Isolation, culture and plantlet regeneration from cotyledon and mesophyll protoplasts of two pickling cucumber (Cucumis sativus L.) genotypes.

Optimal protoplast yields from cotyledons (2.0×10(6) protoplasts/ 0.5 g tissue) and from true leaves (5.0×10(6) protoplasts/g tissue) of two Cucumis sativus genotypes were obtained following a 16 h digestion with, respectively, 1.25% pectinase+0.5% Cellulysin and 0.5 % pectinase+ 1.0% Cellulysin. Enzyme solutions were prepared in modified MS medium containing half-strength major salts, full complement of minor salts and vitamins, 2% sucrose and 0.25 M mannitol. A plating density of 3.5-4.0× 10(4) protoplasts/ml or higher was required for sustained division, with first division occurring in 6-7 days, second-third division in 8-9 days, and minicalli formation by day 13. Embedding in 0.4% agarose provided the highest plating efficiency (proportion that formed minicalli) of mesophyll protoplasts, which was 28.3% for genotype 3672 and 15% for genotype 3676. By comparison, liquid culture and droplet culture gave lower plating efficiencies (10-19%). Cotyledon and mesophyll protoplasts of one genotype formed minicalli on MS medium containing 2,4-D/BA at 1.0/2.5 μM and 5.0/5.0 μM, respectively, within 21 days, while mesophyll protoplasts of the second genotype formed minicalli on MS medium containing NAA/BA at 5.0/5.0 μM within 12 days. Shoot buds or somatic embryos were obtained upon subculture of calli to MS medium containing lower concentrations (0.05-0.01 μM) of 2,4-D/BA or NAA/BA and a few plantlets, ca.18, were recovered on hormone-free medium.