Provide a mechanistic link between auxin and plasma membrane H(+)-ATPases (PM H(+)-ATPases, e.g. AHA1 and AHA2), and triggers PM H(+)-ATPases activity by promoting phosphorylation of their C-terminal autoinhibitory domain as a result of PP2C-D subfamily of type 2C phosphatases inhibition, thus leading to the acidification of the apoplast and the facilitation of solutes and water uptake to drive cell expansion (PubMed:24858935, PubMed:27999086). Prevents the apical hook maintenance of etiolated seedlings (PubMed:24858935). Functions as positive effectors of cell expansion through modulation of auxin transport (PubMed:22348445, PubMed:27999086). Interacts with and inhibits PP2C-D subfamily of type 2C phosphatases such as PP2C67/PP2C-D1, PP2C64/PP2C-D5 and PP2C46/PP2C-D6. By auxin (PubMed:22348445, PubMed:27999086). Triggered by brassinosteroids, including brassinolide (BL) (PubMed:30649552). Seedlings over-expressing SAUR19 display auxin-related phenotypes. These phenotypes include increased cell expansion, increased hypocotyl and leaf size, defective apical hook maintenance, altered tropic responses and increased basipetal auxin transport in hypocotyls. Belongs to the ARG7 family.