Direct somatic embryogenesis from pericycle cells of broccoli (Brassica oleracea L. var. italica) root explants

Cotyledon, hypocotyl or root explants of 7-day-old broccoli seedlings were cultured on Murashige and Skoog (MS) agar or liquid medium supplemented with 1.0 mg l−1 2,4-dichlorophenoxyacetic acid (2,4-D). The frequency of direct somatic embryo formation was 100% when root explants were cultured in liquid medium. Histological analysis indicated that somatic embryos were initiated directly from the pericycle cell layers of root explants as early as 1 day after liquid culture. Genotype did not affect the frequency of somatic embryo formation or the number of somatic embryos per explant. All broccoli genotypes examined had 100% somatic embryo induction efficiency, and the number of somatic embryos per 0.8 mm root segment ranged from 22.9 in ‘Luhui’ to 26.0 in ‘Haizi’. The number of normally developed somatic embryos in culture increased with increasing 2,4-D concentration. Plantlet regeneration frequency was the highest (73.3%) when germinated plantlets were transferred to 1/2 strength MS agar medium containing 1.0 mg l−1 6-benzyladenine (BA). When regenerated plantlets were transferred to a greenhouse, approximately 75% survived and there were no morphological differences between regenerated plants and seed-derived controls. The protocols established in this study will benefit large-scale vegetative propagation and transformation-based genetic improvement of broccoli.