Intact roots promote shoot regeneration from hypocotyl independent of exogenous plant growth regulators in eggplant in vitro

Eggplant (Solanum melongena) seedlings cultured in vitro were excised at the center of the hypocotyl to generate decapitated seedlings with intact roots. This modification of the complete decapitation method (CDM) developed in vivo by Harada et al. (2005) allowed in vitro culture (CDM in vitro; CDMi). As controls, rootless hypocotyl segment explants (approximately 1 cm) and cotyledon explants were cultured on media supplemented with 4.4 µM 6-benzyladenin (BA) and 0.2 μM thidiazuron (TDZ), respectively. Cotyledon explants had formed calli 2 weeks after excision but did not develop adventitious buds, despite the use of optimal conditions reported previously for a different eggplant cultivar. Calli formed at the cut ends of hypocotyls 1 week after excision in both CDMi and hypocotyl cultures, and adventitious buds regenerated 1 week earlier in CDMi. Six weeks after excision, CDMi yielded 11.4 adventitious buds per explant, but only 4.1 formed in hypocotyl culture. Moreover, shoots longer than 1 cm developed 2 weeks earlier in CDMi than in hypocotyl culture. The number of shoots per explant was 8.1 in CDMi, but only 2.4 in hypocotyl culture 6 weeks after cutting. All shoots that developed were rooted on MS medium in CDMi, but only 71% of shoots formed roots in hypocotyl culture. These results indicate that intact roots are important for explant shoot regeneration and development, and CDMi is a simple and efficient method for obtaining multiple shoots without the need to determine optimal concentrations of plant growth regulators and overcome inhibition of rooting in the obtained shoots.