Fine structure and development of proteoplasts in primary leaves of mung bean

Summary The developing primary leaves of mung bean seedlings contain plastids, called proteoplasts, which are modified for protein storage. The proteoplast has a large protein inclusion with a granular matrix that is bound by a single membrane. Proteoplasts of this type are located in a layer of tissue one cell thick between the palisade and spongy mesophyll cells. The cell layer containing proteoplasts (P layer) differentiates within a few days after seed imbibition. Proteoplast precursors are distinguished by the development of membrane-bound protein sacs within the stroma. The protein sacs coalesce to form a spherical protein body. The P layer is short lived in primary leaves of seedlings grown in light and degeneration of these cells begins soon after proteoplast differentiation. As the cell layer degenerates, proteoplast contents become very electron dense. Within two days, the P layer breaks down and disappears as adjacent cells enlarge and differentiate. In contrast, this specialized cell layer remains intact, with little change in proteoplast fine structure, over a corresponding period in etiolated seedlings.