Zeaxanthin epoxidase that plays an important role in the xanthophyll cycle and abscisic acid (ABA) biosynthesis. Converts zeaxanthin into antheraxanthin and subsequently violaxanthin. Required for resistance to osmotic and drought stresses, ABA-dependent stomatal closure, seed development and dormancy, modulation of defense gene expression and disease resistance and non-photochemical quencing (NPQ). Through its role in ABA biosynthesis, regulates the expression of stress-responsive genes such as RD29A during osmotic stress and is required for normal plant growth during vegetative development. Is required for late skotomorphogenic growth through its role in the xanthophyll carotenoids neoxanthin, violaxanthin and antheraxanthin biosynthesis. Required for beta-aminobutyric acid (BABA)-induced priming in disease resistance, tolerance to salt and drought stresses and sterility. Participates in NPQ by regulating the level of zeaxanthin in photosynthetic energy conversion. NPQ is a process that maintains the balance between dissipation and utilization of light energy to minimize the generation of oxidizing molecules and the molecular damages they can generate. all-trans-zeaxanthin + 4 H(+) + 2 O2 + 4 reduced [2Fe-2S]-[ferredoxin] = all-trans-violaxanthin + 2 H2O + 4 oxidized [2Fe-2S]-[ferredoxin] Plant hormone biosynthesis; abscisate biosynthesis. Expressed in leaves, stems and flowers, and at lower levels in roots and siliques. By ABA and drought, salt and osmotic stresses. Increased endogenous level of zeaxanthin and reduced level of ABA. Reduced size of leaves, inflorescences and flowers, early flowering, increased number of wilted plants, premature seed germination and reduced osmotic water permeability and ability to close stomata. Reduced susceptibility to virulent isolates of P.parasitica and sensitivity to BABA-induced priming. Plants overexpressing ZEP show increased levels of violaxanthin and ABA and increased tolerance to salt and drought stresses. Intron retention. Intron retention.