Chloroplast-dependent and light-independent expression of the tobacco rbcS promoter - GUS chimeric gene in black spruce

The tobacco rbcS (ribulose bisphosphate carboxylase small subunit) promoter, fused to the beta-glucuronidase (GUS) reporter gene, was delivered to black spruce (Picea mariana (Mill.) BSP) tissues via microprojectile DNA bombardment, and its regulation was studied. The expression of the tobacco rbcS promoter - GUS chimeric gene was dependent on the presence of chloroplasts in black spruce tissues, as demonstrated in two ways: (i) there was no GUS activity expressed in zygotic embryos where no chloroplasts were observed, whereas it was expressed in light- and dark-grown seedlings that contained mature or immature chloroplasts; (ii) a herbicide, Norflurazon, destroyed chloroplast structure in seedlings and inhibited the expression of the tobacco rbcS promoter - GUS chimeric gene. A control chimeric gene, the cauliflower mosaic virus (CaMV) 35S promoter - GUS fusion gene was not inhibited by Norflurazon. Unlike in angiosperms, light had no effect on the expression of tobacco rbcS promoter - GUS chimeric gene. Both light- and dark-grown seedlings showed GUS activity, and expression in dark-grown seedlings was not enhanced by light. These results suggest that the tissue-specific regulation of the rbcS promoter may be conserved between angiosperms and conifers, but that the light regulation of this promoter may not be conserved.