(A) HOPS is recruited from a cytosolic pool to the vacuole membrane, due, in part, to the presence of the phosphoinositide PI3P. HOPS is responsible for tethering vacuoles together and chaperoning membrane-embedded SNARE proteins into a trans-SNARE fusion complex. Vacuole fusion is mediated by the zippering activity of the SNARE complex. As part of the fusion event, HOPS leaves the membrane. Based on this prior knowledge, one would expect fusion to be impossible in the absence of PI3P. (B) When wortmannin, a PI3K inhibitor, is used to deplete PI3P from the guard cells of closed stoma, small vacuoles rapidly fuse.