Sainsbury Laboratory, University of Cambridge, Cambridge, United Kingdom.
PLoS Genet. 2019 Mar 13;15(3):e1008023. doi: 10.1371/journal.pgen.1008023. eCollection 2019 Mar.
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.
植物的茎系统是由胚胎发生过程中建立的初生茎顶端分生组织的作用构建的。在初生分生组织产生的每个叶子的叶腋中，建立了次生腋生茎顶端分生组织。这些腋生分生组织活性的动态调节赋予了茎系统其非凡的形态可塑性。植物适当激活或抑制这些腋生分生组织的能力需要分生组织之间的环境敏感的通信。植物激素生长素的运输网络长期以来一直被认为是这个可调节通信系统的核心参与者，其他系统移动激素，如独脚金内酯和细胞分裂素，部分通过调节生长素运输起作用。直到最近，极性生长素运输流为理解茎中长距离生长素运输提供了一个高电导生长素运输途径，该运输途径主要由生长素输出蛋白 PIN-FORMED1 (PIN1) 主导。然而，最近已经确定了其他在茎中具有重要作用的生长素外排蛋白，包括 PIN3、PIN4 和 PIN7。这些蛋白有助于更广泛的非极性茎生长素运输模式，我们称之为连接性生长素运输（CAT），因为其在茎系统中的通信作用。在这里，我们对 CAT 在茎分枝中的作用进行了遗传分析。我们证明，在拟南芥中，CAT 在独脚金内酯介导的茎分枝控制中发挥重要作用，三重突变体 pin3pin4pin7 能够部分抑制独脚金内酯缺陷突变体的高度分枝表型。相比之下，缺乏表达于腋生分生组织的转录因子 BRANCHED1 (BRC1) 的突变体的分枝表型不受 pin3pin4pin7 的影响。我们进一步证明，类似于 PIN3、PIN4 和 PIN7 在茎中广泛表达的 ABCB19 生长素外排蛋白的突变，对腋芽顶端生长素的加载有非常不同的影响，这表明 ABCB19 参与了生长素的加载。
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