Practice question

Callus is unorganized proliferation of dedifferentiated parenchymatous cells produced in response to wounding or exogenous hormone application in vitro under sterile conditions. When explant such as leaf segment, stem internode, or cotyledon is placed on Murashige and Skoog medium supplemented with balanced auxin like 2,4-D and cytokinin BAP, mature cells dedifferentiate and divide mitotically forming amorphous mass of thin-walled, vacuolated, isodiametric cells lacking organized meristems, vascular pattern, or polarity initially. Callus growth reflects loss of positional information and continuous cell cycle driven by phytohormone signaling activating auxin response factors and cell cycle genes. It appears cream, friable, or compact depending on genotype and conditions and auxin concentration. Though disorganized, callus retains totipotency and can be induced to redifferentiate into shoots via high cytokinin or roots via high auxin, or embryos via somatic embryogenesis. Callus serves as platform for somaclonal variation generation, genetic transformation, secondary metabolite accumulation, and micropropagation cycles for mass production of uniform planting material.