Incidence of natural light stress renders it important to enhance our understanding of the mechanisms by which plants protect themselves from harmful effects of UV-B irradiation, as this is critical for fitness of land plant species. Here we describe natural variation of a class of phenylacylated-flavonols (saiginols), which accumulate to high levels in floral tissues of Arabidopsis. They were identified in a subset of accessions, especially those deriving from latitudes between 16° and 43° North. Investigation of introgression line populations using metabolic and transcript profiling, combined with genomic sequence analysis, allowed the identification of flavonol-phenylacyltransferase 2 (FPT2) that is responsible for the production of saiginols and conferring greater UV light tolerance in planta. Furthermore, analysis of polymorphism within the FPT duplicated region provides an evolutionary framework of the natural history of this locus in the Brassicaceae.

A cauliflower mosaic virus (CaMV) 35S promoter derivative, which is tightly repressed by the Tn 10 encoded Tet repressor in a transient expression system as well as in transgenic plants has been constructed. After treatment of transgenic plants with tetracycline (Tc) the activity of the reporter enzyme beta-glucuronidase (GUS) increased up to 500-fold in tissue culture as well as under greenhouse conditions. Efficient de-repression was achieved by Tc uptake through the roots as well as by Tc treatment of leaves of intact plants. As Tc is not very stable in the plants, this system can also be used for a transient expression of a transgene. This system provides a unique tool for regenerating transgenic plants carrying a repressed transgene and for efficiently de-repressing its activity by a specific inducer at any time point of further development.

We have investigated the use of the Tn10-encoded tet repressor-operator system to regulate the expression of a suitably engineered cauliflower mosaic virus (CaMV) 35S promoter in transgenic tobacco plants. First, a transgenic plant was generated which constitutively synthesizes 600,000 Tet repressor monomers per cell. In a second transformation step, the beta-glucuronidase (gus) gene under the control of a modified CaMV 35S promoter, containing two tet operators, was stably integrated into the plant genome of a tetR+ plant. Expression of the gus gene is repressed 5-fold, if the operators are located flanking the TATA box, and 50- to 80-fold when both operators are positioned downstream of the TATA box. This indicates that Tet repressor-operator complexes can form on plant chromosomes and interfere with transcription. Maximal induction is achieved after 0.5 h upon application of only 0.1 mg/l tetracycline. This fast and efficient induction makes the system useful for specifically inducing expression of transferred genes at different stages of plant development.

No abstract