Auxins represent a major group of phytohormones controlling plant development. The spatio-temporal regulation of auxin gradients is essential for the initiation, growth and correct development of plant organs. Because auxins and their metabolites occur at trace levels in plant tissue, experiments requiring identification plus their selective and specific quantification can be most conveniently achieved using mass spectrometry (MS) and the associated chromatographic methods. With the advent of appropriate liquid-based ionisation techniques, emphasis has moved from the use of gas chromatography as the sample interface to the MS (GC/MS), with its concomitant need for derivatisation, to the more sensitive liquid chromatography tandem mass spectrometry (LC-MS/MS). We describe an optimized liquid chromatography electrospray-ionisation quadrupole time-of-flight (LC-ESI-QTOF) methodology for the quantification of auxins. While the solvent extraction of young Medicago truncatula (M. truncatula) roots, as described herein, is relatively straightforward, older, woody or oily plant tissues may also be analyzed with appropriate modification to remove interferences and/or enhance extraction efficiency. In our hands, the analytical assay has proved sufficiently sensitive for the quantification of auxins to investigate their roles in various organogenic events, such as root nodulation in M. truncatula. Further increases in sensitivity can be expected with the use of the latest generation of instruments.
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