Auxin Signaling-Mediated Apoplastic pH Modification Functions in Petal Conical Cell Shaping

The flowers of angiosperm species typically contain specialized conical cells. Although substantial progress has been achieved regarding the mechanisms underlying flower development, little is known about how petal cells achieve final conical shape. Here, we use 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) as a fluorescent pH indicator for analyzing the apoplastic pH of conical cells in Arabidopsis and show that normal conical cell expansion requires auxin signaling and apoplastic pH changes. By combining imaging analysis and genetic and pharmacological experiments, we demonstrate that apoplastic acidification and alkalization correlate with an increase and decrease in tip sharpening of conical cells, respectively. Initial expansion of conical cells is accompanied by decreased apoplastic pH, which is associated with increased auxin signaling. Decreased auxin levels, transport, or signaling abolishes cell wall acidification and causes reduced tip sharpening and heights of conical cells. These findings provide an insight into apoplastic pH regulation of conical cell expansion.