Plant stress response: Abscisic acid finds its receptor
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The RNA-binding protein FCA is a receptor for abscisic acid (ABA) which regulates flowering in Arabidopsis by mRNA processing.
The phytohormone abscisic acid (ABA) mediates stress responses in plants. It has been shown to regulate seed drought tolerance and dormancy. Although RNA processing is known to be an important factor, a full understanding of this signaling pathway has remained elusive. A study published in Nature now reveals that FCA, a nuclear RNA-binding protein, is a receptor for ABA that regulates flowering. FCA prevents the accumulation of FLC mRNA, a MADS box transcription factor that represses floral transition.
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Hill and colleagues isolated ABAP1 from a barley complementary DNA expression library screened for ABA receptors. FCA — the Arabidopsis homologue of ABAP1 — was found to bind ABA in vitro. FY, a FCA-binding 3’ RNA processing factor, was also found to be required for FCA function. The FCA/FY complex reduces the amount of functional FCA protein through a negative feedback loop. However, when ABA binds to FCA it prevents FCA from binding to FY. Indeed, binding assays reveal that ABA binds to the carboxy-terminus of FCA through the same tryptophan – tryptophan motif that mediates FY binding. Thus ABA binding causes the FCA/FY complex to dissociate.
FCA undergoes autoregulation by promoting premature cleavage and polyadenylation of its own mRNA through its association with FY. Comparing levels of the full length FCA mRNA versus the truncated form provides a readout of FCA activity. Treatment with ABA induces the accumulation of full length FCA mRNA in plants and delays the onset of flowering. Increased mRNA levels of the floral repressor FLC were also observed. FCA is not required for all ABA signaling pathways. Studies in different genetic backgrounds revealed that FCA is dispensable for seed germination and stomatal response but is required for lateral root formation.
This study sheds light on the regulation of flowering in Arabidopsis as well as providing a receptor for ABA. Other proteins had previously been shown to bind ABA, but FCA is the first bona fide receptor for this key plant hormone.
Clare Garvey
Signaling Gateway
References
- Fawzi A Razem, Ashraf El-Kereamy, Suzanne R Abrams & Robert D Hill. The RNA-binding protein FCA is an abscisic acid receptor. Nature 439, 290 – 294 (2006) | Article | PubMed |
- Julian I Schroeder & Josef M Kuhn. Plant biology: Abscisic acid in bloom. Nature, 439, 277 – 278 (2006) | Article | PubMed |
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