MBF1s regulate ABA-dependent germination of Arabidopsis seeds

Abstract

Transcriptional co-activators of the multiprotein bridging factor 1 (MBF1) controls gene expression by connecting transcription factors and the basal transcription machinery. In Arabidopsis thaliana functions of MBF1 genes have been related to stress tolerance and developmental alterations. Endogenous ABA plays a major role in the regulation of Arabidopsis seed dormancy and germination. Seed dormancy and ABA sensitivity are enhanced in ethylene insensitive mutants suggesting that ethylene signal transduction pathway is necessary to fully develop ABA-dependent germination. In this report we showed that a triple knock-down mutant for Arabidopsis MBF1 genes (abc-) has enhanced seed dormancy and displays hypersensitivity to exogenous ABA. In addition, higher ABA contents were detected in abc- seeds after imbibition. These evidences suggest a negative role of MBF1s genes in ABA-dependent inhibition of germination. The participation of MBF1s in ethylene signal transduction pathway is also discussed.

Figure 1. Mutations in AtMBF1 genes enhance seed dormancy. Seeds from WT, abc-, and abc- +c plants were surface-sterilized and stratified or not at 4°C for 2 d in the dark to break dormancy. Seeds were plated on ATS medium with 0.8% agar and placed on a growth chamber at 23°C with a 16 h-light photoperiod. (A) The percentage of germination was scored after 24 h. (B) The percentage of 0.7 stage according to Boyes et al. was evaluated after 48 h. Approximately, 100 seeds were processed per line in each experiment. Data are mean values (± SE) of five independent experiments. Different letters indicate a significant difference at p < 0.05 (Tukey's test).

Figure 2.abc- mutant is hypersensitive to ABA during germination. (A) Sterilized and stratified WT, abc-, abc- +c and ein3-1 seeds were plated on ATS agar medium supplemented with the indicated concentrations of ABA and incubated in the growth chamber. (B) Sterilized and stratified WT, abc- and abc- +c seeds were plated on ATS agar supplemented with ABA, ACC or the combination of both as indicated. In (A) and (B) the percentage of germination was scored after 24 h. Approximately, 100 seeds were processed per line in each experiment. Data are mean values (± SE) of five independent experiments. Different letters indicate a significant difference at p < 0.05 (Tukey's test).

Figure 3.abc- mutant accumulates ABA in seeds. Seeds from WT, abc-, and abc- +c plants were imbibed in sterile water for the designated times and assayed for ABA content by radioimmunoassay. Each sample was assessed twice. The results presented are the mean value of two biological replicates ± SE.