The effects of abscisic acid on gene expression during wheat germination

Graduation date: 1985Germination of immature (Stage III) and mature (Stage V) wheat (Triticum aestivum L. var. Chinese Spring and Yamhill) embryos is prevented by the inclusion of 10⁻⁴ M abscisic acid (ABA) in culture medium. In addition, Stage III embryos develop into mature embryos in culture medium containing ABA. In the absence of ABA, Stage III embryos precociously germinate and yield mature wheat plants indistinguishable from those produced by Stage V embryos. Germination is characterized by synthesis of a number of specific proteins, including the small subunit of ribulose-1,5- bisphosphate carboxylase/oxygenase ( ssRUBISCO). De novo svnthesis of ssRUBISCO is first detected two days after imbibition. Dark-grown seedlings have 40% of the amount of ssRUBISCO present in light-grown seedlings at this time. Synthesis and accumulation of ssRUBISCO increases in light-grown seedlings by three days, while decreasing in seedlings deprived of :Light. Synthesis and accumulation of ssRUBISCO were monitored by in vivo protein labelling and immunologically using Western blotting. Neither synthesis nor accumulation of this protein is detected when germination is repressed by ABA. Cell-free translations of extant mRNA of Stage V embryos and that of germinating seedlings show a dramatic change in translatable RNA upon imbibition. ABA does not prevent this change in translatable RNA. Both in vivo and in vitro protein synthesis reveal few differences between +ABA- and -ABA-seedlings during early germination. This indicates that ABA is not a general inhibitor of transcript formation, but rather affects specific gene expression. Nick-translated cDNA homologous to wheat ssRUBISCO mRNA is used to determine levels of ssRUBISCO mRNA in germinating seedlings. This message is newly transcribed during the first day of germination. As germination progresses, an increasing percentage of the poly A+ population is found as ssRUBISCO mRNA. After three days of ABA treatment, the quantity of mRNA for ssRUBISCO does not increase and is equal to the amount found in one-day seedlings germinated in the absence of ABA. ABA appears to influence differential gene expression by varying the levels of specific gene transcripts