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Abstract
Plants develop unorganized cell masses like callus and tumors in response to various
biotic and abiotic stimuli. Since the historical discovery that the combination of
two growth-promoting hormones, auxin and cytokinin, induces callus from plant explants
in vitro, this experimental system has been used extensively in both basic research
and horticultural applications. The molecular basis of callus formation has long been
obscure, but we are finally beginning to understand how unscheduled cell proliferation
is suppressed during normal plant development and how genetic and environmental cues
override these repressions to induce callus formation. In this review, we will first
provide a brief overview of callus development in nature and in vitro and then describe
our current knowledge of genetic and epigenetic mechanisms underlying callus formation.