“DAG1 and GAI shared functions in light-mediated seed germination”

Seed germination is the transition of the quiescent embryo into a new photosintetically active plant, and is defined by the radicle protrusion from the seed coat. It is controlled by multiple environmental and endogenous factors. Germination of seeds of Arabidopsis thaliana needs a pulse of red light and is mediated mainly by the photoreceptor phytochrome B (phyB). A key role is also played by phytohormones, such as abscissic acid (ABA) and gibberellin (GA), which play an antagonistic role. In fact, ABA inhibits this process, whereas GA triggers seed germination. PIL5 is a phytocrome-interacting bHLH protein and is the master repressor of light-mediated seed germination. PIL5 directly induces the expression of RGA and GAI, encoding two DELLA proteins, which are negative regulators of GA-mediated processes. We have previously shown that DAG1, a Dof transcription factor, also plays a central role in this light-mediated process. DAG1 acts dowstream of PIL5 and negatively regulates GA biosynthesis by directly repressing the AtGA3ox1 gene. We are currently investigating the functional and molecular relationship between RGA, GAI and DAG1 by using a combined genetic and molecular approach. Our results suggest that RGA and GAI have different functions with respect to DAG1 in repressing seed germination. In fact, our data shows that the expression level of DAG1 and GAI, but not of RGA, are mutually regulated in the seed. Moreover, GAI, similarly to DAG1, is likely to partecipate in the repression of the AtGA3ox1 gene. Interestingly, genetic analyses indicate that DAG1 and GAI could have also a function during embryogenesis, as the dag1gai-t6 double mutant is embryo-lethal. We conclude that DAG1 and GAI may cooperate in both seed germination and embryo development.