Meristem

The Definition of the Meristem

The meristem is a type of plant tissue that is made up of undifferentiated cells that are capable of dividing indefinitely. These meristematic cells are essentially stem cells that are clumped together and can develop into all other types of plant tissues and organs.

Meristematic cells are totipotent stem cells. This is what makes them so unique.

Plant meristematic cells can divide and develop throughout the plant's entire lifespan. This enables plants to keep growing and repairing themselves until they die, unlike most animals.

Fig. 1. If you look at meristematic tissue under a microscope, you will probably see some of the cells undergoing mitosis.

Types of Meristems

Meristem tissues are categorised according to their location in the plant. Apical meristems are found in the root and shoot tips. There are also lateral meristem tissues found in the middle of the plant. Typically, apical meristems support vertical growth, whilst lateral meristems support plant widening.

Apical Meristems

Both root and shoot meristems are formed during development. The cells in this region elongate and divide to support the growth and development of new meristematic cells. These new cells will then differentiate into the required specialised cells.

Fig. 2: Apical meristems are found in the tip of shoots and roots, unsplash.com

Radical Meristem

The radical meristem is a small region of stem cells at the tip of a root. These cells can divide and form new root tissue. An outer part of parenchyma cells, known as the root cap, protects the apical meristem inside the soil from external environmental stressors.

The radical meristem allows roots to grow longer, maximising access to water, nutrients, and minerals in the soil.

Shoot Meristem

Shoot meristems (also known as terminal buds) are found at the ends of all branches and stems. These areas can grow and develop indefinitely, possessing the ability to transform into leaves, branches, flowers and fruits. Shoot meristem tissue aids with reproduction and maximises access to sunlight for photosynthesis.

Lateral Meristems

Some meristem tissues are found in the middle of the plant, rather than at the ends.

The Function of the Meristems

Meristematic tissue supports multiple functions within the plant.

Primary Growth

Cell elongation and division trigger primary growth.

How does this benefit the plant?

• Increased height: the plant gets taller. This enables better access to sunlight for photosynthesis.

• Increased root length: the plant has a greater ability to access water, minerals and nutrients in the soil.

Wound Repair

Meristem cells can form inside injured tissues, differentiating into new, undamaged cells to control wound healing.

Hormone Production

Three different types of plant hormones control the elongation and division of meristem cells. The hormones are summarised in this table.

Table 1: The hormones and their function in plant growth and development.

Shoot and root tips are the production site of auxin, a significant plant hormone. Auxin stimulates cell elongation. This commonly happens in the 'darker' side of the plant stem that faces away from a light source. Longer cells in the darker side of the stem allow the plant to bend towards the light. Furthermore, auxin controls apical dominance. If the apex of the shoot (and, thus, the site of the apical meristem) is removed, auxin is no longer produced. This triggers the lateral meristems to start growing.

Cytokinins are another vital plant hormone produced in apical meristems. They promote cell division.

Producing Identical Plants

Meristem tissues can differentiate into any type of plant cell. As a result, these cells can be used to produce clones of entire plants. This method is a quick and cheap way to prevent the extinction of rare species. Additionally, this cloning method can be used to grow crops of identical plants with positive traits such as disease resistance or a high yield.

Meristem Example: Uses

Meristematic cells can be used to clone plants quickly and economically. How does it work?

Firstly, the meristematic cells are removed from a plant and grown in tissue culture.

The tissue culture is grown in a culture medium, which contains:

• Agar: a jelly-like material derived from seaweed, used for support

• Nutrients: for growth and repair

• Plant hormones: to stimulate growth and cell division

Clones will be genetically identical to the original plant that provided the meristem cells. This is useful for the large-scale cultivation of crops with desirable traits, such as drought tolerance.

Additionally, cloning plants in tissue culture is an easy and effective way of producing new individuals, helping to preserve endangered plant species.

Meristem Tissue: Some Facts

Here are some facts about meristem tissue:

• Meristem tissues are critical for human survival. Essentially, they hold the blueprint for the fuels we burn, the food we eat and the oxygen we breathe!

• Cloning plants using their meristem tissue is a common propagation method. Meristem tissues have been used to clone potatoes, corn, sugarcane, and bananas.

• The cellular and molecular structures of meristem tissues are very similar across many different plant species.

• Meristem tissues allow some trees to live for thousands of years.

• Plant's meristem cells cannot be used in human healthcare. However, they possess antioxidant and anti-inflammatory properties, often used in skincare products.