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The cell. 5. Vesicular traffic.

IN PLANT CELLS

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Plant and animal cells show a similar vesicular traffic organization, but there are some differences.

Vacuoles are prominent organelles in the vesicular traffic of plants.

Plant cells contain the pre-vacuolar organelles, which are not present in animal cells.

Pre-vacuolar organelles can receive vesicles coming from endoplasmic reticulum and Golgi complex. They are one step before vacuoles.

Endocytosis and exocytosis are less intense in differentiated plant cells than in animal cells.

The basic vesicular traffic map of animal cells is also found in plant cells. However, plants show some distinctive features. Vacuoles are prominent organelles with essential functions for the plant cell, and they communicate with other organelles. Furthermore, endocytosis and exocytosis are not so intense as in the animal cells. Plant cell wall is a diffusion barrier between cells, and direct cytoplasm-cytoplasm communication is frequent through cell wall holes known as plasmodesmata, where plasma membranes from adjoining cells are continuous.

Plant cell vesicular traffic

Main pathways of the vesicular traffic in plant cells (modified from Hawes et al., 1999).

Vacuoles are membrane-bound organelles with different sizes, forms and functions (digestion, storage, maintaining hydrostatic pressure, and many others). There are two main types of vacuoles: lytic and storage. Sometimes they can fuse together to become a large central vacuole. Lytic vacuoles are similar in function to animal cell lysosomes because they are in charge of degradation processes. Storage vacuoles are needed during seed germination and during the responses of plants to certain environment changes.

Endoplasmic reticulum is the organelle for the synthesis of proteins, lipids, and some carbohydrates that eventually enter into the vesicular traffic. Like in animal cells, protein synthesis in plant cells is mediated by a signal peptide in the nascent polypeptide chain and by a translocon located in the endoplasmic reticulum membrane. Quality control of protein synthesis is present in this organelle. Molecules packaged in vesicles travel from the endoplasmic reticulum to the Golgi complex. At the same time, there is recycling of molecules from the Golgi complex to the endoplasmic reticulum mediated by vesicles. In plant cells, this bidirectional communication is peculiar because both organelles are very close to each other in the cytoplasm. So close that some authors suggest that instead of vesicles as transporters, sometimes transient membrane bridges between both organelles allow a direct way for molecules to travel from one organelle to the other. Whatever the communication mechanism, there is no a central Golgi complex in plant cells, but many cisternae stacks are scattered through the cytoplasm.

In plant cells, besides the Golgi complex, the endoplasmic reticulum is able to send vesicles to vacuoles. This vesicular pathway has been suggested because some storage vacuoles may contain a high proportion of non glycosylated proteins. However, endoplasmic reticulum and vacuoles are not connected directly, but by an intermediate organelle known as pre-vacuolar compartment. Furthermore, some experiments suggest that there is a direct vesicular traffic between endoplasmic reticulum and plasma membrane.

Vesicles that move from endoplasmic reticulum to the Golgi complex follow the default pathway. The Golgi complex has a relevant role in plant cells because most of the cell wall carbohydrates, but not cellulose molecules, are synthesized in this organelle. From the Golgi complex, vesicles are targeted to vacuoles, both lytic and storage vacuoles, as well as to plasma membrane. There is an intermediate pre-vacuolar compartment between Golgi complex and vacuoles. Pre-vacuolar compartment sends back vesicles to the Golgi complex, like the multivesicular bodies/late endosomes do in animal cells. In differentiated cells, lytic and storage vacuoles can fuse with each other, affecting the vesicular traffic roads. Proteins targeted to vacuoles, either lytic or storage, must have a signal peptide to be recognized in the TGN of the Golgi complex. However, it is unknown the recognizing motif for proteins incorporated in vesicles toward the plasma membrane. The default pathway for vesicles departing from the Golgi complex is toward the plasma membrane, mainly for keeping the homeostasis of this membrana. This is the main function of endocytosis too. In addition, exocytosis releases much of the cell wall molecules.

Endocytic vesicles fuse with pre-vacuolar compartments, with the Golgi complex, or directly with vacuoles. In plant cells, the endocytic pathway is poorly known and it seems less relevant than in animal cells. However, plant cells show endocytosis and contain all the needed molecular machinery for that. Likewise, endosomal compartment is not well characterized and the recycling path toward plasma membrane is not well known. Some authors suggest that there are at least two types of endosomes: early endosomes, located close to the plasma membrane, and multivesicular bodies. Early endosomes are involved in the recycling of the plasma membrane, whereas multivesicular bodies may function as the intermediate pre-vacuolar compartment in the road toward lytic vacuoles. Plant cells do not have lysosomes, and the degradation processes occur in the lytic vacuoles.

In plant cells, there is a membrane-bound organelle present just during cell division. It is known as phragmoplast. It is the cell structure that gives rise to the two plasma membranes that separate the two new cells during cytokinesis, and it is also in charge of the formation of the middle lamina of the cell wall, located between both plasma membranes. Phragmoplast is formed by the work of cytoskeleton, mainly microtubules, which directs vesicles from the two Golgi complexes, one per new cell, to an intermediate space between the two mitotic nuclei. In this space, vesicles coming from the two Golgi complexes fuse to form the phragmoplast. This is the default pathway for vesicles coming out from the Golgi complex during cell division.

Bibliography

Hawes, C.R., Brandizzi, F., Andreeva, A.V. Endomembranes and vesicle trafficking. 1999. Current opinion in plant biology. 2:254-461.


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Updated: 2016-08-11. 11:55