Phylogenetic tree of B-type RR family members. The full-length protein sequences of poplar B-type RRs deduced from cDNA sequences (EMBL: FN908138 to FN908145) were aligned using Muscle  with those of Arabidopsis (ARRs), soybean (GmRRs), maize (ZmRRs) and rice (OsRRs), obtained from respective genome databases (Additional file 2). Maximum likelihood tree reconstructions were performed with MEGA 5  using the JTT substitution model . The C-type ARR22 from Arabidopsis was used as outgroup. Numbers indicate bootstrap support (1000 replicates).