plastid
The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the Cell (biology), cells of plants, algae, and some other eukaryotic organisms. They are considered to be intracellular endosy ...
that develop from
proplastid
The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cells of plants, algae, and some other eukaryotic organisms. They are considered to be intracellular endosymbiotic cyanob ...
s that have not been exposed to light, and convert into
chloroplast
A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...
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 (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants th ...
s (Angiosperms) grown either in complete darkness, or in extremely low-light conditions.
Etymology
The word "
etiolate
Etiolation is a process in flowering plants grown in partial or complete absence of light. It is characterized by long, weak stems; smaller leaves due to longer internodes; and a pale yellow color (chlorosis). The development of seedlings in t ...
d" (from French word étioler — "straw") was first coined by
Erasmus Darwin
Erasmus Robert Darwin (12 December 173118 April 1802) was an English physician. One of the key thinkers of the Midlands Enlightenment, he was also a natural philosopher, physiologist, slave-trade abolitionist, inventor, and poet.
His poems ...
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
tubule
In biology, a tubule is a general term referring to small tube or similar type of structure. Specifically, tubule can refer to:
* a small tube or fistular structure
* a minute tube lined with glandular epithelium
* any hollow cylindrical body stru ...
s and may contain
ribosome
Ribosomes ( ) are macromolecular machines, found within all cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to ...
s and plastoglobules inside. The latter are rich with
carotenoids
Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, ...
violaxanthin
Violaxanthin is a xanthophyll pigment with an orange color found in a variety of plants. Violaxanthin is the product of the epoxidation of zeaxanthin where the oxygen atoms are from reactive oxygen species (ROS). Such ROS's arise when a plant ...
, 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
Protochlorophyllide,KEGG compound database entr/ref> or monovinyl protochlorophyllide, is an intermediate in the biosynthesis of chlorophyll ''a''. It lacks the phytol side-chain of chlorophyll and the reduced pyrrole in ring D. Protochloroph ...
which is rapidly converted into
chlorophyllide
Chlorophyllide ''a'' and Chlorophyllide ''b'' are the biosynthetic precursors of chlorophyll ''a'' and chlorophyll ''b'' respectively. Their propionic acid groups are converted to phytyl esters by the enzyme chlorophyll synthase in the final st ...
by the
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
protochlorophyllide reductase
In enzymology, protochlorophyllide reductases (POR) are enzymes that catalyze the conversion from protochlorophyllide to chlorophyllide ''a''. They are oxidoreductases participating in the biosynthetic pathway to chlorophylls.
There are t ...
upon exposure to light. Following this, chlorophyllide is converted to
chlorophyll
Chlorophyll (also chlorophyl) 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 allow plants to a ...
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 growth and differentiation, but also affect apical dominance, axillary bud growth, and lea ...
s and
gibberellin
Gibberellins (GAs) are plant hormones that regulate various developmental processes, including stem elongation, germination, dormancy, flowering, flower development, and leaf and fruit senescence. GAs are one of the longest-known classes of p ...
s. The structure of PLB itself is almost immediately disrupted, and
thylakoid
Thylakoids are membrane-bound compartments inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thyl ...
and grana development is started in reaction to light:
photosystem I
Photosystem I (PSI, or plastocyanin–ferredoxin oxidoreductase) is one of two photosystems in the photosynthetic light reactions of algae, plants, and cyanobacteria. Photosystem I is an integral membrane protein complex that uses ...
activates within 15 minutes,
photosystem II
Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. Within the photosystem ...
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.
See also
*
Plastid
The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the Cell (biology), cells of plants, algae, and some other eukaryotic organisms. They are considered to be intracellular endosy ...
**
Chloroplast
A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...
**
Chromoplast
Chromoplasts are plastids, heterogeneous organelles responsible for pigment synthesis and storage in specific photosynthetic eukaryotes. It is thought that like all other plastids including chloroplasts and leucoplasts they are descended from sym ...
**
Leucoplast
Leucoplasts (λευκός leukós "white", πλαστός plastós "formed, molded") are a category of plastid and as such are organelles found in plant cells. They are non-pigmented, in contrast to other plastids such as the chloroplast.
Lacki ...
***
Amyloplast
Amyloplasts are a type of plastid, double-enveloped organelles in plant cells that are involved in various biological pathways. Amyloplasts are specifically a type of leucoplast, a subcategory for colorless, non-pigment-containing plastids. Amylop ...
***
Elaioplast
Elaioplasts are one of the three possible forms of leucoplasts, sometimes broadly referred to as such. The main function of elaioplasts are synthesis and storage of fatty acids, terpenes, and other lipids, and they can be found in the embryonic le ...
***
Proteinoplast
Proteinoplasts (sometimes called ''proteoplasts'', ''aleuroplasts'', and ''aleuronaplasts'') are specialized organelles found only in plant cells. Proteinoplasts belong to a broad category of organelles known as plastids. Plastids are special ...
**
Gerontoplast A gerontoplast is a plastid that develops from a chloroplast during the senescing of plant foliage. Gerontoplast development is generally seen to be the process of grana being unstacked, loss of thylakoid membranes, and large accumulation of pl ...