Transient expression of a reporter gene changes significantly during somatic embryogenesis in alfalfa

Transient expression of the β -glucuronidase (GUS) gene driven by a 35S constitutive promoter was investigated during somatic embryo development in alfalfa (Medicago sativa L.) following micro-projectile bombardment. The level of gene expression varied with the different developmental stages and changed dramatically within the stages. During embryogenic callus induction (stage I), expression was low in explants freshly transferred to culture, but increased when cells started to divide and show competence for somatic embryogenesis. Gene expression, however, decreased when extensive callus formation began and reached the lowest observed level when callus was fully developed. Reporter gene expression was consistently low during embryogenic cell proliferation (stage II). In contrast, high levels of reporter gene expression were detected in non-embryogenic suspension cultures during cell proliferation. Extremely low reporter gene expression was detected in embryonal tissues and young embryos at the embryo development and maturation stage (stage III). Expression increased as the somatic embryos developed and reached the highest level when the embryos reached maturity. Transient expression levels were similar in excised petiole tissue across 10 alfalfa genotypes, which included both diploid and tetraploid genotypes. In general, reporter gene expression was found to be higher in more organised mature tissues and organs than in less-organised or young tissues. The developmental status of the cells and tissues appears to be an important factor in the degree of transient expression of the introduced gene. Changes of gene expression during embryogenesis and factors affecting gene expression are discussed. Key words: GUS expression, Medicago sativa, bombardment, forage