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.