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Identification of a protein repressor (GBF1) of the AtCathB3 gene that is important not only in the germination sensu stricto of the A. thaliana seeds but also in the subsequent reserve mobilization.
Germination begins with the uptake of water by the quiescent seed and is completed when the embryo breaks the seed coat and surrounding tissues thus allowing radicle protrusion. Seedling growth, until photosynthesis is fully established, is considered a post-germination process. In both phases, germination sensu stricto and reserve mobilization (post-germination), the expression of different types of hydrolases takes place in a controlled way and this control takes place mainly at the level of transcription. Protein hydrolysis catalyzed by cathepsin B-like proteases has been considered mainly a post-germination process.
In this paper, we have demonstrated that the AtCathB3 gene, encoding a cathepsin B-like protease, is not only important for reserve mobilization but it is also crucial during germination sensu stricto, since its mutants (KO and over-expressors) are altered significantly in its germination kinetics. We have also explored the transcriptional regulation of this gene and have identified a conserved cis- motif (the CathB3-element) in the promoter sequences of the Brassicaceae CathB3 orthologous genes that is specifically recognized in trans- by the transcription factor GBF1. By a novel FISH microscopic experiment, we have demonstrated that AtCathB3 and GBF1 genes are co-expressed in the same cells of the germinating embryo before the radicle protrusion. Moreover, during post-germination AtCathB3 expression is extended to the cotyledons. These microscopic data have been selected as the cover illustration of the volume 65 (8) of the Journal of Experimental Botany where this article has been recently published (see figure).