Etioplasts

Etioplasts are plastids that form in plant tissues grown in darkness and represent an intermediate stage in chloroplast development, occurring prominently in etiolated seedlings and other dark-grown tissues. In contrast to chloroplasts, etioplasts lack organized thylakoid membranes and photosynthetic activity, and they accumulate pigments in a distinct internal structure known as the prolamellar body.

The prolamellar body is a latticed, tubular-vesicular membrane system located inside the etioplast. It contains high levels of protochlorophyllide, a light-dependent pigment, and little to no chlorophyll. Carotenoids are present as well, but the overall photosynthetic pigment content remains low. This specialized membrane arrangement is a characteristic feature of etioplasts and reflects their readiness to rapidly transition to photosynthetic chloroplasts upon light exposure.

Light triggers the conversion of etioplasts into chloroplasts through a light-dependent pathway. Protochlorophyllide is reduced to chlorophyllide by protochlorophyllide oxidoreductase (POR), an enzyme with multiple isoforms (PORA, PORB, PORC) that operate in the plastid. Subsequent steps rapidly assemble thylakoid membranes and synthesize chlorophyll a, transforming the etioplast into a mature chloroplast with grana and stroma lamellae.

Etioplasts occur broadly in dark-grown tissues across many plant species and have long served as a model system for studying plastid development and the chloroplast biogenesis program. They illustrate how light governs organelle maturation and the plant’s readiness to initiate photosynthesis once exposed to illumination.