Vacuoles: features, structure and functions

Features

The eukaryotic cell is composed by numerous organelles and structures that work in synergy to perform the vital functions of the cell and to ensure its interaction with other cells in the tissues. In the plant cell (Fig. 1) there are the vacuoles, special organelles assigned to the reserve, the defense and the metabolism of secondary metabolites and harmful substances.
The name of this organelle comes from the Latin vacuum (vacuum), since in the early years of microscopy they were referred to as empty spaces.

Plant cell, vacuoles
Figure 1 – Representation of a plant cell. [Source: Mariana Ruiz]

Vacuoles can occupy up to 90% of the cell volume of a plant cell. They are in fact well visible under the optical microscope, and contain different types of substances such as ions, sugars and metabolites, or even material imported into the cell by endocytosis. For example, during the ripening of the seeds, large quantities of protein are stored in the vacuoles for the storage of the seeds, previously synthesized at the level of the endoplasmic reticulum. The numerous substances accumulated in the vacuole cause an osmotic effect, which recall water, and this is why the vacuole is a largely swollen structure, and also responsible for the turgor of the cell. The vacuole has also been found in fungi: in particular, it directly participates in the long-distance transport of nutrients through the mycelium, and through autophagy participates in the induction of vital morphogenetic processes.

Structure

The vacuole is bounded by a membrane called tonoplast, in which there are several transport proteins and aquaporins. Through these, the vacuole accumulates reserve proteins, lithic enzymes, repellents and harmful compounds. Some soluble proteins are transported to vacuoles through the Golgi apparatus, while others follow an independent route through the precursor accumulation vesicles (PAC) (Fig. 2). Inside the vacuole, the maturation is completed.

Vacuoles pathway from endoplasmatic reticulum
Figure 2 – Golgi independent pathway in protein storage vacuoles. (a) Electron microscope image of pumpkin seed cell development; (b) Microscopic image of PAC vesicles; (c) Pathway representation [Source: Plant Vacuoles, Annual Review of Plant Biology, 2018]

Some types of vacuoles, called lithic or vegetative, contain lithic enzymes, such as proteinases and nucleases, and defense proteins. These protect against infections by viruses and avirulent bacteria.
Plants also develop specialized vacuoles, such as those in the inner integuments of the seed coating that accumulate flavonoids, which protect the embryo from UV rays. Other substances that can be found are myrosinases, substances repellent towards herbivores, and that therefore defend the cell.

Functions

The functions of the vacuoles are multiple:

  • Accumulation of substances: in the vacuole are accumulated reserve substances, carbohydrates, water, mineral salts, metabolites, protein reserves, pigments (eg. anthocyanins) and fruit aromas.
  • Regulation of the cellular volume: thanks to the water recalled in the vacuoles by osmosis, the plant cell increases in volume, and therefore also increases the leaf surface capable of receiving light and substances useful to photosynthetic activity.
  • Defense against herbivorous predators and pathogens: the vacuole contains real repellents for herbivorous animals, such as the aforementioned myrosinases, but also alkaloids, chitinases and protease inhibitors. Thanks to these substances, the plant also defends itself from potentially pathogenic microorganisms and fungi.
  • Seizure of harmful compounds: the plants, not having an efficient excretory system, use the vacuoles to seize inside the toxic compounds.
  • Digestion: especially in the lithic vacuoles are found acid hydrolases and other enzymes that digest many molecules for a turnover of the components of these. In addition, these enzymes are also responsible for the programmed cell death of the plant cell. Rupture of the tonoplast releases these enzymes and induces hypersensitive cell death.

Original article here – Translated by Luigi Gallucci

Bibliography

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