Formation of Callus from Pith Explant:




Callus formation is a common phenomenon observed when a part of the pith from the stem of a plant is used as an explant and cultured on a nutrient medium. The process of callus formation involves several cellular and molecular events that ultimately lead to the development of an undifferentiated mass of cells called callus.




Cellular Processes Responsible for Callus Formation:




1. Dedifferentiation:


When the pith explant is cultured on a nutrient medium, it undergoes dedifferentiation, which refers to the process of reverting from a differentiated state to an undifferentiated state. The cells in the pith explant lose their specialized functions and acquire the ability to divide and proliferate, leading to the formation of a mass of undifferentiated cells.




2. Cell Division and Proliferation:


Following dedifferentiation, the undifferentiated cells in the pith explant start dividing rapidly through cell division. This increased cell division results in the proliferation of cells, leading to the formation of a callus. The callus consists of numerous cells that continuously divide and multiply, contributing to its undifferentiated nature.




3. Hormonal Control:


The formation of callus is also regulated by plant hormones, particularly auxins and cytokinins. Auxins promote cell division and stimulate callus formation, while cytokinins enhance cell differentiation and inhibit callus formation. The balance between these hormones determines the fate of the cells in the pith explant. High auxin to cytokinin ratio promotes callus formation, while a low ratio promotes differentiation.




4. Somatic Embryogenesis:


In some cases, callus formation can also lead to a process called somatic embryogenesis. During somatic embryogenesis, the undifferentiated cells in the callus undergo further differentiation and organization, resembling the development of embryos. This process can be induced by manipulating the hormonal balance and environmental conditions.




Conclusion:


In summary, the formation of an undifferentiated mass of cells called callus from the pith explant involves dedifferentiation, cell division, hormonal control, and potentially somatic embryogenesis. Understanding these processes is essential in plant tissue culture and genetic engineering techniques, where callus formation is utilized for various applications such as micropropagation, genetic transformation, and the production of secondary metabolites.