The activation of Arabidopsis axillary buds involves a switch from slow to rapid committed outgrowth regulated by auxin and strigolactone.

Arabidopsis thaliana (Arabidopsis) shoot architecture is largely determined by the pattern of axillary buds that grow into lateral branches, the regulation of which requires integrating both local and systemic signals. Nodal explants - stem explants each bearing one leaf and its associated axillary bud - are a simplified system to understand the regulation of bud activation. To explore signal integration in bud activation, we characterised the growth dynamics of buds in nodal explants in key mutants and under different treatments. We observed that isolated axillary buds activate in two genetically and physiologically separable phases: a slow-growing lag phase, followed by a switch to rapid outgrowth. Modifying BRANCHED1 expression or the properties of the auxin transport network, including via strigolactone application, changed the length of the lag phase. While most interventions affected only the length of the lag phase, strigolactone treatment and a second bud also affected the rapid growth phase. Our results are consistent with the hypothesis that the slow-growing lag phase corresponds to the time during which buds establish canalised auxin transport out of the bud, after which they enter a rapid growth phase. Our work also hints at a role for auxin transport in influencing the maximum growth rate of branches.

Connective auxin transport contributes to strigolactone-mediated shoot branching control independent of the transcription factor BRC1.

The shoot systems of plants are built by the action of the primary shoot apical meristem, established during embryogenesis. In the axil of each leaf produced by the primary meristem, secondary axillary shoot apical meristems are established. The dynamic regulation of the activity of these axillary meristems gives shoot systems their extraordinary plasticity of form. The ability of plants to activate or repress these axillary meristems appropriately requires communication between meristems that is environmentally sensitive. The transport network of the plant hormone auxin has long been implicated as a central player in this tuneable communication system, with other systemically mobile hormones, such as strigolactone and cytokinin, acting in part by modulating auxin transport. Until recently, the polar auxin transport stream, which provides a high conductance auxin transport route down stems dominated by the auxin export protein PIN-FORMED1 (PIN1), has been the focus for understanding long range auxin transport in the shoot. However, recently additional auxin exporters with important roles in the shoot have been identified, including PIN3, PIN4 and PIN7. These proteins contribute to a wider less polar stem auxin transport regime, which we have termed connective auxin transport (CAT), because of its role in communication across the shoot system. Here we present a genetic analysis of the role of CAT in shoot branching. We demonstrate that in Arabidopsis, CAT plays an important role in strigolactone-mediated shoot branching control, with the triple pin3pin4pin7 mutant able to suppress partially the highly branched phenotype of strigolactone deficient mutants. In contrast, the branchy phenotype of mutants lacking the axillary meristem-expressed transcription factor, BRANCHED1 (BRC1) is unaffected by pin3pin4pin7. We further demonstrate that mutation in the ABCB19 auxin export protein, which like PIN3 PIN4 and PIN7 is widely expressed in stems, has very different effects, implicating ABCB19 in auxin loading at axillary bud apices.

Connective Auxin Transport in the Shoot Facilitates Communication between Shoot Apices.

The bulk polar movement of the plant signaling molecule auxin through the stem is a long-recognized but poorly understood phenomenon. Here we show that the highly polar, high conductance polar auxin transport stream (PATS) is only part of a multimodal auxin transport network in the stem. The dynamics of auxin movement through stems are inconsistent with a single polar transport regime and instead suggest widespread low conductance, less polar auxin transport in the stem, which we term connective auxin transport (CAT). The bidirectional movement of auxin between the PATS and the surrounding tissues, mediated by CAT, can explain the complex auxin transport kinetics we observe. We show that the auxin efflux carriers PIN3, PIN4, and PIN7 are major contributors to this auxin transport connectivity and that their activity is important for communication between shoot apices in the regulation of shoot branching. We propose that the PATS provides a long-range, consolidated stream of information throughout the plant, while CAT acts locally, allowing tissues to modulate and be modulated by information in the PATS.

An INDEHISCENT-Controlled Auxin Response Specifies the Separation Layer in Early Arabidopsis Fruit.

Seed dispersal is an important moment in the life cycle of a plant species. In Arabidopsis thaliana, it is dependent on transcription factor INDEHISCENT (IND)-mediated specification of a separation layer in the dehiscence zone found in the margin between the valves (carpel walls) and the central replum of the developing fruit. It was proposed that IND specifies the separation layer by inducing a local auxin minimum at late stages of fruit development. Here we show that morphological differences between the ind mutant and wild-type fruit already arise at early stages of fruit development, coinciding with strong IND expression in the valve margin. We show that IND-reduced PIN-FORMED3 (PIN3) auxin efflux carrier abundance leads to an increased auxin response in the valve margin during early fruit development, and that the concomitant cell divisions that form the dehiscence zone are lacking in ind mutant fruit. Moreover, IND promoter-driven ectopic expression of the AGC kinases PINOID (PID) and WAG2 induced indehiscence by expelling auxin from the valve margin at stages 14-16 of fruit development through increased PIN3 abundance. Our results show that IND, besides its role at late stages of Arabidopsis fruit development, functions at early stages to facilitate the auxin-triggered cell divisions that form the dehiscence zone.