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Somatic embryogenesis (SE) can be readily induced in leaf explants of the Jemalong 2HA genotype of the model legume  Medicago truncatula  by auxin and cytokinin, but rarely in wild-type Jemalong. Gibberellic acid (GA), a hormone not included in the medium, appears to act in Arabidopsis as a repressor of the embryonic state such that low ABA (abscisic acid): GA ratios will inhibit SE. It was important to evaluate the GA effect in  M. truncatula  in order to formulate generic SE mechanisms, given the Arabidopsis information. It was surprising to find that low ABA:GA ratios in  M. truncatula  acted synergistically to stimulate SE. The unusual synergism between GA and ABA in inducing SE has utility in improving SE for regeneration and transformation in  M. truncatula.  Expression of genes previously shown to be important in  M. truncatula  SE was not increased. In investigating genes previously studied in GA investigations of Arabidopsis SE, there was increased expression of  GA2ox  and decreased expression of  PICKLE , a negative regulator of SE in Arabidopsis. We suggest that in  M. truncatula  there are different ABA:GA ratios required for down-regulating the  PICKLE  gene, a repressor of the embryonic state. In  M. truncatula  it is a low ABA:GA ratio while in Arabidopsis it is a high ABA:GA ratio. In different species the expression of key genes is probably related to differences in how the hormone networks optimise their expression.

plos one

An Unusual Abscisic Acid and Gibberellic Acid Synergism Increases Somatic Embryogenesis, Facilitates Its Genetic Analysis and Improves Transformation in Medicago truncatula