Quantitative and qualitative study of endogenous and exogenous growth regulators in eggplant (Solanum melongena) microspore cultures

In eggplant microspore embryogenesis, embryos are produced and then transformed into undifferentiated calli, instead of developing as true embryos. This is the main current bottleneck that precludes this process from being efficient. In this work we aimed to shed light on the factors involved in the successful in vitro development of eggplant haploid embryos by evaluating the role of growth regulators (GRs) in this process. We analyzed the endogenous levels of different GRs, including auxins, cytokinins and gibberelins, as well as salicylic, jasmonic and abscisic acid, in microspores and microspore-derived embryos at different culture stages. We also analyzed the same GR profiles in leaf and anther wall tissues of different eggplant backgrounds. Finally, we assessed the application of different GR combinations to the culture medium. Our results showed that in eggplant there are no genotype-specific endogenous GR profiles that can be associated to a high embryogenic response. Instead, the embryogenic response seems related to different GR accumulation patterns during in vitro culture. The changes observed in the endogenous levels of salicylic and abscisic acid were not related to the embryo transition. There were, however, changes in the levels of indole acetic acid and dihydrozeatin. The best GR combination to promote callus production was 0.5 mg/L 1-naphthaleneacetic acid (NAA) and 0.5 mg/L 6-benzylaminopurine (BAP). A 20% reduction of NAA and BAP reduced embryo production but produced structures more anatomically similar to embryos. These results shed light on the role of GRs during the development of microspore-derived embryos in eggplant microspore cultures.