Avoiding prolonged exposure to which PGR reduces somaclonal variation?
Among auxins, 2,4-dichlorophenoxyacetic acid stands out for strong callus inducing activity but also high genotoxicity. At 1 to 3 mg per liter it stimulates rapid cell division through auxin response factors ARF activation, yet its stable structure persists intracellularly, generating reactive intermediates interfering with mitotic spindle assembly and DNA repair. Long-term exposure triggers hypermethylation of cytosine in CCGG sites, reactivation of dormant transposable elements like Tos17, and increased incidence of aneuploidy and polyploidy observed in barley and rice cultures. Habituação and loss of embryogenic competence correlate with duration on 2,4-D. Replacing it promptly with NAA or IAA during differentiation and avoiding prolonged maintenance significantly preserves fidelity. Therefore culture guidelines recommend minimizing exposure to 2,4-D and monitoring subculture duration to curb epigenetic and genetic instability responsible for somaclonal variation. Molecular mechanism involves generation of reactive oxygen species and interference with topoisomerase II during mitosis, leading to chromosome bridges. Transcriptomic studies show upregulation of DNA methyltransferases MET1 and chromomethylases under 2,4-D stress, causing hypermethylation of repetitive sequences. Monitoring culture age and replacing 2,4-D early prevents accumulation of such lesions that reduce regeneration potential.
Ref: Evans Amer J Bot 1984 2,4-D genotoxic; Phillips Theor Appl Genet 1994; NCBI NBK153356 auxin toxicity; PubMed 24254045 epigenetic instability; Taylor & Francis tissue culture variation.