Though root architecture modifications may be critically important for improving phosphorus (P) efficiency in crops, the regulatory mechanisms triggering these changes remain unclear. In this study, we demonstrate that genotypic variation in GmEXPB2 expression is strongly correlated with root elongation and P acquisition efficiency, and enhancing its transcription significantly improves soybean yield in the field. Promoter deletion analysis was performed using six 5' truncation fragments (P1-P6) of GmEXPB2 fused with the GUS reporter gene in transgenic hairy roots, which revealed that the P1 segment containing 3 E-box elements significantly enhances induction of gene expression in response to phosphate (Pi) starvation. Further experimentation demonstrated that GmPTF1, a bHLH transcription factor, is the regulatory factor responsible for the induction of GmEXPB2 expression in response to Pi starvation. In short, Pi starvation induced expression of GmPTF1, with the GmPTF1 product not only directly binding the E-box motif in the P1 region of the GmEXPB2 promoter, but also activating GUS expression in a dosage dependent manner. Further work with soybean transgenic composite plants showed that, altering GmPTF1 expression significantly impacted GmEXPB2 transcription, and thereby affected root growth, biomass and P uptake. Taken together, this work identifies a novel regulatory factor, GmPTF1, involved in changing soybean root architecture through regulation the expression of GmEXPB2. These findings contribute to understanding the molecular basis of root architecture modifications in response to P deficiency, and, in the process, suggest candidate genes and a promoter region to target for improving soybean yield through molecular breeding of P efficiency.
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