A plant callus is a mass of undifferentiated plant cells. This unorganized tissue plays a role in a plant’s ability to heal and regenerate. Understanding callus formation offers insights into plant biology and its applications.
What is a Plant Callus?
A plant callus is an unstructured, soft mass of plant cells. These cells are largely undifferentiated, meaning they lack specialized tissues like roots, stems, or leaves. Callus cells are often thin-walled parenchyma cells that actively divide.
They possess totipotency, the ability to develop into any type of plant tissue or even an entire new plant under suitable conditions. This characteristic makes callus a tool in plant biotechnology. The appearance of callus can vary, with some types being compact and green, while others are friable, yellowish-white, and easily separated.
Natural Formation and Healing
Plants naturally form callus as a response to injury, as a protective and healing mechanism. When a plant is wounded or cut, cells near the damaged area, particularly parenchyma cells in the cortex, begin to divide rapidly. This division produces a mass of new cells that cover and protect the wound, sealing off the injury.
Natural callus formation is also important in grafting, a technique where parts of two different plants are joined. In a successful graft, callus from both the scion (the upper part) and the rootstock (the lower part) grow together, forming a “callus bridge.” These undifferentiated cells then differentiate into vascular tissues, like xylem and phloem, establishing a connection between the two plant parts and allowing for nutrient and water transport.
Callus in Plant Propagation
Callus finds application in plant tissue culture, also known as micropropagation, a method growing new plants from small plant parts called explants. In this laboratory setting, explants are sterilized and placed on a specialized culture medium containing nutrients and plant growth regulators. This controlled environment induces cells at the cut edges of the explant to divide and proliferate, forming a callus.
Once a callus is established, it can be manipulated to regenerate whole plants through processes like somatic embryogenesis or organogenesis. Somatic embryogenesis involves the formation of embryos from non-reproductive cells within the callus, while organogenesis leads to the development of specific organs like shoots or roots. This technique is useful for multiplying plant species that are difficult to propagate by traditional methods or for producing disease-free plants.
The Role of Hormones in Callus Formation
The formation and development of callus are influenced by plant hormones, primarily auxins and cytokinins. These plant growth regulators are supplemented in specific ratios in the tissue culture medium to initiate and control callus growth. Auxins generally promote cell enlargement and division, while cytokinins stimulate cell division and differentiation.
An intermediate ratio of auxin to cytokinin typically encourages callus induction. For example, in many cereal crops, 2 mg/L of 2,4-dichlorophenoxyacetic acid (2,4-D), a synthetic auxin, is used for inducing embryonic callus. The balance between these two hormones determines whether the callus remains undifferentiated or proceeds to form roots or shoots.