Data from: Auxin steers root cell expansion via apoplastic pH regulation in Arabidopsis thaliana

Plant cells are embedded within cell walls, which provide structural integrity, but also spatially constrain cells, and must therefore be modified to allow cellular expansion. The long-standing acid growth theory postulates that auxin triggers apoplast acidification, thereby activating cell wall-loosening enzymes that enable cell expansion in shoots. Interestingly, this model remains heavily debated in roots, because of both the complex role of auxin in plant development as well as technical limitations in investigating apoplastic pH at cellular resolution. Here, we introduce 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) as a suitable fluorescent pH indicator for assessing apoplastic pH, and thus acid growth, at a cellular resolution in Arabidopsis thaliana roots. Using HPTS, we demonstrate that cell wall acidification triggers cellular expansion, which is correlated with a preceding increase of auxin signaling. Reduction in auxin levels, perception, or signaling abolishes both the extracellular acidification and cellular expansion. These findings jointly suggest that endogenous auxin controls apoplastic acidification and the onset of cellular elongation in roots. In contrast, an endogenous or exogenous increase in auxin levels induces a transient alkalinization of the extracellular matrix, reducing cellular elongation. The receptor-like kinase FERONIA is required for this physiological process, which affects cellular root expansion during the gravitropic response. These findings pinpoint a complex, presumably concentration-dependent role for auxin in apoplastic pH regulation, steering the rate of root cell expansion and gravitropic response.

植物细胞包埋于细胞壁之中，细胞壁不仅为细胞提供结构完整性，同时也会在空间上限制细胞，因此必须对其进行修饰以允许细胞扩张。长期以来的酸生长理论假定，生长素会触发质外体酸化，进而激活细胞壁松弛酶，使地上部的细胞得以扩张。有趣的是，该模型在根系中仍存在广泛争议，这既源于生长素在植物发育中复杂的调控作用，也受制于在细胞分辨率下研究质外体pH的技术局限性。本研究引入8-羟基芘-1,3,6-三磺酸三钠盐（8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt，HPTS）作为一种适用于拟南芥（Arabidopsis thaliana）根系细胞分辨率下质外体pH（进而反映酸生长）检测的荧光pH指示剂。借助HPTS，我们证实细胞壁酸化可触发细胞扩张，这一过程与生长素信号的先期上调存在关联。降低生长素水平、阻断生长素感知或抑制生长素信号，均可同时消除胞外酸化与细胞扩张现象。上述研究结果共同表明，内源生长素可调控根系中的质外体酸化与细胞伸长起始过程。与此相反，内源或外源生长素水平升高会诱导胞外基质发生瞬时碱化，进而抑制细胞伸长。类受体激酶FERONIA是该生理过程的必需组分，其参与调控向重力性响应过程中的根系细胞扩张。上述研究结果明确揭示了生长素在质外体pH调控中具有复杂且大概率为浓度依赖性的作用，该作用可调控根系细胞扩张速率与向重力性响应。