In a screen for recessive mutations in Arabidopsis that cause hypersensitivity to abscisic acid (ABA) in germination assays, Hugovieux et al. identified a gene (ABH1) encoding a protein with sequence similarity to messenger RNA cap binding proteins (CBP). In yeast and humans, CBPs are a heterodimeric complex of CBP80 and CBP20. ABH1 interacted with a second Arabidopsis protein called AtCBP20, indicating that the plant CBPs have structural similarity to yeast and human CBPs. Cap-binding activity was measured in a gel-shift assay and depended on expression of both AtCBP20 and ABH1. The abh1 mutants were not significantly altered for auxin, ethylene, brassinosteroid, or jasmonate signaling. DNA array analysis demonstrated that in abh1 mutants, the expression of only a small subset of genes is affected, including a negative regulator of ABA signaling, AtPP2C. Several of the known genes that are down-regulated in abh1 are implicated in signaling pathways for stress and oxidative responses, consistent with known pathways involving ABA. Finally, analysis of guard cell responsiveness to drought and ABA showed that the abh1 mutants were more drought tolerant and exhibited increased calcium transients in response to ABA, consistent with increased sensitivity to ABA. The exact mechanism by which ABH1 regulates ABA signaling remains to be determined, but a CBP that selectively controls the expression of a subset of genes represents a potentially new mechanism for posttranscriptional regulation.
V, Hugovieux, J. M. Kwak, J. I. Schroeder, An mRNA cap binding protein, ABH1, modulates early abscisic acid signal transduction in Arabidopsis. Cell 106, 477-487 (2001). [Online Journal]