Pimenta Lange, M. J.; Lange, T.; parra-martinez, A. C.; Schwarze, J.

Under natural growth conditions, plants are not usually exposed to the high-energy ultraviolet C range (UV-C, 100-280 nm) of the solar spectrum, as this is absorbed by the ozone layer. However, low doses of UV-C radiation can trigger stress responses in plants. Nevertheless, it is not yet fully understood how UV-C light affects plant development at the hormonal level. Here we show that a single one-min UV-C light pulse (20 W/m2) alters gibberellin (GA) homeostasis in Arabidopsis in two phases: initially, the level of GA12 - a key precursor of the final part of gibberellin biosynthesis - is reduced. Consistent with this, the transcript levels of the CPS, KS and KAO2 genes, which encode enzymes involved in the initial parts of gibberellin biosynthesis, decrease. The level of the plant hormone GA4 also decreases initially, probably due to the reduced GA12 precursor levels. However, in a second phase, the endogenous GA4 levels rise in UV-C treated plants relative to control plants. This increase leads to an early onset of flowering, as well as increased growth and fertility, in UV-C-treated Arabidopsis plants. The GA signalling mutant gdella does not exibit wild-type phenotypic responses to UV-C treatment, indicating that GA signalling is essential for the UV-C response. To further narrow down the responsible steps in the GA-signalling pathway, we tested the kao1 and kao2 mutants, which are both impaired in early gibberellin biosynthesis. Neither mutant displays phenotypic responses to the UV-C treatment, indicating that both genes are required for mediating the UV-C response. In contrast, the quintuple 2-oxidase mutant C19--2oxqM exhibits responses to UV-C treatment similar to the wild-type, suggesting that the five catabolic 2-oxidases that act on C19-GAs play a negligible role in regulation GA-hormone levels for growth and development in this case.