Peroxidase genes differentially respond to auxin during the formation of adventitious roots in soybean hypocotyl

In our investigation, auxins (IAA, IBA and NAA) effectively promoted rooting in soybean hypocotyls. The activity of anionic peroxidase (POX) (pI 3.7) and cationic POX (pI 8.5) was significantly suppressed by exogenous auxins on day 2 (the inductive phase). Some particular anionic POXs (pI 4.0 and pI 5.3) significantly increased in IBA-treated tissues as compared with the control when the incubation time was prolonged to day 3 and day 4 (the initiation phase). We sequenced 5′-flanking region of pI 8.5 and pI 5.3 POX genes using the PLACE and PlantCARE databases to identify several potential cis-regulatory elements. The pI 8.5 POX gene promoter contained two sites that were homologous to sequences commonly found in auxin response elements; motifs ARF/AuxRE and CATATGGMSAUR. During the inductive phase, the activity of pI 8.5 POX was significantly suppressed by the exogenously applied auxins. The pI 8.5 POX gene promoter contained both ARF/AuxRE and CATATGGMSAUR motifs that responded to auxins earlier than the pI 5.3 POX gene. Hence, the pI 8.5 POX gene might belong to primary auxin response genes. The pI 5.3 POX gene, which responded to auxins a day or two later, contained only ARF/AuxRE motif. Moreover, unlike pI 8.5 and pI 3.7 POXs that were suppressed by auxins, the pI 5.3 POX was induced or enhanced by the applied auxins, especially IBA. The pI 5.3 POX might generate H₂O₂ which caused the auxin-induced growth at the initiation phase during the formation of adventitious root in soybean hypocotyls.