Changes in mRNA level rhythmicity in the leaves of Sinapis alba during a lengthening of the photoperiod which induces flowering.

A previous study has shown that mRNAs exhibit complex patterns of diurnal rhythms in their quantity in the leaves of Sinapis alba during an 8 h light/16 h dark short day (SD). In order to determine whether this situation is rapidly modified in plants subjected to an extended light treatment, we have used in vitro translation and two-dimensional polyacrylamide gel electrophoresis, together with a strict gel comparison procedure giving a P = 0.03 certitude level, to analyse the mRNA complement at different times during a 22 h light/2 h dark long day (LD). During this LD, complex changes affected about 10% of the mRNAs. Thirty-four different patterns were observed. Some diurnal rhythms present in SD are not modified by the lengthening of the light period, but most are affected. Moreover, we have shown that some mRNAs presenting a constant quantity under a SD regime show an increase or a decrease during the first hours of the photoperiod lengthening. In Sinapis, this LD also induces flowering. All the changes in mRNA quantity detected thus parallel the photoperiodic induction of flowering in the leaves and are quantitative; no mRNA was shown to appear or to disappear.