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. Expressed in the shoot apical meristems (SAM), root tips, pollen and inflorescences. In young seedlings, expressed in the shoot apical meristem (SAM) and in root tips (PubMed:22897245). In inflorescence, observed only in young floral buds and seeds (PubMed:22897245). Accumulates at the late bicellular pollen stage and levels gradually strengthened from the tricellular pollen stage to 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.