersed in buffer in a flow-through cuvette. A CCD-camera attached to a microscope is used to image markings on the hypocotyl surfaces and to track their movements across the field of view.To illustrate the applicability of this method for analysing mutants we compared the growth responses of tomato wild type with the mutant diageotropica (dgt). We showed that the mutation completely abolished all phases of the rapid growth reaction to applied auxin. This excludes the possibility that the reduced auxin sensitivity reported in the literature is due to a reduced or delayed response. Our technique was modified for use with the tiny Arabidopsis thaliana hypocotyl segments and inflorescence stems. In both systems auxin induced a growth response after a lag phase of 15–20 minutes. The new method will now be used to characterise the physiological effect of the mutations in auxin signalling.