A research team from the University of Geneva (UNIGE) has discovered that
UV-B can be a powerful inducer of flowering, but that a protein called RUP2
blocks their action to prevent early flowering. Photoperiodic flowering
depends on changes in daylength. Some plants flower when the days are
longer, others do when the days are shorter. The perception of the length of
the day by plants is essential to control the onset of flowering in natural
ecosystems and to ensure successful reproduction.

Flowering mechanisms were studied in Arabidopsis, which flowers in
Switzerland mainly in May, when the days get longer. The research was mostly
done in growth chambers where the artificial light does not include UV-B.
The researchers then included this type of radiation, since plants possess
UV-B receptors. Together with researchers from the universities of Lausanne,
Tübingen, and the Helmholtz Zentrum München in Neuherberg (Germany), the
biologists showed that UV-B rays can potentially induce flowering of
Arabidopsis throughout the year. "However, their effect is blocked during
short days by a protein called RUP2," explains Adriana Arongaus, researcher
in the Geneva group and first author of the study.

The researchers analyzed the molecular mechanisms at work. They found that
UV-B can stimulate FT protein, the flowering hormone, regardless of the
season. RUP2, in turn, indirectly inhibits the production of this hormone,
and thus represses flowering. However, when the days lengthen,
photoreceptors in the leaves induce FT proteins and flowering starts despite
the presence of RUP2. This changing balance over the seasons allows the
implementation of a photoperiodic flowering, with RUP2 as the central actor.


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