Transient expression of the uidA gene in Pinus pinea cotyledons: a study of heterologous promoter sequences

Transfer and expression of the beta-glucuronidase gene (uidA) in cultured cotyledons of stone pine (Pinus pinea L.) was obtained by microprojectile bombardment. Conditions for optimum transient expression were established by using plasmid pBI121 delivered by 1.0 micrometer-diameter gold particles, into 1-day-old cultured cotyledons. Helium pressure of 6.2 MPa, microcarrier travel distance of 6 cm, and 0.8 micrograms of plasmid DNA per bombardment, were the best parameters for high levels of transient uidA expression. By using these parameters, 98% of bombarded cotyledons showed beta-glucuronidase activity, with a mean of 63 Gus foci per cotyledon. This system was used to study the expression of uidA gene driven by several heterologous promoters. The expression under the control of the sunflower polyubiquitin gene (UbB1) promoter (delta 1 deletion) was higher (99% of GUS positive cotyledons) than under the control of the CaMV35S promoter, whereas the rice actin and the maize alcohol dehydrogenase gene promoters gave lower uidA expression, as determined histochemically. These results were confirmed by using the GUS fluorometric assay. Use of a deletion of the sunflower polyubiquitin promoter resulted in GUS activity detectable 35 days after bombardment, and significant levels of GUS activity were confirmed at the end of that period. The results will be useful to design protocols for stable transformation and high levels 6f transgene expression in P. pinea.