Enrichment for heterokaryocytes by the use of iso-osmotic density gradients after plant protoplast fusion

Buoyant density differences between different types of protoplasts were used in an iso-osmotic density gradient system to enrich protoplast fusion mixtures for heterokaryocytes. Protoplasts of maize stem and wheat mesophyll, as well as epidermis, stem parenchyma and mesophyll protoplasts of two amphihaploid, light sensitive tobacco mutants were fused with polyethylene glycol using conventional methods and a new rolling tube technique. The protoplast combinations used for fusion involved protoplast types with considerably different buoyant densities. Enriched fractions of maize-wheat heterokaryocytes of intermediate density were recovered which contained up to 31% mostly binucleate heterokaryocytes (a 2–7 fold relative enrichment). Tobacco heterokaryocytes recovered analogously from enriched intermediate fractions readily divided and gave rise to an increased number of light resistant calluses when compared with cultures from non-fractionated fusion mixtures. Maize-wheat fusion products, however, failed to divide.