Transient gene expression in sunflower (Helianthus annuus L.) following microprojectile bombardment

Transient expression of the uidA gene, coding for β-glucuronidase (GUS) and driven by different promoters, has been induced in sunflower cotyledonary explants and immature zygotic embryos of different developmental stages using two particle delivery systems. Explants were evaluated for expression of β-glucuronidase activity 3 days, 2, and 4 weeks post bombardment. Immature embryos were more suitable for transformation than cotyledons derived from mature seeds. The highest level of transient GUS expression after 3 and 14 days was obtained with embryos (≤1.5 mm) precultured for 3 days in the presence of NAA and BAP. After 2 and 4 weeks of culture, increased GUS activities were determined following bombardment with plasmids harboring a doubled CaMV 35S and a stress inducible promoter, respectively. In cotyledons, GUS-expressing cells were mostly observed in the epidermal layer, while in immature embryos they were located between the epidermis and the fourth mesophyll layer. The performance of the two biolistic equipments was comparable. Under any condition, GUS expression declined with increasing culture time.