Abstract
A dominant suppressor of the ABAR overexpressor, soar1-1D, from CHLH/ABAR [coding for Mg-chelatase H subunit/putative abscisic acid (ABA) receptor (ABAR)] overexpression lines was screened to explore the mechanism of the ABAR-mediated ABA signalling. The SOAR1 gene encodes a pentatricopeptide repeat (PPR) protein which localizes to both the cytosol and nucleus. Down-regulation of SOAR1 strongly enhances, but up-regulation of SOAR1 almost completely impairs, ABA responses, revealing that SOAR1 is a critical, negative, regulator of ABA signalling. Further genetic evidence supports that SOAR1 functions downstream of ABAR and probably upstream of an ABA-responsive transcription factor ABI5. Changes in the SOAR1 expression alter expression of a subset of ABA-responsive genes including ABI5. These findings provide important information to elucidate further the functional mechanism of PPR proteins and the complicated ABA signalling network.
Highlights
Pentatricopeptide repeat (PPR) proteins are a class of RNAbinding proteins characterized by the presence of a degenerate 35 amino acid repeat, the pentatricopeptide repeat (PPR) motif, which is repeated in tandem 2–50 times
Immunoblot assays by using this anti-SOAR1 serum showed that the amount of SOAR1 protein in the soar1-1D mutant was enhanced >2-fold compared with the wild-type plants (Fig. 1B), which results from this ‘T-DNA’ insertion (Supplementary Fig. S1)
It is noteworthy that the seeds of the SOAR1 OE lines germinated and their post-germination seedlings continued to grow in the medium containing >200 μM (±) abscisic acid (ABA), while the wild-type Col seeds generally do not germinate if the medium contains >3 μM (±)ABA, and the strong ABA-insensitive ABI2-OE line did not grow in the 100 μM
Summary
Pentatricopeptide repeat (PPR) proteins are a class of RNAbinding proteins characterized by the presence of a degenerate 35 amino acid repeat, the PPR motif, which is repeated in tandem 2–50 times. In the Arabidopsis thaliana genome, 450 putative PPR genes were identified, and >600 PPR genes have been predicted to occur in the rice genome (Small and Peeters, 2000; Lurin et al, 2004; Rivals et al, 2006; Schmitz-Linneweber and Small, 2008).
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