Developmental, stress and ABA modulation of mRNA levels for bZip transcription factors and Vp1 in barley embryos and embryo-derived suspension cultures

The transcription factors VP1 (Viviparous-1), EmBP-1 (Em-binding protein 1) and OSBZ8, originally cloned and analysed in various monocot species, have been implicated in the regulation of the Lea (late embryogenesis-abundant) group 1 genes. We have investigated the modulation of the levels of these mRNAs in barley during embryogenesis, and in developing embryos subjected to various kinds of osmotic stress. The accumulation of mRNA for VP1 and EmBP-1 transcription factors, using cDNAs cloned from barley, starts at 10 and 15 days after anthesis, respectively, whereas Lea B19 mRNA levels are very low or undetectable until 25 days after anthesis during normal development. The EmBP-1 mRNA is predominantly induced in mannitol-stressed immature embryos. Vp1 mRNA was not significantly modulated by ABA, salt or mannitol. Inhibition of ABA biosynthesis by norflurazon showed that the induction of both Vp1 and EmBP-1 mRNAs was ABA-independent. In embryo-derived suspension-cultured cells, neither of the two transcripts would be induced by ABA or osmotic stress, although both OSBZ8 and one member of the Lea B19 family was up-regulated by ABA. Electrophoretic mobility shift assays using a Lea B19.1 probe with an ABRE (abscisic acid-responsive element) similar to that which binds EmBP-1 and OSBZ8 in the wheat and rice Em promoters show that the binding activity is increased by ABA and osmotic stress. Taken together, these data show that both VP1 and EmBP-1 are involved in embryo-specific signal transduction pathways, that they are differentially regulated at the mRNA level, and that EmBP-1 can be induced by osmotic stress independently of any increase in endogenous ABA. The difference in mRNA regulation patterns of OSBZ8 and EmBP-1 may suggest that they are involved in different signal transduction pathways in connection with osmotic stress/ABA regulation of Lea genes.