Essential protein. Required during embryo development, especially at early stages. Involved in chloroplast differentiation. Interacts with NAP6. Present in all organs, with higher levels in aerial parts. During seed development, first expressed in developing endosperm, embryo and suspensor and later restricted to the embryo. In seedlings, confined to apical meristems and vascular tissues. In flowers, mostly present in floral meristem, in filaments of the stamen and in the style. Belongs to the ABC transporter superfamily. ABCI family.