Involved in pollen mitosis II (PMII) regulation during male gametogenesis. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Associated with intracellular membranes and in the tonoplast and endosomes in the germinating pollen tubes. Expressed in the shoot apical meristems (SAM), root tips and inflorescences, and, at low levels, in floral buds and pollen. In young seedlings, expressed in the shoot apical meristem (SAM) and in root tips (PubMed:22897245). In inflorescence, highly observed in the pistil and in seeds, but weakly present during male gametogenesis (PubMed:22897245). Observed at very weak levels from the late bicellular pollen stage to the mature pollen stage (PubMed:22897245). Detected in the germinating pollen tubes (PubMed:22897245). No obvious defects during the vegetative developmental stage (PubMed:22897245). The double mutant lacking both APD1 and APD2 exhibits an increased percentage of bicellular-like pollen at the mature pollen stage (PubMed:22897245). Plants lacking APD1, APD2, APD3 and APD4 are defective for cell division in male gametogenesis resulting in severe abnormal bicellular-like pollen phenotypes (PubMed:22897245).