Cold and Light Control Seed Germination through the bHLH Transcription Factor SPATULA

Plants integrate signals from the environment and use these to modify the timing of development according to seasonal cues. Seed germination is a key example of this phenomenon and in Arabidopsis is promoted by the synergistic interaction of light and low temperatures in dormant seeds. This signaling pathway is known to converge on the regulation of the gibberellin (GA) biosynthetic genes GA3 oxidase ( GA3ox), whose expression is transcriptionally induced by light and cold in imbibed seeds. However, the molecular basis of this response has until now been unknown. Here we show that the bHLH transcription factor SPATULA is a light-stable repressor of seed germination and mediates the germination response to temperature. Furthermore, SPT is required in dormant seeds for maintaining the repression of GA3ox transcription. We also show that the related protein PIL5 represses seed germination and GA3ox expression in the dark. We conclude that SPT and PIL5 form part of a regulatory network coupling seed germination and GA3ox expression to light and temperature signaling in the seed.