Abscisic acid enhances the transcription of wheat-germ agglutinin mRNA without altering its tissue-specific expression.

We used RNA gel-blot analysis, in-situ hybridization, and nuclear run-on transcription to examine the effects of exogenous abscisic acid (ABA) on the spatial distribution of mRNA for the lectin wheat-germ agglutinin (WGA) in developing wheat (Triticum aestivum L. cv. Marshall) embryos and seedlings. When analyzed by RNA gel blots, both developing embryos and seedlings exhibited higher steady-state levels of WGA mRNA after ABA treatment. As determined by in-situ hybridization, incubation of developing embryos in 0.1 mM ABA resulted in accumulation of WGA mRNA in the epidermal and subepidermal cell layers of the radicle and seminal roots and throughout the rootcap and coleorhiza. This spatial distribution was identical to that in control embryos. Nuclear run-on transcription assays indicated that at least part of this increase is attributable to transcriptional induction. Thus, exogenous ABA is capable of inducing increased WGA mRNA accumulation only in cells where it is expressed during normal embryogenesis. When seeds were germinated in the absence of ABA, WGA mRNA was detected only in the rootcap. In contrast, seeds imbibed and germinated in the presence of ABA for 3 d exhibited a spatial distribution of WGA mRNA similar to that observed in developing embryos treated with ABA. In contrast, when ABA was added to 3-d-old seedlings, WGA mRNA was not detected in regions of the root beyond the rootcap. We conclude that exogenous ABA, when applied continuously from imbibition, causes retention of the embryo-specific pattern of WGA mRNA distribution and that the spatial pattern of WGA mRNA expression in roots does not change when ABA is added after germination.