Study of the role of the DOF transcription factor DAG1 in the control of seedling development in Arabidopsis thaliana

Seedling development relies on environmental conditions; indeed, once seeds have germinated, they undergo photomorphogenesis or skotomorphogenesis, depending on the presence or absence of light. Photomorphogenesis is a multi-traits process characterised by inhibition of hypocotyl elongation, open and expanded cotyledons, and chloroplast development, whereas skotomorphogenesis is characterised by long hypocotyls and small unfolded cotyledons. Hypocotyl elongation is influenced by both environmental and hormonal cues and it has been extensively studied as a model for cell expansion. Nevertheless, the molecular network underlying this process is not yet fully elucidated. The Arabidopsis Dof protein DAG1 (Dof Affecting Germination1) is a repressor of seed germination, and a key player of the seed-to seedling transition, a crucial developmental phase positively controlled by light, as well as by the phytohormones ABA (abscissic acid) and GA (gibberellins). Indeed, DAG1 controls the ratio of ABA and GA, which play opposite roles, as ABA represses germination whereas GAs promote it. We have previously shown that inactivation of DAG1 affects inhibition of hypocotyl elongation. Indeed, light-grown dag1 mutant seedlings show significant shorter hypocotyls compared to the wild-type, suggesting that DAG1 is a negative component of this light-mediated process. To gain some insight into the molecular network in which DAG1 is involved, we have analysed the transcriptome profile of both dag1 and wild-type hypocotyls and seedlings. We have identified more than 250 genes that are differentially expressed in dag1 hypocotyls, and the analysis of this data suggests that DAG1 is mainly involved in promoting hypocotyl elongation. In addition, a number of the DE genes identified are correlated to the response to ABA stimulus. ABA plays a role in inhibition of hypocotyl elongation, although the molecular mechanism remains unclear. Therefore, we investigated the effect of ABA on hypocotyl development, and our results showed that ABA negatively controls cell expansion in hypocotyls, by acting on GA metabolism, and repressing auxin biosynthesis. Consistently, addition of exogenous ABA can revert the hypocotyl phenotype of dag1 mutant seedlings. In conclusion, our results prove that DAG1 is likely to be an element of a molecular network which controls cell expansion by modulating hormonal response, namely auxin, ABA and GAs.