SI-induced pollen tubes undergo rapid cytosolic acidification, and artificial alteration of pollen tube [pH]cyt can mimic and prevent SI-induced acidification. A, Pollen tubes labeled with the pH indicator BCECF
AM were treated with either PrsS to provide an incompatible response (SI; n = 152) or a compatible response (compatible; n = 32) or untreated (UT; n = 69). B, Pollen tubes labeled with BCECF
AM were incubated with 50 mm propionic acid (pH 7, n = 57; or pH 5.5, n = 62), pretreated with pH 7 for 10 min before SI induction (pH 7 & SI; n = 29), or untreated (n = 70).