Identification and characterization of S-RNase genes in apple rootstock and the diversity of S-RNases in Malus species

We isolated and confirmed two S-RNases, denoted as mpS1 and mpS2, from apple rootstock ``Marubakaido`` (Malus prunifolia Borkh. Var. ringo Asami). These S-RNases contained and conserved five cysteine residues and two histidine residues, which are essential for RNase activity. The mpS1 showed high similarity to S5 (99.1%) of Malus spectabilis, whereas the mpS2 showed 99.5% nucleotide sequence similarity to S26 of (Malus × domestica) and 99.6% to S35 of (Malus sieversii) when compared with reported S-RNases. In amino acid sequences, the mpS1-RNase was almost similar to the S5-RNase of Malus spectabilis, and the mpS2-RNase was similar to the S35 of Malus sieversii, with only one bp being different from the S26-RNase of Malus × domestica. The 57 S-RNases of Malus species were renamed and rearranged containing the new S-RNases, as mprpS35 (mpS2) and mprpS57 (mpS1), for determining S-genotypes and identifying new alleles from apple species (Malus spp.).