PHYTOCHROME INTERACTING FACTOR1 interactions leading to the completion or prolongation of seed germination.

In Arabidopsis thaliana, the basic Helix Loop Helix transcription factor, PHYTOCHROME INTERACTING FACTOR1 (PIF1) is known to orchestrate the seed transcriptome such that, ultimately, proteins repressing the completion of germination are produced in darkness. While PIF1-mediated control of abscisic acid (ABA) and gibberellic acid (GA) anabolism/catabolism is indirect, PIF1 action favors ABA while discriminating against GA, firmly establishing ABA's repressive influence on the completion of germination. The result is tissue that is more sensitive to and producing more ABA; and is less responsive to and deficient in GA. Illumination of the appropriate wavelength activates phytochrome which enters the nucleus, and binds to PIF1, initiating PIF1's phosphorylation by diverse kinases, subsequent polyubiquitination, and hydrolysis. One mechanism by which phosphorylated PIF1 is eliminated from the cells of the seed upon illumination involves an F-BOX protein, COLD TEMPERATURE GERMINATING10 (CTG10). Discovered in an unbiased screen of activation tagged lines hastening the completion of seed germination at 10°C, one indirect consequence of CTG10 action in reducing PIF1 titer, should be to enhance the transcription of genes whose products work to increase bioactive GA titer, shifting the intracellular milieu from one that is repressive to, toward one conducive to, the completion of seed germination. We have tested this hypothesis using a variety of Arabidopsis lines altered in CTG10 amounts. Here we demonstrate using bimolecular fluorescence complementation that PIF1 interacts with CTG10 and show that, in light exposed seeds, PIF1 is more persistent in ctg10 relative to WT seeds while it is less stable in seeds over-expressing CTG10. These results are congruent with the relative transcript abundance from three genes whose products are involved in bioactive GA accumulation. We put forth a model of how PIF1 interactions in imbibed seeds change during germination and how a permissive light signal influences these changes, leading to the completion of germination of these positively photoblastic propagules.