ABI5 acts downstream of ABI3 to execute an ABA-dependent growth arrest during germination.

The development of a germinating embryo into an autotrophic seedling is arrested under conditions of water deficit. This ABA-mediated developmental checkpoint requires the bZIP transcription factor ABI5. Here, we used abi3-1, which is also unable to execute this checkpoint, to investigate the relative role of ABI3 and ABI5 in this process. In wild-type Arabidopsis plants, ABI3 expression and activity parallel those described for ABI5 following stratification. During this process, transcript levels of late embryogenesis genes such as AtEm1 and AtEm6 are also re-induced, which might be responsible for the acquired osmotic tolerance in germinated embryos whose growth is arrested. ABI5 expression is greatly reduced in abi3-1 mutants, which has low AtEm1 or AtEm6 expression. Cross complementation experiments showed that 35S-ABI5 could complement abi3-1, whereas 35S-ABI3 cannot complement abi5-4. These results indicate that ABI5 acts downstream of ABI3 to reactivate late embryogenesis programmes and to arrest growth of germinating embryos. Although ABI5 is consistently located in the nucleus, chromosomal immunoprecipitation (ChIP) experiments revealed that ABA increases ABI5 occupancy on the AtEm6 promoter.