Physical state of the culture medium triggers shift in morphogenetic pattern from shoot bud formation to somatic embryo in Solanum khasianum

Solanum khasianum is a rich source of steroidal alkaloids that are important secondary metabolites with enormous pharmaceutical uses. Development of plantlets from somatic tissues, under in vitro conditions, takes place both through adventitious shoot bud differentiation or somatic embryogenesis (SE) pathway. We observed that the physical state of medium, solid or liquid, determined the regenerant differentiation patterns from root segment explants in S. khasianum . In the solidified medium, the root segments developed adventitious shoot buds whereas somatic embryos were regenerated in the liquid medium. Varying gradients from liquid to solid medium were further used to confirm the effect of solidified condition on regeneration pathway. Histological analysis of developing shoot buds and somatic embryos was also performed to confirm their development and differentiation patterns. In order to further confirm the developmental pathways, qRT-PCR analysis of the marker genes of SE and shoot regeneration was also performed. While SOMATIC EMBRYOGENESIS RECEPTOR KINASE1 ( SkSERK1 ) expression was significantly up-regulated during the early embryogenic stage, the LATE EMBRYOGENESIS ABUNDANT ( SkLEA ) protein was found to be highly expressed in the mature embryos. Expression of the HISTONE DEACETYLASE ( HDA6 ), a repressor of SE related genes, was highly decreased during embryogenesis in the liquid culture. Furthermore, expression of the ENHANCER OF SHOOT REGENERATION ( ESR ) gene was comparatively increased during shoot regeneration in the culture using solid medium. Our results point out that the physical state of the medium in S. khasianum plays a decisive role in differentiation pattern which was independent of hormonal supplements.