20 julio, 2024

Meristem tissue: what it is, characteristics, functions, histology and types

What is meristem tissue?

He meristem tissue or meristematic tissue is the plant tissue specialized in cell division and plant growth. From the activity of these tissues, all the adult tissues of the plant originate. It is located in the root and in the stem (aerial part of the plant).

They have been classified using various criteria: the moment of appearance in the development of the plant, and by the position they occupy in the body of the plant.

The meristems are also known as buds. These tissues are active throughout the life of the plant, and are responsible for the indeterminate and permanent growth that these organisms present.

Characteristics of the meristematic tissue

Origin

– All plants originate from the activity of a single cell (zygote). When the embryo differentiates, there are certain areas that maintain the ability to divide.

– In the radicle of the embryo, the cells in the subapical position continue to divide. While in the stem, it constitutes the first bud (meristematic zone) of the plant.

– In some cases, already differentiated cells of the plant body can divide again. This occurs when some damage is generated to the plant or by endogenous control.

Cytology

– Meristematic cells are not differentiated. Their size ranges from 10-20 µm and they are isodiametric (with all their sides equal).

– They have a thin primary cell wall made up of pectins, hemicellulose and cellulose.

– The nucleus occupies the largest cell volume, up to 50% of the cytoplasm.

– There is a great abundance of ribosomes. There are also numerous dictyosomes forming the Golgi apparatus. The endoplasmic reticulum is sparse.

– There are many small vacuoles, scattered throughout the cytosol.

– The plastids are not differentiated, so they are called proplastids.

– Mitochondria are very small with few mitochondrial cristae.

hormonal activity

– The activity of the meristematic cells is regulated by the presence of hormones produced by the plant, which regulate the activity of the tissues.

– The hormones that are most directly involved in the activity of meristems are auxins and gibberellins.

– Auxins stimulate the formation and growth of roots. In high concentrations they can inhibit division in the shoot meristems.

– Gibberellins are capable of stimulating cell division in latent meristems. These buds have generally stopped their growth due to the influence of environmental factors. The displacement of gibberellins to these zones breaks the dormancy and the meristem begins its activity.

Functions of the meristematic tissue

– Its function is the formation of new cells. This tissue is in constant mitotic division and will originate all the adult tissues of the plant.

– It is responsible for the growth in length and thickness of stems and roots, and determines the pattern of development of the organs of the plant.

Histology

The meristems located at the apex of the shoot and root tend to be conical in shape. Its diameter is between 80-150 µm.

In the stem, this tissue is located at the apical end. In the root, the meristematic cells are located just above the calyptra, which protects them.

The shoot and root meristems have a particular histological organization, which determines the type and position of the adult tissues to which they will give rise.

Shoot apical meristem (MAV)

The meristematic zone in the aerial part of the plant forms the buds. In the most apical part of the meristem, the less differentiated cells are located. It is known as a promeristem and has a particular configuration.

Two levels of organization can be recognized. At the first level, the ability of cells to divide and the position they occupy in the promeristem are taken into account. Three zones are presented:

Central zone

It is made up of elongated and highly vacuolated cells. These cells have a low rate of division compared to other areas of the promeristem. They are pluripotent: they have the ability to originate any tissue.

peripheral zone

It is located surrounding the cells of the central zone. Cells are small and highly stained. They divide frequently.

Medullary or rib area

It appears just below the central area. The cells are vacuolated and arranged in columns. They make up the pith of the meristem and give rise to the greatest amount of stem tissue.

When cells in the central zone divide, part of the daughter cells moves to the sides. These will form the peripheral zone and will give rise to the leaf primordia.

The cells that form towards the bottom of the central zone, are incorporated into the medullary zone.

At the second level of organization of the AVM, the configuration and division planes of the cells are taken into account. It is known as the tunic-body configuration.

Tunica

It is formed by the two outermost layers of the AVM (L1 and L2). They divide only in the anticlinal plane (perpendicular to the surface).

The outermost layer of the tunic gives rise to the protodermis. This primary meristem will differentiate into the epidermal tissue. The second layer participates in the formation of the fundamental meristem.

Body

It is located below the tunic. It is made up of several layers of cells (L3). They divide in both anticline and periclinal (parallel to the surface) directions.

From the cells of the body the fundamental meristem and the procambium are formed. The first will form the tissues of the cortex and pith of the stem, and the tissues of the mesophyll in the leaf. The procambium will form the primary vascular tissues.

Subapical meristem of the root (MSR)

The meristematic zone of the root has a subapical position, because it is protected by the calyptra, which prevents cell damage when penetrating the soil.

The organization of the MSR is much simpler than that of the MAV. Below the calyptra, there is a group of cells with a low rate of division. These form the quiescent center, which is considered a reserve center for meristematic cells.

Lateral to the quiescent center, one to several layers of initial cells appear, depending on the group of plants.

The outermost layer of initial cells will give rise to the cells of the calyptra. It will also form the protodermis. The innermost layers divide to form the ground meristem and the procambium.

Types of meristematic tissue

The most used criteria to classify the meristems are the position and time of appearance on the plant.

by position

Depending on where they are located in the plant, we have:

apical

They are located in the terminal zones of the roots and stems. In the stem they are in an apical position and in the terminal part of the branches. They form the apical and lateral buds.

In each root there is only one meristem, in a subapical position.

Laterals

They occupy a peripheral position to roots and stems of gymnosperms and dicotyledons. They are responsible for the growth in thickness.

interlayers

They are located on the stem. They are remote from the apical meristems and interspersed with adult tissues. They are responsible for increasing the length of internodes and leaf sheaths in some plants.

adventitious

They are formed in different positions in the body of the plant. It happens in response to various stimuli. Adult cells are able to resume meristematic activity.

Among these are the healing meristems, originated when mechanical damage is caused. There are also meristemoids, isolated meristematic cells that can give rise to structures such as stomata or trichomes.

By time of appearance

In all plants there is a primary growth, which basically consists of the growth in length of the stem and roots. The tissues that are formed originate from the so-called primary meristems.

In some groups, such as gymnosperms and many dicots, secondary growth occurs, which is the increase in diameter of stems and roots.

It occurs mainly on trees and shrubs. The tissues that cause this growth are formed by the activity of secondary meristems.

primaries

They are the protodermis, the fundamental meristem and the procambium.

The protodermis forms the epidermal tissues in roots and shoots. It is always located in the outermost part of the plant.

From the activity of the fundamental meristem, the different types of parenchyma originate. Likewise, mechanical tissues (collenchyma and sclerenchyma) are formed.

On the stem it is located outside and inside the procambium. The internal tissues will form the medulla, and the external ones the primary cortex. In the root it is between the protodermis and the procambium. The tissues that it originates constitute the radical cortex.

The procambium forms the primary vascular tissues (xylem and primary phloem). The cells of this meristem are elongated and vacuolated. In the stem it has a lateral position, while in the roots it occurs in the center of the organ.

secondary

They are the phellogen, or suberous cambium, and the vascular cambium.

The phellogen is formed by the dedifferentiation of adult stem or root cells. In the stem it can originate from any tissue of the primary bark. In the root it is formed from the activity of the pericycle.

This meristem forms, towards the external part of the organ, the suber or cork. Towards the internal part the phelodermis originates. The set of suber, phellogen and phelodermis constitutes the secondary cortex.

Secondary vascular tissues are formed from the division of the vascular cambium. This meristem originates from procambium remains that remain dormant in stems and roots.

Likewise, in the roots the pericycle also participates in its formation. In the stem, parenchyma cells may participate in the formation of the vascular cambium.

The meristem originates, towards the outside, secondary phloem, and internally, secondary xylem. In all cases, more secondary xylem, or wood, is formed.

References

Fletcher, J. (2002). Shoot and floral meristem maintenance in Arabidopsis.
Risopatron, JPM, Y. Sun and BJ Jones (2010). The vascular cambium: molecular control of cellular structure. Protoplasm.
Stearn, K. (1997). Introductory plant biology. WC Brown Publishers.

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