Abstract
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.
Highlights
The co-ordinated regulation of plant growth requires effective communication across the plant body, prioritising growth where it is needed
The plant shoot system can be considered as a population of communicating growing tips, each driven by a shoot apical meristem
We analyse the role of transporters for the plant hormone auxin in this communication network and its ability to make meristem activity decisions
Summary
The co-ordinated regulation of plant growth requires effective communication across the plant body, prioritising growth where it is needed. Application of auxin to the decapitated stump of the primary shoot is able to restore axillary bud inhibition, even in axillary buds located far from the shoot tip [1], suggesting long-range auxin-mediated communication between shoot apices. Consistent with this idea, the young expanding leaves of growing shoots are known to be major sites for auxin biosynthesis [2] and this auxin is exported into the stem and transported towards the root in the polar auxin transport stream (PATS) (reviewed in [3]). Mutation in PIN1 strongly reduces stem auxin transport, demonstrating an important role for PIN1 in the PATS [5]
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