DAG1, a master regulator of the hormonal balance during seed development in Arabidopsis thaliana
The phytohormones abscisic acid (ABA) and gibberellins (GAs) play opposite roles in controlling the plant developmental switch between seed dormancy and germination: ABA establishes dormancy during late stages of seed maturation, while GAs release dormancy and promotes germination, thus counteracting the effects of ABA.
We have previously shown that the Dof transcription factor DAG1 (Dof AFFECTING GERMINATION1) negatively regulates seed germination, and it represses the GA biosynthetic gene GA3ox1 during germination by directly binding to its promoter. We have demonstrated that the DELLA protein GAI (GA INSENSITIVE) cooperates with DAG1 in repressing GA3ox1, and that it directly interacts with DAG1. We also observed an increase of the ABA catabolic gene CYP707A2 in dag1 seeds, suggesting that DAG1 may regulate this gene.
Recently, we have shown that DAG1 represses GA3ox1 also during seed dormancy, as revealed by analysis of GA3ox1 expression in dag1 and wild type dormant seeds. In addition, we show that during seed maturation and dormancy DAG1 negatively controls also the ABA-catabolic gene CYP707A2 by directly binding to its promoter, as revealed by ChIP assay. Consistently, the ABA level is reduced while the levels of bioactive GAs are increased in dag1 mutant seeds, which have a reduced dormancy. Moreover, we have shown that during seed imbibition both the expression of DAG1 and the level of the DAG1 protein are positively regulated by GAs; addition of paclobutrazol, an inhibitor of GA biosynthesis, reduced the activity of DAG1.
Our results point to DAG1 as a key player in the control of the developmental switch between seed dormancy and germination acting on the balance between ABA and GAs, and show that DAG1 exerts its control since the late stages of seed maturation when dormancy is established.