• Osmotic stress influences root system architecture, and polar auxin transport (PAT) is well established to regulate root growth and development. However, how PAT responds to osmotic stress at the molecular level remains poorly understood. In this study, we explored whether and how the auxin efflux carrier PIN-FORMED3 (PIN3) participates in osmotic stress-induced root growth inhibition in Arabidopsis (Arabidopsis thaliana).
• We observed that osmotic stress induces a HD-ZIP II transcription factor-encoding gene HOMEODOMAIN ARABIDOPSIS THALIANA2 (HAT2) expression in roots. The hat2 loss-of-function mutant is less sensitive to osmotic stress in terms of root meristem growth. Consistent with this phenotype, whereas the auxin response is downregulated in wild-type roots under osmotic stress, the inhibition of auxin response by osmotic stress was alleviated in hat2 roots. Conversely, transgenic lines overexpressing HAT2 (Pro35S::HAT2) had shorter roots and reduced auxin accumulation compared with wild-type plants.
• PIN3 expression was significantly reduced in the Pro35S::HAT2 lines. We determined that osmotic stress-mediated repression of PIN3 was alleviated in the hat2 mutant because HAT2 normally binds to the promoter of PIN3 and inhibits its expression.
• Taken together, our data revealed that osmotic stress inhibits root growth via HAT2, which regulates auxin activity by directly repressing PIN3 transcription.

中文翻译：

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渗透胁迫通过调节 PIN-FORMED3 的转录调控来抑制根系生长

• 渗透胁迫影响根系结构，极性生长素转运 (PAT) 已很好地调节根系生长和发育。然而，PAT 如何在分子水平上响应渗透压力仍然知之甚少。在本研究中，我们探讨了生长素外排载体 PIN-FORMED3 (PIN3) 是否以及如何参与渗透胁迫诱导的拟南芥 ( Arabidopsis thaliana ) 根系生长抑制。
• 我们观察到渗透胁迫在根中诱导 HD-ZIP II 转录因子编码基因HOMEODOMAIN ARABIDOPSIS THALIANA2 ( HAT2 ) 表达。hat2功能丧失突变体在根分生组织生长方面对渗透胁迫不太敏感。与这种表型一致，而在渗透胁迫下野生型根中的生长素反应被下调，而在hat2 根中，渗透胁迫对生长素反应的抑制作用得到了缓解。相反，与野生型植物相比，过表达HAT2 ( Pro35S::HAT2 ) 的转基因品系具有较短的根和减少的生长素积累。
• Pro35S::HAT2系中的PIN3表达显着降低。我们确定渗透应激介导的PIN3抑制在hat2突变体中得到缓解，因为 HAT2 通常与PIN3的启动子结合并抑制其表达。
• 总之，我们的数据显示渗透胁迫通过 HAT2 抑制根生长，HAT2 通过直接抑制PIN3转录来调节生长素活性。