Induction of embryogenic competence in somatic plant cells: a review

Somatic embryogenesis may serve in many aspects as a model for zygotic embryo development with the advantage of unlimited source of biological material for cellular, molecular and biochemical studies. Due to this reason, several tissue culture systems have been developed to study the molecular and cellular biology of somatic embryogenesis. One of the basic questions considering the mechanism of somatic embryo induction is related to the importance of the first cell division in the determination of cell fate and embryo polarity. Alfalfa leaf protoplast-derived cells represent a suitable experimental system to study this question: at low 2,4-D concentrations cells become elongated, vacuolated and divide more or less equally forming unorganised callus tissue, while at ten-times higher concentrations small, densely-cytoplasmic cells preferentially divide asymmetrically developing into embryo-like structures. In addition to investigations of cell division characteristics, the system proved to be suitable to describe other cell physiological, molecular parameters associated with the transition of somatic cells to a dedifferentiated and embryogenic state. It is well accepted that the development of the embryogenic/non-embryogenic cell types can be influenced by different treatments (e.g. stress or pH manipulations, 2,4-D concentration and genotype combinations). As a result of the experiments with alfalfa leaf protoplast-derived cells, we could hypothesise that the following factors have key roles in the induction of embryogenic competence: the genotype (1115 non-embryogenic and A2 embryogenic genotypes); high or low 2,4-D concentration plus stress (e.g. heavy metal stress); the DeltapH across the plasma membrane; vacuolar pH regulation; chloroplast dedifferentiation and increased IAA content inside the cells. Herewith we give an overview of these early cellular changes during embryogenic cell development comparing the results obtained using embryogenic alfalfa leaf protoplasts with similar results from other systems used to study somatic embryogenesis.