SLOW MOTION Is Required for Within-Plant Auxin Homeostasis and Normal Timing of Lateral Organ Initiation at the Shoot Meristem in Arabidopsis

The regular arrangement of leaves and flowers around a plant''s stem is a fascinating expression of biological pattern formation. Based on current models, the spacing of lateral shoot organs is determined by transient local auxin maxima generated by polar auxin transport, with existing primordia draining auxin from their vicinity to restrict organ formation close by. It is unclear whether this mechanism encodes not only spatial information but also temporal information about the plastochron (i.e., the interval between the formation of successive primordia). Here, we identify the Arabidopsis thaliana F-box protein SLOW MOTION (SLOMO) as being required for a normal plastochron. SLOMO interacts genetically with components of polar auxin transport, and mutant shoot apices contain less free auxin. However, this reduced auxin level at the shoot apex is not due to increased polar auxin transport down the stem, suggesting that it results from reduced synthesis. Independently reducing the free auxin level in plants causes a similar lengthening of the plastochron as seen in slomo mutants, suggesting that the reduced auxin level in slomo mutant shoot apices delays the establishment of the next auxin maximum. SLOMO acts independently of other plastochron regulators, such as ALTERED MERISTEM PROGRAM1 or KLUH/CYP78A5. We propose that SLOMO contributes to auxin homeostasis in the shoot meristem, thus ensuring a normal rate of the formation of auxin maxima and organ initiation.