This article provides information about the Origin and Development of Plastids!
Origin and Development of Plastids:
Recent studies state that all plastids arise always from pre-existing minute sub-microscopic amoeboid plastids called as proplastids.
The proplastid is considered as stem plastid which gives rise to either leucoplast or immature lamellar plastids, the later may form any type of plastids.
The proplastids are spherical bodies of 0.5µ diameter bounded by double membranes enclosing the dense stroma.
When light is available and the proplastid reaches a diameter of 1 µm, its inner membrane invaginates to form vesicles into the matrix or stroma, which arrange themselves parallelly in the stroma and later these vesicles fuse to form discs or lamellae.
These intrachloroplastic membranes are the thylakoids which, in certain region, pile closely to form the grana few thylakoids remain connected with each other by the tubules or stromal lamellae. In the mature chloroplast the thylakoids are no longer connected to the inner membrane, but the grana remain united by intergranal thylakoids.
In darkness, however, the lamellae breakup into vesicles. If a plant is kept under low light intensity, the reverse sequence of changes takes place. This process is called etiolation, and results in the disorganization of the membranes.
The same phenomenon occurs if the plant is grown from the very beginning in low light intensity. In this case the vesicles of the proplastid aggregate to form one or more prolamellar bodies, which can develop into grana if the plant is again exposed to light.
The plastids never arise denovo (afresh). In Monocotyledons, some of the mature plastids called, elaioplasts, develop from old chloroplasts. In carrot root chromoplasts become developed from amyloplasts.
In algae and ferns the mature plastids give rise to new plastids by division. New plastids also develop by budding from the mature plastids but it occurs only in abnormal conditions, i.e., regeneration of plant from dissected leaves.