Etioplasts are an intermediate type of plastid that develop from proplastids that have not been exposed to light, and convert into

chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...

s upon exposure to light. They are usually found in stem and leaf tissue of

flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Ancient Greek, Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed with ...

s (Angiosperms) grown either in complete darkness, or in extremely low-light conditions.

Etymology

The word " etiolated" (from French word étioler — "straw") was first coined by Erasmus Darwin in 1791 to describe the white and straw-like appearance of dark-grown plants. However, the term "etioplast" did not exist until 1967 when it was invented by John T. O. Kirk and Richard A. E. Tilney-Bassett to distinguish etioplasts from proplastids, their precursors.

Structure

Etioplasts are characterized by the absence of chlorophyll and the presence of a complicated structure called a prolamellar body (PLB). Usually, a single one is present in each etioplast. PLB is composed of symmetrically arranged, tetrahedrally-branched tubules and may contain

ribosome Ribosomes () are molecular machine, macromolecular machines, found within all cell (biology), cells, that perform Translation (biology), biological protein synthesis (messenger RNA translation). Ribosomes link amino acids together in the order s ...

s and plastoglobules inside. The latter are rich with carotenoids, especially lutein and violaxanthin, which may help in transition to chloroplasts. Due to the higher presence of carotenoids than protochlorophyllide, etiolated leaves appear pale yellow instead of just white.

Transition to chloroplast

Every PLB contains protochlorophyllide which is rapidly converted into chlorophyllide by the

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

protochlorophyllide reductase upon exposure to light. Following this, chlorophyllide is converted to

chlorophyll Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words (, "pale green") and (, "leaf"). Chlorophyll allows plants to absorb energy ...

through enzymatic processes. This is stimulated by plant growth hormones:

cytokinin Cytokinins (CK) are a class of plant hormones that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in Cell (biology), cell growth and cellular differentiation, differentiation, but also affect apical ...

s and

gibberellin Gibberellins (GAs) are plant hormones that regulate various Biological process, developmental processes, including Plant stem, stem elongation, germination, dormancy, flowering, flower development, and leaf and fruit senescence. They are one of th ...

s. The structure of PLB itself is almost immediately disrupted, and thylakoid and grana development is started in reaction to light: photosystem I activates within 15 minutes, photosystem II within 2 hours, and after approximately 3 hours an etioplast is completely converted into a functional chloroplast. The transitional stage between an etioplast and a chloroplast which still contains small PLBs interconnected with developing thylakoids, but already has chlorophyll is sometimes called an "etio-chloroplast". Etioplasts were once thought to be laboratory artefacts not found in nature, but that has since been disproven: in cabbage heads, developing inner leaves contain etioplasts due to being shaded by outer leaves; seedlings that naturally germinate underground may also contain etioplasts.

References

Organelles Plant physiology {{cell-biology-stub

Etymology

Structure

Transition to chloroplast

See also

References