Cell polarity is a fundamental feature of all multicellular organisms. In plants, prominent cell polarity markers are PIN auxin transporters crucial for plant development. To identify novel components involved in cell polarity establishment and maintenance, we carried out a forward genetic screening with PIN2:PIN1-HA;pin2 Arabidopsis plants, which ectopically express predominantly basally localized PIN1 in the root epidermal cells leading to agravitropic root growth. From the screen, we identified the regulator of PIN polarity 12 (repp12) mutation, which restored gravitropic root growth and caused PIN1-HA polarity switch from basal to apical side of root epidermal cells. Complementation experiments established the repp12 causative mutation as an amino acid substitution in Aminophospholipid ATPase3 (ALA3), a phospholipid flippase with predicted function in vesicle formation. ala3 T-DNA mutants show defects in many auxin-regulated processes, in asymmetric auxin distribution and in PIN trafficking. Analysis of quintuple and sextuple mutants confirmed a crucial role of ALA proteins in regulating plant development and in PIN trafficking and polarity. Genetic and physical interaction studies revealed that ALA3 functions together with GNOM and BIG3 ARF GEFs. Taken together, our results identified ALA3 flippase as an important interactor and regulator of ARF GEF functioning in PIN polarity, trafficking and auxin-mediated development.

细胞极性是所有多细胞生物的基本特征。在植物中，突出的细胞极性标记是PIN植物生长素转运蛋白，对植物发育至关重要。为了鉴定涉及细胞极性建立和维持的新成分，我们用PIN2：PIN1-HA; pin2拟南芥植物进行了正向基因筛选，该植物在根表皮细胞中异位表达主要在基础上定位于PIN，从而导致引力性根的生长。从屏幕上，我们确定了PIN极性12（repp12）突变的调节剂，该突变体恢复了趋向性根的生长并导致PIN1-HA极性从根表皮细胞的基端向顶端转移。补充实验确定repp12致病突变为氨基磷脂ATPase3（ALA3）中的氨基酸取代，在囊泡形成中具有预测功能的磷脂翻转酶。ala3 T-DNA突变体在许多生长素调节过程，不对称生长素分布和PIN交易中均显示出缺陷。对五倍体和六倍体突变体的分析证实了ALA蛋白在调节植物发育以及PIN转运和极性中的关键作用。遗传和物理相互作用研究表明，ALA3与GNOM和BIG3 ARF GEF一起起作用。两者合计，我们的结果确定ALA3翻转酶是ARF GEF在PIN极性，运输和生长素介导的发育中起作用的重要相互作用者和调节剂。对五倍体和六倍体突变体的分析证实了ALA蛋白在调节植物发育以及PIN转运和极性中的关键作用。遗传和物理相互作用研究表明，ALA3与GNOM和BIG3 ARF GEF一起起作用。两者合计，我们的结果确定ALA3翻转酶是ARF GEF在PIN极性，运输和生长素介导的发育中起作用的重要相互作用者和调节剂。对五倍体和六倍体突变体的分析证实了ALA蛋白在调节植物发育以及PIN转运和极性中的关键作用。遗传和物理相互作用研究表明，ALA3与GNOM和BIG3 ARF GEF一起起作用。两者合计，我们的结果确定ALA3翻转酶是ARF GEF在PIN极性，运输和生长素介导的发育中起作用的重要相互作用者和调节剂。