Enhanced luciferin entry causes rapid wound-induced light emission in plants expressing high levels of luciferase

In-vivo imaging of transgenic tobacco plants (Nicotiana tobacum L.) express ing firefly luciferase under the control of the Arabidopsis phenylalanine a mmonialyase 1 (PAL1)-promoter showed that luciferase-catalyzed light emissi on began immediately after the substrate luciferin was sprayed onto the lea ves and reached a plateau phase after approximately 60 min. This luminescen ce could easily be detected for up to 24 h after luciferin application alth ough the light intensity declined continuously during this period. A strong and rapid increase in light emission was observed within the first minutes after wounding of luciferin-sprayed leaves. However, these data did not co rrelate with luciferase activity analysed by an in-vitro enzyme assay. In a ddition, Arabidopsis plants expressing luciferase under the control of the constitutive 35S-promoter showed similar wound-induced light emission. In e xperiments in which only parts of the leaves were sprayed with luciferin so lutions, it was shown that increased uptake of luciferin at the wound site and its transport through vascular tissue were the main reasons for the rap id burst of light produced by preformed luciferase activity. These data dem onstrate that there are barriers that restrict luciferin entry into adult p lants, and that luciferin availability can be a limiting factor in non-inva sive luciferase assays.