8 julio, 2024

Amyloplasts: what they are, characteristics, functions, structure

What are amyloplasts?

The amyloplasts They are a type of plastid specialized in starch storage and are found in high proportions in non-photosynthetic reserve tissues, such as the endosperm of seeds and tubers. They are unique to plant cells.

Since the complete synthesis of starch is restricted to plastids, there must be a physical structure that serves as a storage site for this polymer. In fact, all the starch contained in plant cells is found in organelles covered by a double membrane.

In general, plastids are semi-autonomous organelles found in different organisms, from plants and algae to marine mollusks and some parasitic protists.

Plastids participate in photosynthesis, in the synthesis of lipids and amino acids, lack chlorophyll, function as a lipid reserve site, are responsible for the coloration of fruits and flowers, and are related to the perception of the environment.

Similarly, amyloplasts participate in the perception of gravity and store key enzymes of some metabolic pathways.

Characteristics and structure of amyloplasts

Amyloplasts are cell organelles present in plants, they are a reserve source of starch and do not have pigments –such as chlorophyll–, so they are colorless.

Like other plastids, amyloplasts have their own genome, which codes for some of the proteins in their structure. This characteristic is a reflection of its endosymbiotic origin.

One of the most outstanding characteristics of plastids is their ability to interconvert. Specifically, amyloplasts can become chloroplasts, so when the roots are exposed to light they acquire a greenish hue, thanks to the synthesis of chlorophyll.

Chloroplasts can behave in a similar way, as they temporarily store starch grains inside. However, in amyloplasts the reserve is long-term.

Its structure is very simple, it consists of a double external membrane that separates them from the rest of the cytoplasmic components. Mature amyloplasts develop an internal membranous system where starch is found.

Amyloplast formation

Most amyloplasts form directly from protoplastids when reserve tissues are developing and divide by binary fission.

In the early stages of endosperm development, proplastids are present in a coenocytic endosperm. Next, the cellularization processes begin, where the proplastids begin to accumulate starch granules, thus forming amyloplasts.

From a physiological point of view, the differentiation process of proplastids to give rise to amyloplasts occurs when the plant hormone auxin is replaced by cytokinin, which reduces the speed at which cell division occurs, inducing accumulation of the starch.

Functions of amyloplasts

starch storage

Starch is a complex polymer with a semi-crystalline and insoluble appearance, product of the union of D-glucopyranoses through glycosidic bonds. Two starch molecules can be distinguished: amylopectin and amylose. The first is highly branched, while the second is linear.

The polymer is deposited in the form of oval grains in spherocrystals and, depending on the region where the grains are deposited, they can be classified as concentric or eccentric grains.

Starch granules can vary in size, some approaching 45 um, and others smaller, around 10 um.

starch synthesis

The plastids are responsible for the synthesis of two types of starch: the transient, which is produced during daylight hours and temporarily stored in the chloroplasts until night, and the reserve starch, which is synthesized and stored in the chloroplasts. amyloplasts of stems, seeds, fruits and other structures.

There are differences between the starch granules present in amyloplasts with respect to the grains that are transiently found in chloroplasts. In the latter, the amylose content is lower and the starch is arranged in structures similar to plates.

perception of gravity

Starch grains are much denser than water and this property is related to the perception of gravitational force. In the course of plant evolution, this ability of amyloplasts to move under the influence of gravity was exploited for the perception of said force.

Briefly, amyloplasts react to the stimulation of gravity by sedimentation processes in the direction in which this force acts, downwards. When the plastids come into contact with the plant cytoskeleton, it sends a series of signals so that growth occurs in the proper direction.

In addition to the cytoskeleton, there are other structures in cells, such as vacuoles, the endoplasmic reticulum, and the plasma membrane, that are involved in the uptake of sedimenting amyloplasts.

In root cells, the sensation of gravity is picked up by columella cells, which contain a specialized type of amyloplasts called statoliths.

The statoliths fall by the force of gravity to the bottom of the columella cells and initiate a signal transduction pathway where the growth hormone, auxin, is redistributed and causes differential downward growth.

metabolic pathways

Previously it was thought that the function of amyloplasts was restricted exclusively to the accumulation of starch.

However, recent analysis of the protein and biochemical composition of the interior of this organelle has revealed a molecular machinery quite similar to that of the chloroplast, which is complex enough to be able to carry out the photosynthetic processes typical of plants.

The amyloplasts of some species (such as alfalfa, for example) contain the necessary enzymes for the GS-GOGAT cycle to occur, a metabolic pathway that is closely related to nitrogen assimilation.

The name of the cycle comes from the initials of the enzymes involved in it, glutamine synthetase (GS) and glutamate synthase (GOGAT). It involves the formation of glutamine from ammonium and glutamate, and the synthesis of glutamine and ketoglutaramate from two glutamate molecules.

One is incorporated into the ammonium and the remaining molecule is carried to the xylem to be used by the cells. In addition, chloroplasts and amyloplasts have the ability to provide substrates for the glycolytic pathway.

References

Cooper GM (2000). The Cell: A Molecular Approach. 2nd edition. Sinauer Associates. Chloroplasts and Other Plastids. Available at: ncbi.nlm.nih.gov
Grajales, O. (2005). Plant Biochemistry Notes. Bases For Its Physiological Application. UNAM.
Pyke, K. (2009). Plastid biology. Cambridge University Press.
Raven, PH, Evert, RF, & Eichhorn, SE (1992). plant biology (Vol. 2). I reversed.
Rose, R.J. (2016). Molecular Cell Biology of the Growth and Differentiation of Plant Cells. CRC Press.
Taiz, L., & Zeiger, E. (2007). plant physiology. Jaume I University

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