Practice question

Colchicine is used in plant tissue culture mainly to induce polyploidy and chromosome doubling rather than as fusogen or wall digesting agent. Alkaloid isolated from Colchicum autumnale binds tubulin dimers preventing polymerization into microtubules forming mitotic spindle fibers during metaphase. Without functional spindle, sister chromatids fail to separate at anaphase, nuclear membrane reforms enclosing doubled chromosome complement leading to endoreduplication tetraploid or doubled haploid formation. Application involves treating haploid plantlets derived from androgenesis gynogenesis or diploid explants with aqueous colchicine 0.05 to 0.5 percent for few hours to few days often with DMSO enhancing penetration then washing thoroughly to remove residual toxicity. Resulting polyploids often exhibit larger cells organs increased vigor greater secondary metabolite production and restored fertility in doubled haploids homozygous at all loci valuable for breeding. Due to colchicine toxicity mutagenicity alternatives like oryzalin amiprophos methyl herbicides inhibiting microtubule organization at lower concentrations are increasingly used. Dosage optimization prevents chimerism aneuploidy and severe growth inhibition allowing recovery of stable polyploid lines after few subcultures.